Class 8

Force And Pressure Concepts

• Force
• Pressure

Force And Pressure Objectives

• A push or pull applied on a body is called force.
• The strength of a force is expressed by its magnitude.
• Force is denoted by the letter ‘P and its unit is Newton (N).
• When two forces act at the same point in opposing directions, the resultant force is equal to the difference between the applied forces and is in the direction of the greater force.
• When two forces act at the same point in the same direction, the resultant force is equal to the sum of the two applied forces.
• The effect of force changes when its direction and magnitude change.
• Force applied brings a change in the state of motion of an object.
• The speed of an object increases if the force applied to the object is in the direction of the motion of the object.
• The speed of an object decreases if the force applied to the object is in the direction opposite to that of the motion of the object
• Force can be of two types: contact force and non-contact force.
• Contact force acts when two or more objects are in direct physical contact with each other and bring about necessary changes.
• Muscular force, frictional force, mechanical force, etc., are examples of contact forces.
• The force exerted by the muscles of a body is called muscular force.
• A force that opposes the motion of an object and arises due to contact between two surfaces is called frictional force.
• The force applied by a machine is called mechanical force.
• Non-contact forces act from a distance without coming into direct contact with the body. Magnetic forces, electrostatic forces, gravitational forces, etc., are a few examples of non-contact forces.
• The force exerted by a magnet is called magnetic force.
• The force exerted by a charged body on another body is called electrostatic force.
• The force with which the Earth attracts all objects towards itself is called gravitational force.
• The amount of offeree acting per unit area is called pressure. The SI unit of pressure is Pascal (Pa). Pressure = Force/Area
• Pressure increases if the area over which the force is applied decreases and vice-versa.
• Liquids do not have a definite shape and hence, the pressure exerted by them depends upon the depth of the liquid column.
• Liquids exert pressure not only on the base of the container but also on its sides.
• The pressure exerted by the atmosphere on an object is called atmospheric pressure.
• Atmospheric pressure decreases with an increase in altitude.

Read and Learn More UP Board Notes for Class 8 Science

Force And Pressure Important Terms And Definitions

Pull: It is the force that is applied while picking up an object
Push: It is the force that is applied while kicking or hitting an object.
Weight: It is the force with which the Earth pulls a body towards itself.

Force

Applying force can move a stationary object or stop a moving object Force can also change the direction j of the motion of an object If the force applied is in the direction of the motion of the object, it increases the: speed of the object.

Any object thrown up in the air falls back to the Earth due to the Earth’s gravitational force. The gravitational force exerted by the Earth on an object is called its weight

Force And Pressure Activity 1

Aim: To find the effect offeree on a stationary object
Procedure:

1. Take a heavy box and try- to push it by yourself.
2. Ask one of your friends to help you push the box in the same direction.
3. Next, push the box and ask your friend to push it from the opposite direction.

Observation: In the first case, it was difficult to push the box alone. However, with the help of a friend, it was easier to move the box in the same direction.
It was difficult to move the box when your friend applied force in the opposite direction.
Conclusion: In step 2, the forces were applied in the same direction and the resultant force was the sum of the two forces. Hence, the box moved easily. In step 3, the forces applied were in opposite directions and the resultant force was the difference of the two forces. The box moved in the direction of the greater force.

Force And Pressure Activity 2

Aim: To observe the effect of force on the motion of a moving object
Procedure:

1. Take a rubber ball and place it on a table.
2. Push the ball gently and observe if the ball begins to move.
3. Push the moving ball again.
4. Place your palm in front of the moving ball and remove it as soon as the ball touches your palm.

Observation: The Dali starts moving when it is pushed gently. When it is pushed again, the speed of the moving ball increases. When pain is placed in its path, the speed of the moving ball decreases.
Conclusion: Force can change the motion of an object and can even bring it to rest.

Force And Pressure Activity 3

Aim: To observe the effect of an applied force on the direction of motion of an object Procedure:

1. Take a ball and push it gently.
2. Place a ruler in front of the moving ball.
3. Observe if the moving ball changes its direction on coming in contact with the ruler.

Observation: The direction of the moving ball changes on coming in contact with the ruler. Conclusion: Force can change the direction of motion of an object.

Force And Pressure Activity 4

Aim: To study the effect offeree on the shape of different objects
Observation table:

Conclusion: Force can change the shape of objects.

Force And Pressure Activity 5

Aim: To observe attraction and repulsion between two magnets Which are not in contact.
Procedure:

1. Take a pair of bar magnets.
2. Place one of the magnets over three round-shaped pencils or wooden rollers.
3. Bring one end of the other magnet near the end of the magnet placed on the rollers. Ensure that the two magnets do not touch each other.
4. Now, bring the other end of the magnet near the same end of the magnet placed on the rollers and note down the observation.

Observation: In the first case, the magnets repelled each other. In the second case, they attracted each other.
Conclusion: Two magnets can attract or repel each other without touching each other. Thus, a magnetic force is a non-contact force.

Force And Pressure Activity 6

Aim: To observe the properties of electrostatic force
Procedure:

1. Take a plastic straw and cut it into two equal pieces.
2. Suspend one of the pieces from the edge of a table with the help of a thread.
3. Hold the other piece of the straw in your hand and rub its free end with a sheet of paper.
4. Bring the rubbed end of the straw close to the suspended straw. Make sure that these two do not touch each other. Observe what happens and note it down.
5. Now, rub the free end of the suspended piece of the straw’ with a sheet of paper.
6. Bring the piece of the straw that was rubbed earlier near the free end of the suspended straw again.

Observation: In the first case, both the straws attract each other due to unlike changes present on them. While in the second case, they repel each other due to the same type of change present in them.
Conclusion: Electrostatic force shows similar properties to a magnet. Like charges repel each other, while unlike charges attract each other. Thus, an electrostatic force is a non-contact force.

Objective Type Questions

A. Multiple Choice Questions.

1. Which of the following is an example of contact force?

1. Magnetic force
2. Electrostatic force
3. Gravitational force
4. Frictional force

2. ‘What is the SI unit of force?

1. meter/second
2. Newton
3. meter/hour
4. Pascal

3. Which force is exerted by the Earth on all objects?

1. Gravitational force
2. Frictional force
3. Electrostatic force
4. Mechanical force

Answers:

1. (4) Frictional force
2. (2) Newton
3. (1) Gravitational force

B. Fill in the blanks.

1. The strength of a force is expressed by its______________.
2. The force exerted by a charged body or another body)’ is__________.
3. Muscular force is also known as__________ force.

Answers:

1. Magnitude
2. Electrostatic force
3. Contact

C. State whether the following statements are true or false.

1. Two objects must interact with each other for any force to act upon them.
2. A charged body attracts another charged body due to gravitational force.
3. A push or pull on a body is called force.

Answers:

1. False
2. False
3. True

Short Answer Type Questions

Question 1. Why force of gravity is termed a non-contact force? Explain.

Answer
The force of gravity can act on distant objects that are not in direct contact with the Earth, for example, falling leaves. Hence, it is termed a non-contact force.

Question 2. Why does an object moving on a smooth surface stop by itself?

Answer
An object moving on a smooth surface stops by itself due to the frictional force, which acts between it and the surface on which it is moving. Friction acts in a direction opposite to that of the motion of the object. The surface that looks smooth has many irregularities on it These irregularities lock with the irregularities on the surface of the moving object and stops it.

Question 3. How does the force apply to change the speed of an object?

Answer
If the force is applied in the direction of motion of the object, the speed of the object increases. However, if the force is applied in the direction opposite to that of the motion of the object, the speed of the object decreases.

Question 4. When is the net force applied equally to zero? Give an example.

Answer
When two forces are applied from opposite directions and are equal in magnitude then the net force applied is equal to zero. A game of tug of war is an example of this.

I 1.2 Pressure

• Force per unit area is called pressure. Just like solids, liquids and gases also exert pressure. The air; around us exerts pressure on us. It is called atmospheric pressure. Astronauts wear a special ‘space suit’! to maintain normal atmospheric pressure for their bodies as there is no atmosphere in space.

Force And Pressure Activity 7

Aim: To determine if the pressure exerted by a liquid at the bottom of the container in which it is kept depends on the amount of liquid present in the container
Procedure:

1. Take a transparent glass tube or a Plastic pipe of about 15 cm in length and 5-5.75 cm in diameter.
2. Take a piece of a thin sheet of a good-quality rubber balloon.
3. Stretch and fasten the rubber sheet tightly over one end of the pipe.
4. Hold the pipe in the middle, keeping it in a vertical position, and pour some amount of water into the pipe through its open end.
5. Note down your observation.
6. Pour some more amount of water into the pipe and note down your observation again.

Observation: When some amount of water is poured in the pipe, the rubber balloon bulges out The bulging increases upon pouring more water in the pipe.
Conclusion: The pressure exerted by a liquid at the bottom of the container in which it is kept depends on the amount of liquid present in the container.

Force And Pressure Activity 8

Aim: To determine that liquid exerts pressure on the walls of the container in which they are kept
Procedure:

1. Take a discarded plastic bottle.
2. Fix a cylindrical glass tube, a few centimeters long near the bottom of the bottle by using molten wax.
3. Cover the mouth of the glass tube with a thin rubber sheet and fill the bottle half with water.
4. Note down your observation.
5. Pour some more water into the bottle and observe the rubber sheet.

Observation: The rubber sheet bulges out. On adding more water in the bottle, the bulging of the rubber sheet increases.
Conclusion: The bulging of the rubber sheet shows that liquids exert pressure on the walls of the container in which they are kept.

Force And Pressure Activity 9

Aim: To find if liquids exert equal pressure at the same depth
Procedure:

1. Take an empty plastic bottle.
2. Drill four holes close to the base of the bottle. Make sure that the holes are at the same height as the tie base.
3. Fill the bottle with water and observe.

Observation: Four different streams of water coming out of the holes fall at the same distance from the bottle.
Conclusion: Water falling at the same distances proves that liquids exert equal pressure at the same depth.

Force And Pressure Activity 10

Aim: To determine the effect of atmospheric pressure on an object
Procedure:

1. Take a good-quality rubber sucker and press it hard on a smooth plane surface.
2. Now, try to pull the rubber sucker off the surface.

Observation: The rubber sucker sticks to the plane surface when pressed hard. It is very difficult to pull the rubber sucker from the plane surface.
Conclusion: The rubber sucker sticks to the plane surface because atmospheric pressure acts on it The rubber sucker can be pulled off from the surface only when the applied force is more than the atmospheric pressure.

Objective Type Questions

A. Multiple Choice Questions.

1. ‘Which of the following statements is correct?
(a) atmospheric pressure increases as height increases
(b) Atmospheric pressure remains the same at all heights
(c) Atmospheric pressure decreases as height increases
(d) atmospheric pressure decreases as height decreases

2. How does pressure exerted by a liquid change with depth?
(a) It decreases with depth
(b) It increases with depth
(c) It does not change with depth
(d) It cannot be determined

3. What is pressure?
(a) Volume/Area
(b) Force/Area
(c) Mass/Area
(d) Density/Area

Answers:

1. (c) Atmospheric pressure decreases as height increases
2. (b) ll increases will deeply
3. (b) Force/Area

B. Fill in the blanks.

1. The SI unit of pressure is_____.
2. The envelope of air all around us is known as__________.

Answers:

1. Pascal (Pa)
2. Atmosphere

C. State whether the following statements are true or false.

1. Pressure depends on the area of contact between two surfaces.
2. ‘When a liquid is put in a vessel, it exerts pressure at its bottom only.

Answers:

1. True
2. False

Short Answer Type Questions

Question 1. Why are nail tips made pointed?

Answer
Pressure is force per unit area. The lesser the area the more is pressure. The tips of the nails are made pointed so that maximum pressure can Degenerate per unit force applied and hence, the nails can penetrate in an object easily.

Question 2. Why do divers wear special suits?

Answer
The pressure deep in the sea is very high and can cause damage to the human body. That is why divers wear specially designed suits to protect themselves from high pressu^.

Question 3. Why do cutting instruments have a sharper edge?

Answer
As pressure is force per unit area, all instruments used for cutting, like knives, blades, etc., have a sharp cutting edge. This decreases the area of contact between the surface of the instrument and the object and the pressure exerted by a given force increases.

Textbook Exercises

Question 1. Give two examples of each of the situations in which you push or pull to change the state of motion of objects.

Answer

1. A batsman hitting a ball (Push)
2. Drawing water from a well (Pull)

Question 2. Give two examples of situations in which applied force causes a change in the shape of an object.

Answer

1. Pressing a lump of dough on a plate.
2. A spring fixed in the seat of a bicycle pressed down due to the weight of the rider.

Question 3. Fill in the blanks in the following statements.

1. To draw water from a well we have to___________ the rope.
2. A charged body__________ an uncharged body towards it
3. To move a loaded trolley we have to___________ it.
4. The north pole of a magnet____________the north pole of another magnet.

Answer:

1. Pull
2. Attracts
3. Push or pull
4. Repels

Question 4. An archer stretches her bow while aiming at the target. She then releases the arrow, which begins to move toward the target. Based on this information, fill up the blanks in the following statements.

1. To stretch the bow, the archer applies a force that causes a change in its.__________
2. The force applied by the archer to stretch the bow is an example of___________ force.
3. The type of force responsible for a change in the state of motion of the arrow is an example of a__________ force.
4. While the arrow-‘ moves toward the target, the forces acting on it are due to
   that due to__________ of air.

Answer:

1. Shape
2. Muscular
3. Contact
4. Gravity, friction

Question 5. In the following situations, identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
Answer:

Question 6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Answer
The force applied by the hammer flattens the piece of iron, i.e., it changes its shape and size.

Question 7. An inflated balloon was pressed against a wall after it had been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the sticking balloon and the wall?

Answer
The electrostatic force is responsible for the attraction between the balloon and the wall.

Question 8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Answer
Forces acting on the plastic bucket are:
(i) Force of gravity acting downwards
(ii) The muscular force of arms acting upwards
The two forces do not bring any change in the state of motion of the bucket because both these forces are equal in magnitude and opposite in direction. Thus, they cancel each other and the net force on the bucket is zero.

Question 9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Answer
When the rocket leaves the launching pad, the ‘following forces act on it:
(i) Gravitational force of the Earth which pulls rocket towards the ground.
(ii) Frictional force due to the Earth’s atmosphere which opposes its mot on.

Question 10. When we press the bulb of a dropper with its nozzle in water, the air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to:
a) Pressure of water
b) Gravity of the Earth
c) Shape of a rubber bulb
d) Atmospheric pressure

Answer:
(d) Atmospheric pressure

Force and Pressure Hots Corner

A. Why do your ears ‘pop’ as you go higher up on mountains?
As we go higher up on mountains, our ears ‘pop’ because the air pressure around the ears decreases, causing an imbalance between the pressure inside and outside the ears.

Practice Exercise

Objective Type Questions

A. Give one word for each of the following.

1. Pressure exerted by the air around us
2. Force acting on a surface per unit area
3. A substance used for reducing friction
4. The force applied by the muscles of a human being or animal
5. A push or pull acting on a body, which changes its state o* rest or motion
6. The force exerted by a charged body on another charged or uncharged body

Answers:

1. Atmospheric pressure
2. Pressure
3. Lubricants
4. Muscular force
5. Force
6. Electrostatic force

B. State whether the following statements are true or false.

1. If two forces act on an object from opposite directions, the resultant force is the sum of both forces.
2. Pressure exerted by a liquid at the base of the tie container in which it is kept depends on the depth of the liquid column.
3. The lesser the area of contact, the lesser is the effect of the force acting on an object
4. Mountaineers often suffer from nose bleeds at high altitudes.

Answers:

1. False
2. True
3. False
4. True

Force and Pressure Short Answer Type Questions

Question 1. Why school bags are provided with broad straps?

Answer
We know that the higher is the area of contact, the lesser is the pressure applied on an object School bags with broad straps occupy larger area and do not exert too much pressure on the shoulders. Hence, children do not feel strain on their shoulders.

Question 2. State where pressure is the greatest and the least inside a bottle filled with water.

Answer
In a bottle filled with water, pressure is the greatest at the bottom of the bottle and it is the least at the top. This is because the pressure exerted by liquids depends upon the depth o{the liquid column.

Question 3. If atmospheric pressure is so high, why are we not crushed by it?

Answer
The body of living organisms is made up of cells, which have fluids that exert pressure from within. The pressure exerted from inside the cell is equal to the atmospheric pressure. Thus, we are prevented from being crushed.

Question 4. Mountaineers always carry oxygen cylinders while climbing a mountain. What will happen if they do not carry it and why?

Answer
Mountaineers carry oxygen cylinders while climbing high mountains to avoid suffocation. As we go to a higher altitude, the layer of air that surrounds us becomes thinner. Due to lesser air, there is a decrease in the number of oxygen molecules per breath and this makes a person suffocate at high altitudes.

Also Read

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 12 Friction
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Some Natural Phenomena Chapter Objectives

• Two objects can be electrically charged by rubbing them with one another.
• American scientist, Benjamin Franklin, showed that clouds have electrical charges on them.
• Electrical charges are of two types Positive charges and Negative charges.
• Like electrical charges repel each other while unlike electrical charges attract each other.
• A charged object can transfer its charge to another object through a metal conductor.
• Earthing protects us from electric shocks that may take place due to leakage of electric current.
• Natural phenomena like lightning, earthquakes, etc. cause large-scale destruction of human life and property.
• Lightning occurs due to electric discharge between the clouds and the Earth or between different clouds.
• The lightning conductor is installed at the highest point of a building to protect them from the damaging effects of lightning.
• During lightning and thunderstorm, we must find a safe place for ourselves.
• During the thunderstorm, we must not travel in open vehicles or try to be in open fields or parks.
• The sudden shaking or trembling of the Earth due to a disturbance caused deep inside the Earth’s crust is known as an earthquake.
• Other reasons of earthquakes are volcanic eruptions, meteor hits or an underground nuclear explosion.
• The outermost layer of the earth is fragmented and each fragment is called a plate.
• Earthquakes generally occur at the boundaries of the Earth’s plates. These boundaries are known as seismic or fault zones.
• A seismograph is used to measure the intensity of an earthquake.
• The power of an earthquake is measured on the Richter scale. An earthquake of intensity 7 or more can cause severe destruction of life and property on the Earth.
• Necessary precautions must be taken to protect ourselves from the earthquakes.

Read and Learn More UP Board Notes for Class 8 Science

Some Natural Phenomena Important terms and definitions

Static charges: Electrical charges produced when two objects are rubbed with one another are known as static charges.
Electroscope: A device used to detect the presence of charge on an object is known as
electroscope.
Earthing: The process of transferring charge of a charged object to the Earth is called earthing.
Seismic waves: The waves produced on the Earth’s surface due to the tremors of the earthquake are known as seismic waves.
Seismograph: An instrument used to record the intensity of the earthquake is called seismograph.

Electrical Charges

Two objects acquire static electrical charge when they are rubbed with one another. Static charges do not move by themselves. There are mainly two types of electrical charges positive charges and negative charges. The presence of charge on an object can be detected by an electroscope.

The process of transferring charge of a charged object to the Earth is known as earthing. It is
provided in a tall buildings to protect us from electric shocks that may take place due to the leakage of electric current.

Some Natural Phenomena Activity 1

Aim: To show that objects get electrically charged by rubbing them with other objects
Precaution: Do not touch the rubbed end of the refill with your hand or with a metallic object.
Procedure:

1. Take a used ball pen refill and rub it vigorously with a piece of polythene.
2. Bring the refill close to small pieces of paper.
3. Repeat this activity with small pieces of dry leaf, husk, and mustard seeds. Record your observations.

Observation: After rubbing the plastic refill with a piece of polythene, the refill attracts bits of paper.
Conclusion: An object can be charged by rubbing it with another object.

Some Natural Phenomena Activity 2

Aim: To charge objects by rubbing them with other objects
Procedure:

1. Collect different objects or materials as given in the observation table.
2. Try to charge each material by rubbing it with other materials.
3. Record your observations in the table.

Observation:

Conclusion: Objects acquire a charge when they are rubbed with other objects.

Some Natural Phenomena Activity 3

Aim: To show that like charges repel each other whereas unlike charges attract each other
Procedure:

1. Inflate two balloons and hang them such that they do not touch each other.
2. Rub both the balloons with a woolen cloth and release them.
3. Observe them carefully.

1. Interaction between like charges
   
2. Now, take a used pen refill and rub it with polythene.
3.  Place this refill carefully in a glass tumbler.
4. Rub another refill with polythene and bring it close to the charged refill.
5. Observe the refills.

Interaction between like charges

8. Now, rub a refill and place it gently in a glass tumbler. Bring an inflated charged balloon near the refill and observe it.

Interaction between unlike charges

Observations:

1. The charged balloon repelled the other charged balloon.
2. The charged refill repelled the other charged refill.
3. However, the charged balloon attracted the charged refill.

Conclusion: Like charges attract each other whereas unlike charges repel each other.

Some Natural Phenomena Activity 4

Aim: To show that electrical charge can be transferred from a charged object to another object through a metal conductor

Procedure:

1. Take an empty jam bottle and a piece of cardboard slightly bigger than the mouth of the bottle.
2. Pierce a hole in the cardboard and insert a metal paper clip in it.
3. Now, open the paper clip as shown below.
4. Cut two strips of aluminum foil of size about 4 cm x 1 cm each.
5. Hang these aluminum foil strips on the paper clip.
6. Insert the paper clip in the cardboard lid such that, it is perpendicular to it.
7. Charge a refill and touch it with the end of the paper clip.
8. Observe aluminum strips carefully.

A simple electroscope

Observations: The strips acquire the same charge from the charged refill and repel each other. They become wide open.
Conclusion: Electrical charge can be transferred from a charged object to another object through a metal conductor.

Some Natural Phenomena Objective Type Questions

A. State whether the following statements are true or false.

1. Objects made up of the same materials acquire different electrical charges.
2. The negative charge is acquired by a glass rod when it is rubbed with silk.
3. Positive charges will always attract negative charges.
4. The motion of electrical charges constitutes an electric current.

Answers:

1. False
2. False
3. True
4. True

B. Multiple Choice Questions.

Question 1. Which of the following will charge an object?

1. Rubbing with another object
2. Wetting with water
3. Drying the object in the Sun
4. All of these

Answers: 1) Rubbing with another object

Question 2. Which of these instruments is used to test whether an object is carrying charge or not?

1. Kaleidoscope
2. Microscope
3. Telescope
4. Electroscope

Answers: 4) Electroscope

C. Fill in the blanks.

1. A body or an object possessing charge is said to be a______ object.
2. The electrical charges generated by rubbing are________in nature.
3. Electrical charge can be transferred from a charged object to another object through a____ conductor.
4. ______is the process of transferring of charge from a charged object to the earth.

Answers:

1. Charged
2. Static
3. Metal
4. Earthing

Some Natural Phenomena Short Answer Type Questions

A. What do you mean by ‘static charge’?

Answers: The electrical charges that are generated by rubbing two objects with one another are known as static charges. This means that these charges do not move by themselves.

B. When you rub an air-filled balloon with your dry hair and press it against the wall, it sticks to the wall. Explain why?

Answers: When an air-filled balloon is rubbed with dry hair, it gets charged. Therefore, the presence of electrical charge on the surface of the balloon makes it stick to the wall.

C. What are the two types of electrical charges? What happens when a glass rod is rubbed with silk?

The two types of electrical charges are positive charges and negative charges. When a glass rod is rubbed with silk, it gets electrically charged. The glass rod acquires a positive charge on it.

Lightning: Causes, Effects, and Safety Measures

Lightning occurs due to the electric discharge between the clouds and the Ear or between different clouds. During the lightning and thunderstorms, it is recommended to rush to a safer place like a building, a car with closed windows, etc. If no shelter is available, we must stay far away from all the trees and squat down on our knees with our heads between our hands. Inside the house, the electrical appliances like computers, TVs, etc., should be unplugged. Taking a shower or bath must be avoided.

Lightning conductors protect buildings from the damaging effects of lightning. The lightning conductor consists of a metallic rod at the top of the building connected to an aluminum or copper cable. The cable connects to a copper plate buried in the Earth. The metallic rod acts as a medium to easily transfer electric charge to the ground.

Some Natural Phenomena Objective Type Questions

A. State whether the following statements are true or false.

1. You should avoid taking a bath during a thunderstorm.
2. It is advised to use an umbrella during lightning.
3. The phenomenon of lightning takes place due to the accumulation of electrical charges in the clouds.

Answers:

1. true
2. False
3. True

B. Multiple-Choice Questions

Question 1. The process of electric discharge can occur between which of the following?

1. Two or more clouds
2. A building and a tree
3. Two buildings on the Earth
4. Either a) or b)

Answers: 1) Two or more clouds

Question 2. Which of these should be done during a thunderstorm?

1. Taking a bath
2. Staying inside a building
3. Using wired phones
4. Standing under the trees

Answers: 2) Staying inside a building

C. Fill in the blanks.

1. During the development of a thunderstorm, the air currents move_____while the water droplets move_____
2. When the negative charges of the clouds and the positive charges of the earth meet, they produce
   streaks of bright light in the form of_____

Answers:

1. Upwards, downwards
2. Lightning

Some Natural Phenomena Short Answer Type Questions

A. Which places are safe for taking shelter during a thunderstorm?

Covered vehicles like cars and buses (with windows and doors closed), buildings, and houses are safe places for taking shelter during a thunderstorm.

B. Which position is recommended during a thunderstorm – lying on the ground or squatting?

Squatting is a safer position during a thunderstorm. While squatting, place your hands on your knees with your head between your hands.

Some Natural Phenomena Long Answer Type Questions

A. Explain how lightning occurs between two clouds during a storm.

Answers: During a thunderstorm, the air currents move upward while the drops of water move downwards. These movements cause the separation of charges such that the positive charges are collected near the upper surface of the clouds and the negative charges are collected near the lower surface of the

cloud. At the same time, a positive charge is accumulated near the ground. When the accumulated charges become large in magnitude, the positive and negative charges meet and produce lightning.

Accumulation of charges leading to lightning

Earthquakes: Causes, Effects, and Protective Measures

Earthquakes cause huge damage to life and property. Earthquake occurs due to the motion of Earth’s plates. The Earth’s plates collide and move past one another or slide over one another.

Movement of earth’s plates

A seismograph records the seismic waves produced due to the tremors of the earthquake on the Richter scale. A seismograph consists of a vibrating rod or a pendulum, which vibrates when the tremors of an earthquake occur. The pen attached to the vibrating system records the seismic waves on the paper moving under it.

Necessary precautions must be taken to protect ourselves from earthquakes. We should avoid the use of heavy construction materials in the highly earthquake-prone areas. The buildings constructed must be ‘Quake Safe’. If we are inside the house, we must take shelter under a table and stay away from heavy objects. If we are outside the house, we must move to a clear spot.

Some Natural Phenomena Objective Type Questions

A. State whether the following statements are true or false.

1. 1Earthquakes can also cause floods, landslides, and tsunamis.
2. The fault zones are the most susceptible regions to earthquakes.
3. An earthquake measuring 8 on the Richter scale is mild and quite harmless.

Answers:

1. True
2. True
3. False

B. Multiple-Choice Questions

Question 1. Which of the following instruments is used to record seismic waves during an earthquake?

1. Seismograph
2. Richter scale
3. Electroscope
4. Barometer

Answers: 1)Seismograph

Question 2. Most earthquakes are caused by disturbances in which of these parts of the earth?

1. Crust
2. Mantle
3. Inner core
4. All of these

Answers: 1)Crust

C. Fill in the blanks.

1. Each fragment of the Earth’s crust is called a____
2. The outermost layer of the Earth is known as_____

Answers:

1. Plate
2. Crust

Some Natural Phenomena Short Answer Type Questions

A. How does a seismograph work?

Answers: A seismograph consists of a vibrating rod or a pendulum, which vibrates when tremors occur. The pen attached to the vibrating system records the seismic waves on the paper moving under it. Scientists construct a complete map of the earthquake by studying these waves.

Some Natural Phenomena Long Answer Type Questions

A. What is an earthquake? How is the magnitude of an earthquake expressed?

Answers: The sudden shaking or trembling of the Earth due to a disturbance caused deep inside the Earth’s crust is known as an earthquake. The magnitude of an earthquake is expressed in the Richter scale. The earthquakes measuring up to 3 on the Richter scale are said to be mild and do not cause any damage. On the other hand, earthquakes measuring 7 or more on the Richter scale are very severe and can destroy the entire area where they occur.

Some Natural Phenomena Textbook Exercises

Select the correct option in Questions 1 and 2.

Question 1. Which of the following cannot be charged easily by friction?

1. A plastic scale
2. A copper rod
3. An inflated balloon
4. A woollen cloth

Answers: (2) A copper rod

Question 2. When a glass rod is rubbed with a piece of silk cloth the rod

1. and the cloth both acquire a positive charge.
2. becomes positively charged while the cloth has a negative charge.
3. and the cloth both acquire a negative charge.
4. becomes negatively charged while the cloth has a positive charge.

Answers: (2) becomes positively charged while the cloth has a negative charge.

Question 3. Write T against true and F against false in the following statements:

1. Like charges attract each other.
2. A charged glass rod attracts a charged plastic straw.
3. Lightning conductors cannot protect a building from lightning.
4. Earthquakes can be predicted in advance.

Answers:

1. F
2. T
3. F
4. F

Question 4. Sometimes, a crackling sound is heard while taking off sweaters during winter. Explain.

Answers: Woolen clothes get charged when they get rubbed with the human body. When these clothes are taken off, the electric discharge takes place between the sweater and the body. This electric discharge produces a crackling sound.

Question 5. Explain why a charged body loses its charge if we touch it with our hand.

The human body is a conductor of electricity. When we touch a charged body with our hand, the charge of the charged body passes to the Earth through our body. Thus, the charged body loses its charge.

Question 6. Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?

Answers: The destructive energy of an earthquake is measured in the Richter scale. Yes, an earthquake that measures 3 on the Richter scale will be recorded by a seismograph. Such an earthquake is mild and does not cause any damage to life and property.

Question 7. Suggest three measures to protect ourselves from lightning.

Answers:

Three safety measures to protect ourselves from lightning during a thunderstorm are:

• Stay indoors or under a covered area
• Do not take a shower or bath
• Do not use electrical appliances or wired phones at home

Question 8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon.

Answers: The same materials get the same charges on rubbing and the same charges to repel each other. Therefore, balloons on rubbing are similarly charged and they repel each other. The charged and the uncharged balloons do have the same charges on them. Therefore, they attract each other.

Question 9. Describe with the help of a diagram an instrument that can be used to detect a charged body.

Answers: An electroscope can be used to detect if a body is charged or not. When a charged body touches the metal conductor, the metal strips repel each other and open wide. This proves that the body is charged.

A simple electroscope

Question 10. List three states in India where earthquakes are more likely to strike.

Answers: In India, the earthquake-prone areas are Kashmir, Gujarat, and Rajasthan.

Question 11. Suppose you are outside your home and an earthquake strikes. What precautions would you take to protect yourself?

Answers: During an earthquake, if we are outside the home, we should find a clear spot, away from buildings, trees, and overhead power lines. If in a car or a bus, we should not come out and ask the driver to drive slowly to a clear spot.

Question 12. The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.

Answers: Carrying an umbrella is not a good idea during a thunderstorm because it has metal wires and a metal rod in it which can provide an easy path for the electric discharge from the clouds. Hence, it increases the risk of electrocution in case of lightning strikes.

Some Natural Phenomena Hots corner

A. A glass rod can be charged by rubbing when held in hand. However, an iron rod cannot be charged in the same way. Can you explain why?

Answers: A glass rod is an insulator. Hence, it does not transfer the electric charge produced on its surface through our hand and the body to the Earth. However, an iron rod is a good conductor of electricity. As soon as it gets charged by rubbing, the electric charges produced on its surface flow through our hand and body into the Earth. Therefore, the iron rod remains uncharged.

Some Natural Phenomena Practice Exercises

Objective Type Questions

A. Circle the odd one out.

1. Polythene, silk, woolen cloth, wet hair
2. Seismic zone, safe zone, fault zone, weak zone
3. Plastic refill, glass rod, balloon, metal rod

Answers:

1. Wet hair
2. Safe zone
3. Metal rod

B. Give one word for the following.

1. The device is used to protect buildings from the damaging effects of lightning.
2. The device is used for detecting the presence of charge on an object.
3. The sudden shaking or trembling of the Earth.
4. Waves are produced during an earthquake.

Answers:

1. Lightning conductor
2. Electroscope
3. Earthquake
4. Seismic waves

C. State whether the following statements are true or false.

1. If you are indoors during an earthquake, it is best to rush outside immediately.
2. We should go to the roof of tall buildings when it is lightning.
3. During an earthquake, the Earth’s plates collide with each other.

Answers:

1. True
2. False
3. True

D. Fill in the blanks.

1. Earthquakes tend to occur at the_____of the Earth’s plates.
2. _____is the rapid flow of charge through air between the two oppositely charged clouds.
3. Lightning occurs when____and_____ charges meet.
4. Two charged plastic refills will_____each other.

Answers:

1. Boundaries
2. Electric discharge
3. Positive, negative
4. Repel

Some Natural Phenomena Short Answer Type Questions

A. Give two common examples of static electricity from your daily life.

Answers:

Two common examples of static electricity are:

1. When the television sets are turned on for a long time a layer of charge develops on their screens. This is due to the presence of static electricity.
2. When we brush our hair, they start getting attracted to the hairbrush. This is because they have acquired charge in the form of static electricity.

B. Which regions of the Earth are more prone to earthquakes?

Answers: The regions that fall on the boundaries of the Earth’s plates are weak or dangerous zones. Earthquakes are more likely to occur in these zones. These weak zones are also called seismic or fault zones.

C. Do earthquakes bring some changes to the Earth’s surface? Explain.

Answers: Yes, earthquakes can bring some geographical changes to the Earth’s surface. During an earthquake, the Earth starts shaking. Sometimes, deep and large slits appear on the Earth’s surface. Earthquakes with high intensity are so strong that new islands and coral beaches may be formed. Some freshwater springs have also been formed due to earthquakes.

Also Read

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 11 Force and Pressure
• Chapter 12 Friction
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Friction Concepts

• Force of Friction and factors affecting friction
• Friction: A Necessary Evil
• Increasing and reducing friction
• Fluid Friction

 Friction Objectives

• The force that opposes the relative motion between two objects in contact is called friction.
• Friction always acts in the direction opposite to the direction of the applied force.
• Friction is caused due to the irregularities on both the surfaces which are in contact
• The force of friction depends on the nature of the surfaces in contact and the mass of the objects in contact
• The force of friction is greater on rough surfaces as compared to smooth surfaces.
• Friction causes wear and tear, and wastage of energy.
• Friction can be considered as a ‘necessary evil’ because our normal life would not function properly without friction.
• Friction can be reduced by either polishing the surfaces or using lubricant wheels call bearings. Friction can be increased by increasing the roughness of the surface and mass of the object Friction is of three types static friction, sliding friction, and rolling friction.
• The frictional force exerted by fluids is called fluid friction.
• Fluid friction depends on the shape of the object and the nature of the fluid.
• The shape of the bodies of birds and fishes is such that they overcome fluid friction easily using less energy. Such shapes are called streamlined.

Read and Learn More UP Board Notes for Class 8 Science

Friction Important terms and Definitions

Ball bearing: Ball Bearings are rolling elements that use balls to reduce friction between the moving parts of the bearing.
Drag: The force of friction exerted by fluids is called drag.
Static friction: The force required to overcome friction when an object starts moving from rest is called static friction.
Sliding friction: The force required to keep an object moving with a uniform speed is called sliding friction.
Rolling friction: The force required to keep an object moving when it rolls over the other is called rolling friction.

12.1 Force of Friction and factors affecting friction

• The force that opposes the relative motion of two objects, when their surfaces are in contact with each other is called friction. If we push a ball forward, the force of friction acts in the backward direction and tends to stop the ball. Friction is caused by the irregularities present on the surfaces which are in contact Thus, rough surfaces exert greater frictional force as compared to smooth surfaces. The force of friction acting on a body depends upon the mass of the body and the nature of the surface. The force of friction increases if the two surfaces are pressed harder on each other.
• The force required to move an object from rest is called static friction. The force required to keep an object in motion is called sliding friction. The force exerted when two objects roll over each other is called rolling friction. The value of static friction is more than sliding and rolling friction. The value of rolling friction is less than sliding friction.

Friction Activity 1

Aim: To demonstrate the presence of friction
Procedure:

1. Gently push a book on the table.
2. After the book stops, repeat the activity by using the book from the opposite side.

Observation: When pushed the book slides for some time and then stops.
Conclusion: The motion of the book on the table is opposed by the frictional force. The force which opposes the motion of one body over the other is called friction.

Friction Activity 2

Aim: To demonstrate that rough surfaces exert greater frictional force as compared to smooth surfaces
Procedure:

1. Take a string around a brick and pull the brick by a spring balance.
2. Note down the reading on the spring balance when the brick begins to move.
3. Now, wrap a polythene sheet around the brick and repeat the activity.

Observation: The fading on the spring balance for the uncovered brick was more than that for the brick covered with polythene.
Conclusion: When polythene was not wrapped on the brick the surface of the brick was rough. So, the brick experienced greater frictional force. When polythene was wrapped around the brick the surface of the brick became smooth. So, the brick experienced lesser frictional force. Thus, rough surfaces exert greater frictional force as compared to smooth surfaces.

Friction Activity 3

Aim: To determine that friction depends on the nature of the surfaces in contact
Procedure:

1. Make an inclined plane on a smooth floor, or on a table as shown in the diagram below.
2. Put a mark with a pen at any point on the inclined plane.
3. Let a pencil cell move down from this point
4. Note down the distance the pencil cell moves on the table before coming to rest.
5. Spread a piece of cloth over the table but make sure that there are no wrinkles on the cloth.
6. Repeat the activity.

Observation:
The pencil cell rolls to a greater distance on the smooth floor or table than the floor or table covered with cloth.
Conclusion:
The resistance offered by the smooth table or floor is less as compared to the resistance offered by the cloth. Thus, the pencil cell covers a shorter distance on the cloth. The distance covered by the pencil cell depends on the nature of the surface on which it moves. Thus, friction depends on the nature of the surfaces in contact.

Objective Type Questions

A. State whether the following statements are true or false.

1. Rolling friction is less than sliding friction.
2. Rolling friction exists between the surfaces when the body rolls over another.
3. Friction depends on gravity.

Answers:

1. True
2. True
3. False

B. Fill in the blanks.

1. The frictional force__________ with an increase in the force applied.
2. The force required to move an object from rest is called____________ friction.
3. The value of rolling friction is___________ then static friction.

Answers:

1. Increases
2. Static
3. Less

Short Answer Type Questions

12.2 Friction: A Necessary Evil

• Friction plays an important role in our daily life. It would be impossible to walk f there was no friction between the soles of ou^ shoes and the ground. Matchsticks are lit due to the force of friction between the match-head and the rough side of the matchbox. Cars and buses are able to move on the road because of friction between the tires and the road. Braking a vehicle is possible due to friction between the brake lining and the wheels of the vehicle.
• However, the tires of vehicles and the soles of shoes wear out due to friction. Friction causes a waste of energy; this is because anything that moves has to overcome the force of friction. This energy is converted into heat energy that, in turn, increases the temperature. The heat produced in the moving parts of machinery due to friction results in wear and tear of the parts. Friction reduces the speed of moving vehicles to a great extent.
  Thus, friction is considered a necessary evil.

Objective Type Questions

A. State whether the following statements are true or false.

1. Friction causes a waste of energy.
2. Meteors get burnt due to the friction of air.
3. A rough surface experiences less wear and tear.

Answers:

1. True
2. True
3. False

B. Fill in the blanks.

1.  A bicycle comes to rest when we stop pedaling due to_________.
2.  ‘When we rub both of our hands with each other, they become warm due to__________
   produced by __________.

Answers:

1. Friction
2. Heat, Friction

C. Multiple Choice Questions.

1. Which of the following devices was used in ancient times to keep track of time?
(a) Digital Watch
(b) Stop Watch
(c) Wall Clock
(d) Sundial

2. Which of the following is true for two pendulums A and B of lengths 5 cm and 10 cm respectively?
(a) Both have the same time period
(b) A has a greater time period than B
(c) B has a greater time period than A
(d) Time period of A and B cannot be determined

Answer:

1. (d) Sundial
2. (c) B has a greater time period than A

Short Answer Type Questions

Question 1. Why are spaceships provided with heat shields?

Answer
When spaceships enter the Earth’s atmosphere they experience friction. The heat shields protect; them from the heat generated due to friction during their entry into the earth’s atmosphere.

12.3 Increasing and reducing friction

• Friction is caused by the roughness of the surfaces. friction can be reduced by: polishing the surfaces to make them smooth, using suitable lubricants or grease, using wheels or ball bearings, and giving an object a streamlined shape.
• Friction can be increased by increasing the roughness of the surface or the mass of the object. Spikes and grooves are provided in the soles of the shoes of athletes to increase friction.

Friction Activity 4

Aim: To demonstrate that rolling friction is smaller than sliding friction
Procedure:

1. Place a book on the table and push it from one side.
2. Place three or four cylindrical pencils on the table parallel to each other.
3. Now, place the books over these pencils as shown in the diagram below.
4. Push the book and observe the pencils rolling as the book moves.

Observation:
It was easier to move the books when placed on cylindrical pencils.
Conclusion:
The value of rolling friction is less than sliding friction.

Objective Type Questions

A. State whether the following statements are true or false.

1. Spikes in shoes are provided to decrease friction.
2. Lubricants are oil-like substances used to reduce friction.

Answers:

1. False
2. True

B. Multiple-Choice Questions

1. Which of the following methods is used to increase friction?
(a) Polishing
(b) Roughening the surface
(c) Sfr’eamlining
(d) Lubrication

2. Use of bail bearings between hubs and the axles of ceiling fans is an example of______.
(a) Rolling Friction
(b) Static Friction
(c) Sliding Friction
(d) Kinetic Friction

Answers:

1. (b) Roughening the surface
2. (c) Sliding Friction

Short Answer Type Questions

Question 1. How are ball bearings useful to reduce friction?

Answer
The value of rolling friction is less than si ding friction. Ball bearings convert sliding friction into rolling friction and thus reduce friction. They are used in most of industrial machinery that has moving parts, car axles, etc.

12.4 Fluid Friction

• Fluids exert friction on the objects moving through them. This friction is called fluid friction or drag. The fluid friction exerted on a body depends on the shape of the moving body, the velocity of the moving body, and the nature of the fluid. Objects with streamlined shape experience less fluid friction. It is for this reason that boats and ships are streamlined. Automobiles are narrow from the front, so as to reduce drag in the air. The streamlined shape of fish enables them to move easily in water by minimizing fluid friction. Airplanes, rockets, and missiles have streamlined shapes to move in the air easily.

 Friction Objective Type Questions

A. State whether the statements are true or false.

1. Fluids exert force on the bodies moving through them. This force is called surface tension.
2. Fluid friction is also called drag.

Answer:

1. False
2. True

B. Multiple Choice Questions.

1. Which of the following factor has no effect on fluid friction?
(a) Mass of the object
(b) Shape of the object
(c) Velocity of the Object
(d) Nature of the fluid

2. Airplanes and rockets have___________ shape to reduce friction.
(a) Circular
(b) Triangular
(c) Streamlined
(d) None of the above

Answer:

1. (a) Mass of the object
2. (c) Streamlined

 Friction Short Answer Type Questions

Question 1. What is the reason behind the shape of airplanes?
Answer
The objects with streamlined shape experience less fluid friction. Airplanes have to streamline shape so that they can overcome the frictional force of the air.

Textbook Exercises

Question 1. Fill in the blanks.

1. Friction opposes the__________ between the surfaces in contact with each other.
2. Friction depends on________of the surfaces.
3. Friction produces___________ .
4. Sprinkling of powder on the carom board__________ friction.
5. Sliding friction is___________ than the static friction.

Answers:

1. relative motion
2. nature
3. heat
4. decreases
5. less

Question 2. Four children were asked to arrange forces due to rolling, static, and sliding frictions in decreasing order. Their arrangements are given below. Choose the correct arrangement.

1. Rolling, static, sliding
2. Rolling, sliding, static
3. Static, sliding, rolling
4. Sliding, static, rolling

Answer

3. Static, sliding, rolling

Question 3. Alida runs her car on a dry marble floor, wet marble floor, newspaper, and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be

1. Wet marble f our, dry marble floor, newspaper, and towel.
2. Newspaper, towel, dry marble floor, wet marble floor.
3. Towel, newspaper, dry marble floor, wet marble floor.
4. Wet marble floor, dry marble floor, towel, newspaper.

Answer:

4.  Wet marble floor, dry marble floor, newspaper, and towel.

Question 4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of the frictional force acting on it.
Answer:

The frictional force acting on it is in the upward direction.

Question 5. You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer:

Soapy water will make walking more difficult Soapy floor acts as a smooth surface as it has no irregularities in it and soap is also very slippery in nature.

Question 6. Explain why sportsmen use shoes with spikes.
Answer:

Sportsmen use shoes with spikes to get a better grip on the ground. Spikes act as irregularities and help in increasing friction.

Question 7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer:

Seema will experience more frictional force since the heavier box applies greater pressure on the floor as compared to the lighter box

Question 8. Explain why sliding friction is less than static friction.
Answer:

When a body is at rest, then the force acting between the two surfaces in contact is more due to irregularities on the two surfaces locked into one another. However, when a body is moving or sliding, the contact points do not get enough time to lock into one another. So sliding friction is always less than static friction.

Question 9. Give examples to show that friction is both a friend and a foe.
Answer: Friction is considered a friend. It is because of friction that:

1. We can write with a pen or pencil.
2. A teacher can write with chalk on the blackboard.
3. We can walk on the floor.
4. We can fix a nail on the wall.

Friction is considered a foe. It is because of friction that

1. Things experience wear and tear.
2. Heat is produced in machines that damage their parts.
3. The speed of the vehicle is reduced due to friction.

Question 10. Explain why objects moving in fluids must have special shapes.
Answer: When objects move through fluids, they have to overcome friction acting on them. In this process, they lose energy. Efforts are made to minimize friction by giving special shapes to the objects.

Friction Hots Corner

Question 1. Why do tires in the vehicles have designs and patterns?

Answer
Tires are the vehicles have designs and patterns (treads) with grooves on the surface to increase
friction and channel away water that, act as a lubricant on the road.

Question 2. Why it takes much longer time and distance to stop a moving ship in water than a moving car on the road?

Answer
The friction between the surface of water and the ship is very less due to the streamlined shape of the ship and hence the ship takes more time to stop and covers a large distance. On the contrary, the friction between the tires and the surface of the road is quite large. Hence, a car stops immediately; when brakes are applied.

Practice Exercises

 Friction Objective Type Questions

A. Fill in the blanks.

1. Grooves are made in the tyres of vehicles to__________ friction.
2. Polishing a rough surface________ friction.
3. Friction always acts in the direction_________ to the motion of the object
4. Liquids and gases are called__________.
5. The___________ of both the surfaces with each other causes friction.

Answers:

1. Reduce
2. Reduces
3. Opposite
4. Fluids
5. Interlocking

B. State whether the following statements are true or false.

1. Friction exerted on an object does not depend on its mass.
2. It is easier to roll an object than to slide it.
3. Friction produces heat.
4. Friction can be increased by drying the surfaces in contact.
5. If two surfaces are pressed harder, the force of friction increases.

Answer:

1. False
2. True
3. True
4. True
5. True

C. Give reasons for each of the following.

1. A ball rolling on the ground stops after some time.
2. The shoes of athletes have grooves on their soles.
3. Machine parts are oiled occasionally.
4. It is difficult to write on glazed paper.
5. Airplanes and cars are given streamlined shapes.

Answer:

1. A ball rolling on the ground stops after some time because of the friction between the ball and the surface of the ground.
2. Spikes and grooves are provided in the soles of the shoes of athletes to increase friction.
3. Machine parts are oiled occasionally to reduce friction. This in turn reduces wear and tear of the machine parts.
4. It is difficult to write on glaze paper because the smooth surface of the glazed paper reduces friction.
5. Airplanes and cars have streamlined shapes to reduce frictional force.

 Friction Short Answer Questions

Question 1. Why are the handles of the bikes provided with rough surfaces?

Answer
The handles of bikes are provided with rough surfaces so as to provide a better grip due to friction.

Question 2. Why does a horse require more force to start a tonga than to keep it moving?

Answer
In the first few steps when the horse starts the tonga, it works against state friction. After that tonga begins to move, the horse has to work only against rolling friction. The value of static friction is more than rolling friction. Thus, the horse applies more force to start a tonga than to keep it moving.

Also Read

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 11 Force and Pressure
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Reaching The Of Adolescence Chapter Objectives

• Adolescence is the period of transition from childhood to adulthood during which the human body undergoes changes leading to reproductive maturity. It starts from the age of II and continues till 18 or 19 years of age.
• Sweat and sebaceous glands become more active during adolescence causing acne and pimples on the face of boys and girls during this period.
• Reproductive organs become fully functional during adolescence. In boys, testes start producing sperm and in girls, ovaries enlarge and start releasing eggs.
• Larynx is the voice-producing area present in the throat. Puberty leads to change in the voice of boys and girls. Boys; voice becomes deep and those of girls become high-pitched.
• Hormones produced by endocrine glands control the changes that take place during adolescence.
• Endocrine glands secrete hormones directly into the bloodstream. Hence, they are also known as ductless glands.
• Testosterone, the male hormone, is released by the testes. It helps in sperm production.
• Estrogen, the female hormone, is released by the ovaries and it enables eggs to mature in the ovaries. Testosterone and estrogen are known as sex hormones. They also help in the development of secondary sexual characters.
• Thyroid gland, located in the throat region, produces thyroxine.
• Thyroxine controls the rate of metabolism, growth, and respiration.
• Goitre, a disease of the thyroid gland, occurs due to the deficiency of iodine in diet.
• Adrenalin, produced by adrenal glands, helps the body to adapt to stress, anger, embarrassment, or worry.
• All human beings have 23 pairs of chromosomes in the nuclei of their cells. Two chromosomes out of these are sex chromosomes.

Read and Learn More UP Board Notes for Class 8 Science

Reaching The Of Adolescence  Important Terms And Definitions

Adam’s apple: The protruding part of the throat in boys is referred to as the Adam’s apple.
Secondary Sexual Characters: Features that help to distinguish males from females such as facial and chest hair in males and breasts in females are known as secondary sexual characters.
Ovulation: The release of a mature egg from one of the ovaries after every 28 to 30 days is called ovulation.
Menstruation: It is the bleeding that occurs in females every month and continues for 3-5 days.
Menarche: It the first menstrual flow that occurs at puberty.

Adolescence and Puberty

Adolescence is the period during which the human body undergoes changes leading to reproductive
maturity. Adolescence starts at age of II and continues till 18 or 19 years of age. Puberty refers to the process of physical changes that happens to child’s body leading to sexual maturity. The child changes into an adult capable of sexual reproduction.

Reaching The Adolescence Activity 2

Aim: To observe the increase in the height of boys and girls with age
Procedure:
1. Use the data given in Activity I and draw a graph.
2. Mark age (in years) on the X-axis and percentage of full height on the Y-axis.
3. Highlight the point that represents your age.
4. Calculate the percentage of height you have already reached.
5. Calculate the height you might eventually reach. Tally your graph with the one shown below.

Conclusion: The rate of growth in height varies in different individuals.

Reaching The Of Adolescence Objective Type Questions

A. State whether the following statements are true or false.

1. Boys have high-pitched voices.
2. Genes inherited from parents influence the height of an individual.
3. Acne in teens is caused due to the increased activity of sweat and oil glands.
4. Testes in males, start producing sperm during puberty.
5. Ovaries of a girl start releasing ovum soon after birth.
6. During adolescence, the body’s muscles grow more prominently in boys than in girls.

Answers:

1. False
2. True
3. True
4. True
5. False
6. True

B. Multiple Choice Questions.

Question 1. At what age do boys and girls attain their maximum height?

1. 15 years
2. 20 years
3. 18 years
4. 16 yearsAnswers:  3) 18 years

Question 2. What is the protruding part of throat in boys called?

1. An apple
2. Adam’s apple
3. Adam’s box
4. None of theseAnswers: 2) Adam’s apple

Question3. Due to which of the following reasons does the voice of boys change during puberty?

1. Increased activity of oil glands
2. Development of sex organs
3. Brain’s increased capacity for learning
4. Growth of voice boxAnswers: 4) Growth of voice box

Question 4. What other name is given to adolescents?

1. Adults
2. Teenagers
3. Newborn babies
4. None of these Answers: 2)Teenagers

Question C. Fill in the blanks.

1. Changes in the human body during adolescence mark the onset of______
2. The period between childhood and adulthood is known as_______
3. On an average, girls reach puberty at the age of______years.
4. ________and sebaceous glands become more active during puberty.

Answers:

1. Puberty
2. Adolescence
3. 11-13
4. Sweat

Reaching The Of Adolescence Short Answer Type Questions

A. Is the adolescence and puberty the same?

Answers: Adolescence and puberty are not the same. Puberty is the process of physical changes by which adolescents reach sexual maturity, while adolescence is the period of psychological and social transition from childhood to adulthood.

Hormones and their Role in Reproduction

Answers: Hormones are chemical substances which control the changes that take place in human beings during adolescence. The testes in males secrete testosterone or the male hormone at the onset of puberty. It helps in the production of sperm in testes. In females, the ovaries release estrogen or the female hormone which makes the breasts develop.

Activity 3

Aim: To identify the importance of consuming iodised salt
Procedure:
Collect information regarding the importance of consuming iodised salt from doctors, magazines and the internet.
Observation: Thyroid gland needs iodine to produce thyroxine, which regulates the metabolic activities of the body.
Conclusion: lodine is essential for the production of thyroxine hormone, which regulates body’s metabolic activities. lodine deficiency causes a disease known as goitre.

Reaching The Of Adolescence Objective Type Questions

A. State whether the following statements are true or false.

1. Hormones are released by endocrine glands.
2. Thyroxine is a sex hormone.
3. Both males and females develop hair under arms and in the pubic region.
4. The male hormone testosterone is secreted by the pituitary gland.

Answers:

1. True
2. False
3. True
4. False

B. Multiple Choice Questions.

Question 1. Which hormone helps in the production of sperms?

1. Estrogen
2. Testosterone
3. Thyroxine
4. AdrenalinAnswers: 2) Testosterone

Question 2. Which of the following is the female hormone?

1. Estrogen
2. Testosterone
3. Thyroxine
4. AdrenalinAnswers: 1) Estrogen

Question 3. Which of the following are the milk-secreting glands that develop inside breasts?

1. Endocrine glands
2. Pituitary gland
3. Thyroid
4. Mammary glandsAnswers: 4) Mammary glands

Question 4. What is the role of adrenalin?

1. Maintains blood sugar level
2. Maintains body temperature
3. Maintains the salt balance in the blood
4. Maintains reproductive healthAnswers:  3) Maintains the salt balance in the blood

Question 5. Which element is essential for the formation of thyroxine hormone?

1. Sodium
2. lodine
3. Bromine
4. MagnesiumAnswers: 2) lodine

C. Fill in the blanks

1. Adolescence lasts up to_____ years of age.
2. The specific body part to which a hormone reaches and the body part reacts to the hormone is called the_______
3. _____is also called the voice box.
4. Growth hormone is produced by the_____gland.
5. Hormones are the secretions of the_____gland.

Answers:

1. 19
2. target site
3. larynx
4. pituitary
5. endocrine

Reaching The Of Adolescence Short Answer Type Questions

A. Why endocrine glands are called ductless glands?

Answers: Endocrine glands are known as ductless glands because they release their hormones directly into the bloodstream. Thyroid and pituitary glands are examples of endocrine glands.

B. Why iodine is important for us?

Answers: lodine is important for the production of thyroxine hormone, which regulates the metabolism of our body. Deficiency of iodine causes a disease called goitre.

C. Write the main characteristics of hormones.

Main characteristics of hormones are:

1. Hormones are secreted by the endocrine glands.
2. They have specific functions and hence, they affect specific organs only.
3. Hormones are required in very little quantity.

D. Give the names of hormones secreted by the following endocrine glands.

1. Thyroid gland   2. Adrenal gland   3. Pancreas

1. Thyroid gland secretes thyroxine
2. Adrenal gland secrets adrenalin
3. Pancreas secretes insulin and glucagon

Phases of Reproduction in Human Beings

In females, at the onset of puberty, ovaries mature and start releasing an egg every 28 to 30 days. Pregnancy occurs when an egg gets fertilised by a sperm.

Reaching The Of Adolescence Activity 4

Aim: To identify food items which are responsible for growth and development during adolescence
Note: This answer may vary from one student to another. Here is what a sample answer may look like.
Procedure:

1. Create a group and note down the food items which you had taken on the previous day.
2. Identify the items that enable proper growth.

Conclusion:

1. A balanced diet includes carbohydrates, fats, vitamins, proteins, minerals, water and fibers in appropriate quantities.
2. Food items such as burgers, chips, tinned snacks are not good for health and should be consumed only occasionally.

Reaching The Of Adolescence Activity 5

Aim: To spread awareness about the benefits of a balanced diet
Procedure:

1. Prepare posters, charts depicting items that make-up a balanced diet.
2. Display these charts and posters in your class.

Conclusion:
A balanced diet consists of all the nutrients, carbohydrates and fats, vitamins, water, minerals, and fibers, in the proper proportion. It is needed to provide energy to the body, to facilitate the formation of blood, bones, and teeth, to regulate body functions, to initiate chemical reactions in the body, and to have a healthy digestive system.

Reaching The Of Adolescence Activity 6

Aim: To prepare a report on the advantages of regular exercise
Procedure:

1. Find out how many of your classmates exercise regularly and how many don’t.
2. Observe and note down the difference in their fitness level.

Observation:

1. The classmates who exercise regularly are energetic, more alert and have a better immune system.
2. The classmates who do not exercise are lethargic, lack focus and are often unwell. Conclusion: Physical exercises such as walking, running and playing outdoor games increase physical and mental health.

Reaching The Of Adolescence Objective Type Questions

A. State whether the following statements are true or false.

1. Females attain reproductive maturity between the ages of 10 to 12 years.
2. Menopause occurs around the age of 30 to 40 years.
3. Testosterone is a female hormone.
4. Sex chromosome in human males consists of two X chromosomes.
5. Reproductive life of a woman lasts till menopause.

Answers:

1. True
2. False
3. False
4. False
5. True

Reaching The Of Adolescence Multiple Choice Questions.

Question 1. Which of the following is the process wherein one of the ovaries produces an egg or ovum, after it matures, in every 28 to 30 days?

1. Menstruation
2. Ovulation
3. Menarche
4. Menopause

Answers: 2)Ovulation

Question 2. Which of the following is not included in the menstrual cycle?

1. Release of egg
2. Maturing of egg
3. Fertilisation
4. dShedding of uterus wall

Answers: 3)Fertilisation

Question 3. How many pairs of sex chromosomes are present in human beings?

1. One
2. Two
3. Three
4. Four

Answers: 1) One

Question 4. Where does fertilisation in human beings take place?

1. Fallopian tube
2. Uterus
3. Vagina
4. All of these

Answers: 1) Fallopian tube

C. Fill in the blanks.

1. ________is the first menstrual flow.
2. The bleeding that occurs in females after every 28 to 30 days is called_______
3. Insulin is secreted by the_______
4. Human beings have_________pairs of chromosomes.
5. The male hormone testosterone is secreted by_______
6. In human beings, the_______is responsible for determining the sex of the child.

Answers:

1. Menarche
2. Menstruation
3. Pancreas
4. 23
5. Testes
6. Sex chromosome

Reaching The Of Adolescence Short Answer Type Questions

A. Why is adolescent pregnancy harmful?

Answers: It is harmful because a teenage mother is neither mentally nor physically capable of taking the responsibility of motherhood and coping with its challenges. Early motherhood leads to health problems in both the mother and the child.

B. What is the meaning of reproductive health?

Answers: According to the World Health Organization (WHO), reproductive health means a total well being in all aspects of reproduction, i.e., physical, emotional, behavioural and social health.

Textbook Exercises

Question 1. What is the term used for secretions of endocrine glands responsible for changes taking place in the body?

Answers: Hormones

Question 2. Define adolescence.

Answers: The period during which the human body undergoes changes leading to reproductive maturity is called
adolescence. It begins at the age of 11 years and ends between the age of 18 to 19 years.

Question 3. What is menstruation? Explain.

Answers: In females, an ovum matures and is released by one of the ovaries once in about every 28 to 30 days. If the egg is not fertilized by a sperm, it is released out from the body along with the thickened lining of the uterus. This causes monthly bleeding in women and is called menstruation.

Question 4. List changes in the body that take place at puberty.

Answers:

The following changes take place in the body at puberty:

• The most noticeable change is the increase in the height of adolescents.
• The voice box or the larynx grows during puberty resulting in a change in voices of both the boys
  and girls.
• The increased activity of sweat and sebaceous glands during puberty results in acne and pimples
  on the face.
• The male sex organs (testes and penis) develop fully during this period and testes start producing
  sperms. In girls, the ovaries expand, eggs mature and ovaries start releasing matured eggs.

Question 5. Prepare a table having two columns depicting names of endocrine glands and hormones secreted by them.

Question 6. What are sex hormones? Why are they named so? State their functions.

Answers: Testosterone and estrogen are called sex hormones. Testosterone is the male hormone secreted by testes at the start of puberty. Estrogen is the female hormone secreted by ovaries. It makes the egg mature. These hormones help in the development of secondary sexual characters and hence are called sex hormones.

Question 7. Choose the correct option.

(a) Adolescents should be careful about what they eat, because:

1. Proper diet develops their brains.
2. Proper diet is needed for the rapid growth taking place in their body.
3. Adolescents feel hungry all the time.
4. Taste buds are well-developed in teenagers.

Answers:  (2) Proper diet is needed for the rapid growth taking place in their body

b)Reproductive age in women starts when their:

1. Menstruation starts
2. Breasts start developing
3. Body weight increases
4. Height increases

Answers: (1) Menstruation starts

(c) The right meal for adolescents consists of:

1. Chips, noodles, coke
2. Chapatti, dal, vegetables
3. Rice, noodles and burger
4. Vegetable cutlets, chips and lemon drink

Answers:  (2) Chapatti, dal, vegetables

Question 8. Write notes on:

1. Adam’s apple
2. Secondary sexual characters
3. Sex determination in the unborn baby

Answers:

1. Adam’s Apple: The protruding part of the throat in boys is the growing voice box. It is called the Adam’s apple.
2. Secondary sexual characters: Features which help in distinguishing a man from a woman are called secondary sexual characters. In males, the secondary sexual characters are growth of hair on face, chest, under arms, and pubic region. In females, the development of breasts and growth of hair in the under arm and pubic regions are the secondary sexual characters.
3. Sex determination in the unborn baby: The sex of an unborn child depends on whether the zygote has XX or XY chromosomes. If the zygote has one X and one Y chromosome, it develops into a male child. On the other hand, if the zygote has two X chromosomes, it develops into a female child.

Question 9. Word game: Use the clues to work out the words.

Across:

1. Protruding voice box in boys
2. Glands without ducts
3. Endocrine gland attached to brain
4. Secretion of endocrine glands
5. Pancreatic hormone
6. Female hormone

Down:

1. Male hormone
2. Secretes thyroxine
3. Another term for teenage
4. Hormone reaches here through bloodstream
5. Voice box
6. Term for changes at adolescence

Across:

1. Adam’s apple
2. Endocrine
3. Pituitary
4. Hormone
5. Insulin
6. Estrogen

Down:

1. Testosterone
2. Thyroid
3. Adolscence
4. Target site
5. Larynx
6. Puberty

Question 10. The table given below shows the data on the likely height of boys and girls as they grow in age. Draw graphs showing the height and age of both boys and girls on the same graph paper. What conclusions can be drawn from these graphs?

The graph is given below:

It can be seen from the graph that girls at the age of 12 are generally taller than boys. At 16 years, the height of both is the same but eventually boys gain height and become taller than girls.

Reaching The Of Adolescence Hots corner

A. How will a human body be affected if the thyroid gland is removed from the body?

Answers: Thyroid gland releases thyroxine, which regulates metabolism, maturation and growth.. If it is removed, the growth and metabolic activities of human beings will be adversely affected.

Reaching The Of Adolescence Practice exercises

Objective Answer Type Questions

A. Select the odd one out

1. Endocrine gland, Pituitary gland, Adrenals, Oil gland
2. Testosterone, Estrogen, Insulin, Ovaries
3. AIDS, Goitre, Diabetes, Menstruation

Answers:

1. Oil gland
2. Ovaries
3. Menstruation

B. Give one word for the following.

1. The master gland in our body.
2. The milk secreting gland in human females.
3. The hormone secreted by the adrenals.
4. The process in which mature ovum is released from the ovary.

Answers:

1. Pituitary gland
2. Mammary Glands
3. Adrenalin
4. Ovulation

C. State whether the followire true or falseng statements a.

1. During puberty, the voice box grows more in boys than in girls.
2. Goitre is caused due to the lack of thyroxine.
3. Mother’s chromosomes are responsible for the sex of the child.
4. The adrenals are responsible for causing acne in teenagers.

Answers:

1. True
2. True
3. False
4. False

D. Fill in the blanks.

1. ______is a female hormone_____
2. _____is a male hormone.
3. Salivary gland is a type of_____ gland.

Answer:

1. Estrogen
2. Testosterone
3. Exocrine

Reaching The Of Adolescence Short Answer Type Questions

A. It is assumed that an 8-years old child will attain about 72% of his or her full height at the end of the growth period. If the child is 108 cm tall now, what would his or her height at the end of the growth period?

Answers:

Current age of child = 8 years
Current height of child = 108 cm
According to the formulas
Full height of the child =
current height of child
% of full height
108
= .*100=
12
-x100
150 cms.

Also Read

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 11 Force and Pressure
• Chapter 12 Friction
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Chemical Effects Of Electric Current Concepts

• Electric Current
• Chemical Ejects of Electric Current
• Electroplating

Chemical Effects Of Electric Current Abstract

• Based on electrical conductivity, materials can be classified as conductors or insulators.
• Substances that allow an electric current to pass through them are called conductors, for example, copper, iron, etc.
• Substances that do not allow electric current to pass through them are called insulators, for example, wood, rubber, etc.
• Under certain circumstances, most of the substances can conduct electricity. Hence, substances through which electric current can flow easily are good conductors, while the substances through which electric current can pass negligibly or in a very small quantity are poor conductors.
• The passage of electric current through conducting liquids causes chemical changes in the conducting liquid.
• This phenomenon is called the chemical effect of electric current.
• The process of depositing a layer of desired metal on another material with the help of electricity is called electroplating.

Read and Learn More UP Board Notes for Class 8 Science

Important Terms And Definitions

Anode: The electrode which is connected to the positive terminal of a batter/ is known as the anode. Cathode: The electrode which is connected to the negative terminal of a battery is known as the cathode. Electrodes: Both anode and cathode are collectively called electrodes.
Electrolysis: The process of decomposition of chemicals in a solution or a liquid by passing electric current through it is called electrolysis.
Electrolytes: The substances which give ions in a solution are called electrolytes.
Light Emitting Diode (LED): It glows even when a small amount of current passes through the electric circuit

Electric Current

• Electric current or electricity can flow through different types of substances including liquids. Most of the liquids that conduct electricity are solutions of acids, bases and salts.
  

Chemical Effects of Electric Current Activity 1

Aim: To test whether lemon juice or vinegar is a good or bad conductor of electricity
Procedure:
1. Collect a few small plastics or rubber caps from the discarded bottles and clean them.
2. Pour one teaspoon of lemon juice or vinegar into one cap. Then, bring your tester over this cap and let the ends of the tester dip into lemon juice or vinegar as shown below.
3. Take care that the ends are not more than I cm apart but at the same time, they do not touch each other.
4. Check to see if the bulb of the tester glows or not

Observation: The bulb glows when dipped in the image here lemon juice or vinegar.
Conclusion: Lemon juice or vinegar is a good conductor of electricity.

Chemical Effects of Electric Current Activity 2

Aim: To prepare an electromagnetic tester based on the magnetic effect of current Precaution: Wash and wipe dry the ends of the test- after testing each liquid.
Procedure:
1. Take the tray of a discarded matchbox
2. Wrap an electric wire a few times around the tray and place a small compass needle inside it
3. Connect one free end of the wire to the terminal of a battery.
4. Leave the other end free. Take another piece of the wire and connect it to the other terminal of the batter.

5. Join the free ends of two wires momentarily. The compass needle should show deflection. Your tester with two free ends of the wire is ready.
6. Repeat ‘Activity 2’ above using this tester. You will notice that there is a deflection in the compass needle the moment the free ends of the tester are dipped in the lemon juice.
7. Take out the ends of the tester from the lemon juice. Dip them in water and then wipe them dry.
8. Repeat the activity with other liquids such as milk, honey, etc.
9. In each case, observe whether the magnetic needle shows deflection or not.

Observation:

Conclusion: Lemon juice, vinegar, tap water, milk, curd water and soda are all good conductors of electricity. However, vegetable oil, honey, kerosene oil and distilled water are all bad conductors of electricity.

Chemical Effects of Electric Current Activity 3

Aim: To show that distilled or pure water is a poor conductor of electricity
Procedure:

1. Take about two teaspoons full of distilled water in a clean, dry plastic or rubber cap of a bottle.
2. Use the tester to test if distilled water conducts electricity.
3. Dissolve a pinch of common salt in the distilled water and test again.

Observation:

1. The bulb of the tester does not glow when the tester is put into distilled water.
2. When a pinch of salt is dissolved in the distilled water and the tester is immersed in it, the bulb glows.

Conclusion: Distilled or pure water is a poor conductor of electricity.

Chemical Effects of Electric Current Activity 4

Aim: To show that water containing any salt, acid or alkali is a good conductor of electricity
Procedure:

1. Take three clean plastic or rubber caps of bottles.
2. Pour about two teaspoonfuls of distilled water into each of them.
3. Add a few drops of lemon juice or dilute hydrochloric acid to the distilled water in the first cap.
4. In the second cap, add a few drops of a base such as caustic soda or potassium iodide.
5. Add a small amount of sugar to the distilled water in the third cap and dissolve it.
6. Test to see which of these solutions conduct electricity.

Observation: Solutions in the first and second cap conduct electricity, while the one in the third does not conduct electricity.
Conclusion: Water containing any salt, acid or alkali is a good conductor of electricity.

Objective Type Questions

A. State whether the following statements are true or false.

1. All liquids allow electric current to flow through them.
2. The deflection of the needle of a magnetic compass kept near an electric circuit indicates the flow of electric current.
3. An LED glows even when a small amount of electric current is passed through it.
4. Adding sugar to distilled water makes it a good conductor of electricity.

Answers:

1. False
2. True
3. True
4. False

B. Multiple Choice Questions.

1. Which of the following does not allow the passage of electric current through them?
(a) Tap water
(b) Vinegar
(c) Distilled water
(d) Seawater

2. ‘Which of these substances is a poor conductor of electricity?
(a) Wood
(b) Iron
(c) Copper
(d) Graphite

Answers:

1. (c) Distilled water
2. (a) Wood

C. Fill in the blanks.

1. Materials that do not allow electric current to easily pass through them are called__________
conductors of electricity.
2. Tap water is a__________ conductor of electricity.
3. An LED consists of two wires called_________.

Answers:

1. Poor
2. Good
3. Leads

Chemical Effects of Electric Current Activity-4 Short Answer Type Questions

Question 1. What are electrodes?

Answer
Electrodes are electrical conductors that are used to make contact with the non-metallic part of a circuit such as a conducting liquid, air or a semiconductor.

Question 2. Explain how an LED should be connected to a circuit

Answer
LED or light emitting diode comprises a bulb with two wires (called leads), where one lead is longer than the other. The longer lead is connected to the positive terminal of the battery and the shorter lead is connected to the negative terminal of the battery.

Chemical Effects of Electric Current

• The passage of electric current through conducting liquids causes chemical changes in the liquids. This phenomenon is called the chemical effect of electric current. The chemical reactions or changes vary depending on the type of solution and electrodes used.

Chemical Effects Of Electric Current Activity 5

Aim: To show the production of gases during electrolysis of water
Procedure:
1. Carefully take out carbon rods from two discarded batteries.
2. Clean their metal caps with sandpaper.
3. Wrap copper wires around the metal caps of the carbon rods and connect them to a battery.

Note: Instead of carbon rods, you may take bio-iron nails about 6 cm long.
4. Pour a cupful of water into a glass or a plastic bowl.
5. Add a teaspoonful of salt or a few drops of lemon juice to the water to make it more conducting.
6. Immerse the electrodes in this solution. Ensure that the metal caps of the carbon rods are outside the water.
7. Wait for 3-4 minutes.
8. Observe the electrodes.
Observation: Gas bubbles are seen at the ends of the electrodes.
Conclusion:
1. Due to the passage of electric current, water is broken down into its constituent gases, i.e., hydrogen and oxygen. This process of breaking up a*i electrolyte by passing an electric current through it is known as electrolysis.
2. Oxygen bubbles are formed on the positively charged electrode (anode) and hydrogen bubbles are formed on the negatively charged electrode (cathode).

Objective Type Questions

A. State whether the following statements are true or false.

1. When electric current passes through copper sulphate solution, copper gets deposited at the anode.
2. Electric current can bring a chemical change in a liquid solution.

Answers:

1. False
2. True

B. Multiple Choice Questions.

1. ‘What does a positively charged electrode know as?
(a) Cathode
(b) Anode
(c) Electrolyte
(d) Electrolysis

2. When electrodes are immersed in water and electric current is passed through them, on which terminal of the battery oxygen bubbles are formed?
(a) Positive terminal
(b) Negative terminal
(c) Both the terminals
(d) Neither of the terminal

Answers:

1. (b) Anode
2. (a) Positive terminal

C. Fill in the blanks.

1.  When electrodes are immersed in water and an electric current is passed through them, bubbles of
   __________and_________are produced.
2. The closed path through which electric current ‘lows is called a________.
3. The negative terminal of a batter)’ or negatively charged electrode is also known as the_______.

Answers:

1. Oxygen, hydrogen
2. Circuit
3. Cathode

Chemical Effects Of Electric Current Activity 5 Short Answer Type Questions

Question 1. During electroplating, the copper deposited on the plate connected to the negative terminal comes from the solution. How is this loss of copper from the solution compensated?

Answer
The loss of copper from the solution is compensated by the other plate. An equal amount of copper gets dissolved in the solution and the process keeps on going. This means that copper gets transferred from one plate to another.

Question 2. Solid NaCI does not conduct electricity but when dissolved in water, it conducts electricity. Why?

Answer
Solid NaCI consists of ions that cannot move. However, when solid NaCI is dissolved in water, the ions separate and can move easily. Thus, solid NaCI cannot conduct electricity, whereas NaCI solution can conduct electricity.

14.3 Electroplating

• Electroplating is the process of depositing a layer of a desired metal on another material with the help of electricity. Artificial jewellery is made with coatings of less expensive metals such as iron and aluminium with costlier metals, such as silver and gold to give them a rich look.

Chemical Effects Of Electric Current Activity 6

Aim: To demonstrate the process of electroplating
Materials Required: Two copperplates approximately 10 cm 4 cm in size, copper sulphate, distilled water, a beaker, dilute sulphuric acid, sandpaper and batter)’
Procedure:
1. Take 250 ml of distilled water in a clean and dry beaker.
2. Dissolve two teaspoonfuls of copper sulphate in rt. Also, add a few drops of dilute sulphuric acid to the copper sulphate solution to make it more conducting.
3. Clean copper plates with a sandpaper.
4. Rinse the plates with water and let them dry/.
5. Connect the copper plates to the terminals of a batter)- and immerse them into the copper sulphate solution.
6. Allow the electric current to pass through the circuit for abo it 15 minutes. Now, remove the electrodes from the solution and observe them.

Observation: Copper metal gets deposited on the plate at the negative terminal of the battery.

Objective Type Questions

A. State whether the following statements are true or false.

1. During electroplating, the object to be electroplated is connected to the positive terminal of the batter).
2. Copper and iron are the only metals that can be used for electroplating purposes.
3. A coating of zinc is deposited on iron to protect it from corrosion.

Answers:

1. False
2. False
3. True

B. Multiple Choice Questions.

1. Electroplating is based on which of these effects of electricity?
(a) Magnetic effect
(b) Physical effect
(c) Heating effect
(d) Chemical effect

2. Which of these is true about electroplating?
(a) It is a method of making metal plates.
(b) It is the process of coating a layer of metal over another metal by using an electric current.
(c) It is the technique of making plates that do not conduct electricity,
(d) None of these

Answers:

1. (d) Chemical effect
2. (b) ft is the process of coating a layer of metal over another metal by using electric current

C. Fill in the blanks.

1.__________is used to coat bath taps, bicycle handlebars, etc., and give them a shiny look.
2.________is one of the most common applications of chemical effects of electric current.

Answers:

1. Chromium
2. Electroplating

Chemical Effects Of Electric Current Activity 6 Textbook Exercises

Question 1. Fill in the blanks.

1. Most liquids that conduct electricity are solutions of_________,__________ and_________
2. The passage of an electric current through a solution causes__________effects.
3. If the current passes through a copper sulphate solution, copper gets deposited on the plate connected
   to the__________terminal of the battery.
4. The process of depositing a layer of any desired metal on another material using electricity
   is called____________.

Answers:

1. Acids, bases, salts
2. Chemical
3. Negative
4. Electroplating

Question 2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?

Answer
The needle shows deflection because the solution is a good conductor of electricity and electric current has a magnetic effect

Question 3. Name three liquids, which when tested in the manner given below, may cause the magnetic needle to deflect.
Answer: Tap water, lime water and vinegar cause the magnetic needle to deflect.

Question 4. The bulb does not glow in the setup shown in the figure given below. List the possible reasons. Explain your answer.
Answer:

In the given setup, if the bulb does not glow, it may be because the bulb may be fused or the connection of the wires may be loose. Replace the bulb and check once again. If the bulb still does not glow, check the connection of the wires. However, after tightening the connections and testing the bulb, if it still does not glow, then the only possible reason can be that the solution is a poor conductor of electricity.

Question 5. A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightfor liquid A, while it glows very dimly for liquid B. You would conclude that

1. liquid A is a better conductor than Liquid B
2. liquid B is a better conductor than Liquid A
3. both liquids are equally conducting
4. conducting properties of liquids cannot be compared in this manner

Answer:

1. liquid A is a better conductor than liquid B.

Question 6. Does pure water conduct electricity? If not, what can we do to make it conduct?

Answer
No, pure water does not conduct electricity. However, adding some salt, acid or alkali can make pure water a good conductor of electricity.

Question 7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.

Answer
Tap water is a good conductor of electricity. Hence, to avoid electric shock, firemen shut off the main electrical supply before using water hoses.

Question 8. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?

Answer
Drinking water is generally free of salts and minerals, whereas seawater is rich in salts and minerals. Thus, seawater is a better conductor of electricity than drinking water. Hence, the compass needle deflects more in the case of seawater as compared to drinking water

Question 9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpours?

Answer
Explain. No. it is not at all safe to perform outdoor electrical repairs when it is raining. Rather, it is dangerous because the electrician may get an electric shock as rainwater is a good conductor of electricity.

Question 10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester To her surprise she found that the compass needle showed deflection. What could be the reason?

Answer
Though rainwater is as pure as distilled water it may be contaminated by the impurities suspended in the atmosphere. These impurities make rainwater a better conductor of electricity. This is the reason that the compass needle showed deflection.

Question 11. Prepare a list of objects around you that are e electroplated.

Answer
Bicycle rims, door handles, taps, metallic pens, artificial jewellery, utensils, belts, and buckles.

Question 12. The process that you saw in Activity 7 is used for the purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod j is sought to be transferred to the thin copper plate. Which electrode should be attached to the • positive terminal of the battery and why?

Answer
The impure copper rod should be attached to the positive terminal because copper ions will get drawn towards the negative terminal and will be deposited there. So, the copper from the impure rod will get deposited on the pure copper plate.

Chemical Effects of Electric Current Hots Corner

Question 1. We are advised not to touch electrical appliances with wet hands. Why?

Answer
Small quantities of mineral salts are naturally present in water. The salts make water a good conductor of electricity. Therefore, it may be dangerous to touch any electrical appliance with wet hands because of the potential risk of being electrocuted.

Practice Exercise Objective Type Questions

A. Give one word for the following.

1. Substances that do not allow an electric current to pass through them
2. The metal rods dipped in liquids to which batteries are attached in a circuit
3. Substances that conduct electricity in a liquid state or when dissolved in water and break up chemically during the process

Answers:

1. Insulators
2. Electrodes
3. Electrolytes

B. State whether the following statements are true or false.

1. Electric bulbs glow due to the chemical effect of electricity.
2. Some liquids are good conductors of electricity and some are poor conductors.
3. Electrolysis is a chemical change.
4. Bath taps are electroplated to prevent them from corrosion.
5. A solution in which very few mobile ions are present is a strong electrolyte.

Answers:

1. False
2. True
3. True
4. True
5. False

C. Circle the odd one out.

1. Plastic, copper, aluminium, graphite
2. Lemon juice, tap water, vegetable oil, and common salt solution

Answers:

1. Plastic
2. Vegetable oil

D. Fill in the blanks.

1. Electroplating __________rusting of objects.
2. is an electrolyte and____________ is a non-electrolyte.
3. Distilled water is a____________ conductor of electricity.
4. Carbonic acid is a____________ electrolyte.

Answers:

1. Prevents
2. Sulphuric acid, ethyl alcohol
3. Poor
4. Weak

Chemical Effects Of Electric Current Short Answer Type Questions

Question 1. Name four liquids that can conduct electricity.

Answer
Sea water, vinegar, soda and milk

Question 2. A pencil sharpened at both ends can be used as a conductor in laboratories. Why?

Answer
A pencil sharpened at both ends can be used as a conductor because pencil lead is made up of graphite and graphite is a good conductor of electricity.

Question 3. Why high voltage electric current is dangerous for our body?

Answer
The brain and heart send tiny electrical impulses generated within the body. If high voltage electric current passes through our body, the brain or heart could stop working. Thus, high-voltage electric current is dangerous for our bodies.

 

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 11 Force and Pressure
• Chapter 12 Friction
• Chapter 13 Sound
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Sound Concepts

•  Sound and its Production
•  Propagation of Sound
• Characteristics of Sound

Sound Abstract

• Sound is produced by a vibrating object.
• The to-and-fro motion of an object is termed a vibration. Vibration is an example of oscillatory motion.
• In human beings, the sound is produced by the voice box, which is also called the larynx.
• Two cords called the vocal cords, are stretched across the larynx in such a way that there is a narrow slit between them for the passage of air. When air is forced through this slit vocal cords vibrate and sound is produced.
• Sound needs a medium to propagate. It cannot travel in a vacuum.
• Sound is received by our outer ear from where it travels down a canal.
• At the end of the ear canal is a tightly stretched, thin membrane called the eardrum.
• The eardrum is just like a stretched rubber sheet. When sound strikes the eardrum, it vibrates. The eardrum then sends vibrations to the inner ear.
• From the inner ear, the signals are further transmitted to the brain and we can hear.
• The number of oscillations per second is called the frequency. It is measured in hertz (Hz).
• The maximum displacement of a vibrating body from its mean position is called amplitude.
• The time taken to complete one oscillation is called the period.
• Amplitude and frequency are the two important properties of any sound.
• The amplitude of the vibrating body producing the sound determines the loudness of the sound. The larger the amplitude of the vibration, the louder the sound is produced.
• Frequency determines the shrillness or pitch of a sound.
• The higher the frequency of vibration, the higher the pitch.
• Sound can also be classified as audible and inaudible.
• Sound of frequencies less than about 20 vibrations per second (i.e. 20 Hz) and more than about
• 20,0 vibrations per second (i.e. 20 kHz) are not audible to the human ear. Such sounds are called inaudible sounds.
• Thus, for a normal human ear, the audible frequency range is roughly between 20 and 20,000 Hz. A sound that is pleasing to the ear is called a musical sound, while an unpleasant sound is called noise.
• Noise pollution may cause many health-related problems such as lack of sleep, hypertension {high blood pressure), anxiety, etc.
• A person who is continuously exposed to a loud sound may suffer from temporary or even permanent hearing impairment.
• Planting trees along the roadside and in the surroundings can reduce noise pollution.

Read and Learn More UP Board Notes for Class 8 Science

Sound and its Production

• A sound is a form of energy which is produced by a vibrating body. In human beings, the larynx helps in; the production of sound. It is situated in the neck at the upper end of the windpipe. Sound is controlled; by the vocal cords. The quality of sound produced from loose and thick vocal cords is different from the ’ sound produced by tight and thin vocal cords.

Sound Activity 1

Aim: To create and feel vibrations
Procedure:
1. Take a metal plate (or a shallow frying pan) and hang it at a convenient place in such a way that it does not touch any walls.
2. Now strike it with a stick and touch the plate or pan gently with your finger. Note down your observation.
3. Again strike the plate with the stick and hold it tightly with your hands immediately after striking. Note down your observation again.
4. Touch the plate after it stops producing sound. Note down your observation.

Observations: When the plate was struck, sound and vibrations produced were felt on touching the plate gently. The sound produced vanished on holding the plate tightly. When the plate stopped producing the sound, no vibrations were felt.
Conclusions: When a metal object is struck vibrations are produced. Sound is also produced by a vibrating object.

Sound Activity 2

Aim: To produce sound Dv creating vibrations
Procedure:
1. Take a rubber band and put it around the longer side of a pencil box
2. Insert two pencils between the box and the stretched rubber and then pluck the rubber band somewhere in the middle. Note down your observations.

Observations: On plucking, the band vibrates and a sound is produced.
Conclusion: Sound is produced by a vibrating object.

Sound Activity 3

Aim: To create waves by producing vibrations
Procedure:
1. Take a metal dish and pour water into it. Strike it at its edge with a spoon
2. Note down your observations.

2. Strike the plate again and then touch it. Note down your observations.
3. Strike the dish again and look at the surface of the water.
4. Then hold the dish and note down your observations.
Observations: In steps I and 2, both sound and vibrations were produced. In step 3, waves were seen on the water’s surface. But on holding the dish, vibrations stopped and there were no waves.
Conclusion: Vibrations produce sound as well as waves.

Sound Activity 4

Aim: To make an Ektara and identify its vibrating part
Procedure:
1. Take a hollow coconut shell or an earthen pot and make a musical instrument Ektara.
2. Play this instrument and identify its vibrating part.

Observations: The vibrating part is its strings.
Conclusion: The vibrating part of an Ektara is its string, which produces sound vibrating.

Sound Activity 5

Aim: To make a Jaitarang
Procedure:
1. Take 6-8 metal bowls or tumblers and fill them with water up to different levels, increasing gradually from one end to the other.
2. Then take a pencil and strike the bowls gently, in succession. Note down your observation.

Observations: Different types of sounds are produced on vibrating bowls filled with water up to different levels.
Conclusion: Vibrations produce sound. The sound produced varies with the amount of water present in a bowl.

Sound Activity 6

Aim: To find out how vocal cords produce sound
Procedure:

1. Take two rubber strips of the same size. Place them one above the other and stretch them tight.
2. Now blow air through the gap between them
3. Record your observation.

4. You can also take a piece of paper with a narrow slit and hold it between your fingers
5. Now’ blow’ through the slit and listen to the sound.
Observation: As the air blows through the stretched rubber strips or the piece of paper, a sound is produced.
Conclusion: In human beings, sound is produced by the voice box or the larynx. Two cords called the vocal

Objective Type Questions

A. State whether the following statements are true or false.

1. The to-and-fro motion of an object is called vibration.
2. In human beings, the sound is produced by vocal cords.
3. ‘When we pluck the string of an instrument, like the sitar, not only its string but the whole instrument vibrates.
4. Women have longer vocal cords than men.

Answer

1. True
2. True.
3. True
4. False

B. Multiple choice questions.

1. What does a vibrating body transfer to the adjacent molecules in a medium?
(a) Energy
(b) Force
(c) Speed
(d) Pressure

2. What is the length of the vocal cord in men?
(a) 10 mm
(b) 15 mm
(c) 20 mm
(d) 25 mm

Answers:

1. (a) Energy
2. (c) 20mm

C. Fill in the blanks.

1. Sound is a form of_________
2. Sound is produced when objects__________
3. The voice box is also called the_________
4. The voice box is situated in the neck at the upper end of the________

Answers:

1. Energy
2. Vibrate
3. Larynx
4. Windpipe

Sound Short Answer Type Questions

Question 1. How do human beings produce voices?

Answer
In human beings, the sound is produced by the voice box, which is also called the larynx. Two cords called the vocal cords, are stretched across the larynx in such a way that there is a narrow slit between them for the passage of the air. When air is forced through this slit, the vocal cords vibrate and sound is produced.

 Propagation of Sound

• Sound needs a medium for its propagation. Sound can travel through solids, liquids, and gases.  Sound cannot travel through a vacuum. The outer part of the ear, which is shaped like a funnel, – receives the sound. From here, the sound travels through the middle ear and reaches the inner ear; which transmits signals to the brain. That is how we hear.

Sound Activity 7

Aim: To observe that sound needs a medium to travel
Procedure:
1. Take a dry metal tumbler and place a cell phone inside it
2. Ask your friend to call you on that cell phone from another cell phone. Listen to the ring carefully.
3. surround the nim of the tumbler with your hands and put your mouth on the opening between your hands.

4. Indicate to your friend to call on that cell phone again. Listen to the ring while sucking air from the tumbler.
5. Remove the tumbler from your mouth. Note down your observation.

Observations: The sound became fainter as the air was sucked from the tumbler. The sound became loud again after removing the mouth.
Conclusion: As the air was sucked, a partial vacuum was created inside the tumbler and the sound became fainter. Once air entered the tumbler again, the sound became louder. This proves that sound needs a medium to travel.

Sound Activity 8

Aim: To determine if sound can travel through liquids
Procedure:
1. Take a bucket or a bathtub and fill it with clean water.
2. Take a small bell in one hand. Shake this bell inside the water to produce sound. Make sure that the bell does not touch the body of the bucket or the tub.
3. Place your ear gently on the water surface and note down your observations.

Observations: The sound of the bell is heard.
Conclusion: Sound can travel through liquids.

Sound Activity 9

Aim: To determine if sound can travel through solids
Procedure:
1. Take a meter scale or a long metal rod and hold its one end to your ear.
2. Ask your friend to gently scratch or tap the other end of the scale.

Observations: The sound of the scratch is heard.
Conclusion: Sound can travel through solids.

Sound Activity 10

Aim: To observe how an eardrum works
Procedure:
1. Take a tin can and cut its ends.
2. Stretch a piece of rubber balloon across one end of the can and fasten it with a rubber band.
3. Put four or five grains of dry cereal on the stretched rubber and ask your friend to speak “Hurray, Hurray’1 from the open end.

Observations: On speaking, grains jump up and down.
Conclusion: Grains jump up and down due to the vibrations. The vibrations produce movements. The eardrum is just like a stretched rubber sheet. Sound vibrations make the eardrum vibrate and thus, we can hear sound.

Objective Type Questions

A. State whether the following statements are true or false.

1. Sound needs a medium for its propagation.
2. The eardrum sends vibrations directly to the brain.

Answers:

1. True
2. False

B. Multiple-Choice Questions

1. What is the name of the membrane present in the ear?
(a) Ear foil
(b) Eardrum
(c) Ear sheet
(d) Hammer

2. In which of the following mediums does sound travel the fastest?
(a) Water
(b) Air
(c) Iron
(d)Vacuum

Answers:

1. (b) Eardrum
2. (c) Iron

C. Fill in the blanks.

1. The_______sends vibrations to the inner ear.
2. Sound travels the_________in solids and_______in gases.

Answers:

1. Eardrum
2. Fastest, Slowest

Sound Short Answer Type Question

Question 1. How do we hear?
‘When sound enters into our ear, it travels through a canal at the end of which there is a tightly stretched, thin membrane called the eardrum. When sound strikes the eardrum, it vibrates. The eardrum then sends vibrations to the inner ear. From there, the signal is further transmitted to the brain. That is how we hear.

 Characteristics of Sound

• Amplitude and frequency are two important properties of sound. Frequency determines the shrillness or pitch of a sound. The male voice has a low pitch, whereas the female voice has a high pitch. Scientifically, a musical sound is produced by regular vibrations, and noise is produced by irregular vibrations. Too much noise in our surroundings is harmful to us and is known as noise pollution.

Sound Activity 11

Aim: To find the relation between the loudness and amplitude of the sound
Procedure:
1. Take a metallic tumbler and a tablespoon.
2. Strike the tablespoon gently at the brim of the tumbler.
3. Hear the sound produced.
4. Now’ Dang the spoon on the tumbler and hear the sound produced again.
5. Next suspend a small thermocol ball touching the rim of the tumbler.
6. vibrate the tumbler by striking it and check how far the ball is displaced.
7. Strike the tumbler gently and then with some force.
8. Compare the displacements (or amplitudes} of vibrations in both cases.

Observation: In steps, I and 2, the sound produced was loud when the tumbler was struck hard. In steps 3 and 4, the vibrations produced were more when force was applied.
Conclusion: The loudness of the sound was more when the amplitude of the vibration of the tumbler was greater. Thus, the loudness of sound is proportional to its amplitude.

Objective Type Questions

A. State whether the following statements are true or false.

1. The number of oscillations per second is called the amplitude of oscillation.
2. Amplitude and frequency are two important: properties of any sound.
3. The larger the amplitude of vibrations, the louder the sound produced.
4. For a normal human ear, the audible frequency range is roughly between 200 and 20,000 Hz.

Answers:

1. False
2. True
3. True
4. False

B. Multiple-Choice Questions

1. Which of the following determines the shrillness or pitch of a sound?
(a) Frequency
(b) Amplitude
(c) Period
(d) None of these

2. ‘Which characteristic of sound distinguishes a male and a female voice?
(a) Loudness
(b) Period
(c) Pitch
(d) Amplitude

Answers:

1. (a) Frequency
2. (c) Pitch

C. Fill in the blanks.

1. The higher the frequency of vibration, the__________is the pitch.
2. The maximum displacement of a vibrating body from its mean position is called___________.
3. Sounds of frequencies less than about___________ GHz are not audible to the human ear.
4. A complete to-and-fro motion of a vibrating body about its mean position is called a /an_________.

Answers:

1. Higher
2. Amplitude
3. 20
4. Oscillation

Sound Textbook Exercises

Question 1. Choose the correct answer.
Sound can travel through

(a) Gases only
(b) Solids only
(c) Liquids only
(d) Solids, liquids, and gases

Answer

(d) Solids, liquids, and gases

Question 2. Voice of which of the following is likely to have a minimum frequency?

(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman

Answer

(c) A man.

Question 3. In the following statements, tick (T) against those which are true, and (F) against those which are false.

1. Sound cannot travel in a vacuum.
2. The number of oscillations per second of a vibrating object is called its period.
3. If the amplitude of the vibration is large, the sound is feeble.
4. For human ears, the audible range is 20 Hz to 20,000 Hz.
5. The lower the frequency of vibration, the higher the pitch.
6. An unwanted or unpleasant sound is termed as music.
7. Noise pollution may cause partial hearing impairment

Answers:

1. True
2. False
3. False
4. True
5. False
6. False
7. True

Question 4. Fill in the blanks with suitable words.

1. The time taken by an object to complete one oscillation is called.
2. Loudness is determined by the vibration.
3. The unit of frequency is.
4. Unwanted sound is called
5. The Shrillness of a sound is determined by the vibration.

Answer:

1. Period
2. Amplitude
3. Hertz (Hz)
4. Noise
5. Frequency.

Question 5. A pendulum oscillates 40 times in 4 seconds. Find its period and frequency.

Answer

Frequency=Number of oscillations/Time in seconds

Frequency = 40/4

Frequency = 10 Hz

Time period =1/Frequency

Time period = 1/10

Period — 0.1 s

Question 6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the period of the vibration?

Answer

Frequency = 500 vibrations per second – 500 Hz
We know, that time period =1/Frequency
Time period = 1/500
Time period = 2 x 10 3 s

Question 7. Identify the part that vibrates to produce sound in the following instruments:
(a) Dholak (b) Sitar (c) Flute

Answer

(a) Stretched membrane
(b) String
(c) Air column

Question 8. What is the difference between noise and music? Can music become noise sometimes?
Answer:

Differences between noise and music are:

Yes, music can become noise. When music is too loud or the sounds produced by the instruments are not in sync with each other then music becomes noise.

Question 9. List sources of noise pollution in your surroundings.

Answer
Sources of noise pollution are:

1. Noise of vehicles
2. Loudspeakers
3. Working machines
4. Bursting of crackers
5. Air coolers
6. Radios and televisions at high volumes
7. Kitchen Appliances
8. Hawkers etc.

Question 10. Explain in what ways noise pollution is harmful to humans.

Answer
Too much noise in our surroundings is known as noise pollution. It is very harmful to us and can cause irritability, loss of concentration, high blood pressure, headache, stress, sleep disturbances, etc. It can even damage hearing permanently.

Question 11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Answer
My parents should buy the house, which is located three lanes away from the roadside because a roadside house has many disadvantages such as:

1. A lot of noise due to passing vehicles.
2. Smoke and dust are produced by running vehicles.
3. Sound of loud horns of vehicles at the time of traffic jams.

Question 12. Sketch the larynx and explain its function in your own words.
Answer:

Functions of the larynx: When air passes through the vocal cords, they vibrate to produce sound. Vocal cords may become loose/thick or tight/thin on vibration, thus producing different Types of sounds.

Question 13. Lightning and thunder take place in the sky at the same time and the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?

Answer
The speed of light is 3 * 10® m/s, while that of sound is only 330 m/s. So, although lightning and thunder take place simultaneously in the sky and at the same distance from the Earth, lightning is seen first.

Sound Hots Corner

1. Can two people talk on the Moon in the same way as they can on the Earth?

Answer
No, there is no air or atmosphere on the Moon and sound needs a medium to travel. Therefore, the sound from one person will not travel to the ear of the other person. Hence, we cannot talk on the Moon in the same way as we do on the Earth.

Practice Exercise

A. Give one word for the following.

1. The position where the bob of a pendulum stops
2. Number of oscillations per second
3. The sound produced by regular vibrations
4. The factor on which the loudness of a sound depends
5. Unit in which the loudness of a sound is measured

Answers:

1. Mean position
2. Frequency
3. Music
4. Amplitude
5. Decibel

B. State whether the following statements are true or false.

1. Sound travels at different speeds in different substances.
2. Scientifically, a musical sound is produced by regular vibrations.
3. The male voice has a high pitch.
4. The time period is denoted by the letter T.
5. Loudness of a sound is inversely proportional to the amplitude of the vibration.

Answers:

1. True
2. True
3. False
4. True
5. False

Sound Short Answer Type Questions

Question 1. Sound cannot travel through a vacuum. Why?

Answer
Sound is a form of energy and is produced by a vibrating body. The vibrating body transfers its energy to the neighboring molecules in the medium, which in turn pass on their energy to the other; molecules. Since no molecules are present in a vacuum, there is no transfer of energy. Thus, sound \ cannot travel in a vacuum.

Question 2. The bob of a pendulum oscillates twice per second. Find its period and frequency.

Answer
Frequency = Number of oscillations/Time (in seconds)

Frequency = 2/1 = 2 Hz.

Time period = 1/Frequency

Time period = — = 0.5 s

Question 3. Write any three methods to reduce noise pollution.

Answer

Three methods to reduce noise pollution are:

1. The use of loudspeakers in public places and residential areas should be banned.
2. Vehicles should be fitted with effective silencers and should not be allowed to play loud music.
3. Planting more trees on the roadsides reduces noise pollution as they absorb the sound.

Question 4. What are ultrasounds?

Answer
Sound frequencies more than 20,000 Hz (20 k Hz) are called ultrasounds. Human beings cannot perceive ultrasonic sounds. Hence, ultrasounds are also called ‘inaudible’ sounds.

Also Read

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 11 Force and Pressure
• Chapter 12 Friction
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Light Abstract

• All the objects or surfaces reflect light.
• When light from an object enters our eyes, we can see that object. The light may have been emitted by the object or reflected by it.
• The ray of light which strikes a surface is called the incident ray. The ray of light which comes back after reflection from the surface is known as the reflected ray.
• The point at which the incident ray strikes the surface is called the point of incidence.
• The perpendicular drawn to the reflecting surface at the point of incidence is called the normal.
• The angle between the normal and the incident ray is called the angle of incidence. It is denoted by Zi.
• The angle between the normal and the reflected ray is known as the angle of reflection. It is denoted by Zr.
• Reflection from a smooth surface such as a mirror is called regular reflection.
  When the rays reflected from a surface are not parallel, the reflection is known as diffused or irregular reflection. It takes place on rough surfaces.
• The splitting of white light into its constituent colors is known as the dispersion of light.
• Important parts of an eye are the cornea, iris, pupil, lens, retina, and optic nerves.
• Light enters the eye through the cornea. It helps protect the eye and focus the light.
• The amount of light entering into the eye is controlled by the iris. It is the iris that gives distinctive color to the eyes.
• The size of the pupil increases or decreases depending on the brightness of the light falling on the eye.
• The lens in our eyes forms images of the objects on a layer called the retina. The impression of an image persists on the retina for about 1/16th of a second.
• The retina contains many nerve cells. These cells are of two types: cones and rods.
• Sensory cells are not present at the junction of the optic nerve and the retina. Thus, no vision is possible at that spot. This spot is called the blind spot.
• Myopia or short-sightedness is an eye defect in which people can see near objects clearly but cannot see distant objects so clearly.
• Hypermetropia or long-sightedness is an eye defect in which people cannot see near objects
  clearly but they can see distant objects quite well.
• A cataract is an eye defect that occurs particularly during old age. The eyesight becomes foggy during a cataract because the eye lens becomes cloudy.
• The Braille system enables visually challenged people to read and write. This system uses patterns of raised dots to represent letters and numerals.

Read and Learn More UP Board Notes for Class 8 Science

Laws of Reflection

We can see objects because of the light that falls on them. When the light from an object enters our eyes, we see that object. The bouncing back of light rays from the surface of an object is called the reflection of light. The two laws of reflection are:

• The angle of incidence is equal to the angle of reflection.
• The incident ray, the reflected ray, and the normal drawn at the point of incidence, all lie in the same plane.

Light Activity 1

Aim: To verify the first law of reflection i.e. the angle of incidence is equal to the angle of reflection
Procedure:

1. Fix a white sheet of paper on a drawing board or a table.
2. Take a comb and close all its openings, except one in the middle. You can use a strip of black paper for this purpose.
3. Hold the comb perpendicular to the sheet of paper.
4. Throw light from a torch through the opening of the comb from one side. With a slight adjustment of the torch and the comb, you will see a ray of light along the paper on the other side of the comb.
5. Keep the comb and the torch steady.
6. Place a strip of plane mirror in the path of the light ray. Notice that after striking the mirror, the ray of light is reflected in another direction.
7. Draw lines showing the position of the plane mirror, the incident ray and the reflected ray on the paper with the help of your friends.
8. Remove the mirror and the comb. Draw a line making an angle of 90° to the line representing the mirror at the point where the incident ray strikes the mirror.
9. Measure the angle of incidence and the angle of reflection. Repeat the activity several times by changing the angle of incidence.
10. Note the observations in the observation table.

(a) Arrangement for showing reflection

 

(b) Drawing the normal

 

(c) Angle of incidence and the angle of reflection

Observation:

Conclusion: The angle of incidence is always equal to the angle of reflection.

Light Activity 2

Aim: To verify the second law of reflection, i.e. the incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane
Procedure:

1. Fix a white sheet of paper on a drawing board in such a way that a small portion of it protrudes a little beyond the edge of the drawing board.
2. Place the plane mirror strip on the sheet of paper and hold it vertically with a mirror stand.
3. Throw a ray of light on the mirror using a torch.
4. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper.
5. Bend that part of the projected portion on which the reflected ray falls.
6. Now, bring the projected portion of the paper back to its original position and observe.

 

Incident ray, reflected ray, and the normal at the point of incidence lie in the same plane

Observation: When the paper is bent, the reflected ray is not seen.
The entire sheet fixed on the drawing board represents a plane. The bent paper represents a new plane in which the incident ray and the normal lie.

Conclusion: The incident ray, the normal at the point of incidence, and the reflected ray lie in the same plane.

Light Activity 3

Aim: To show the image formed by a plane mirror
Procedure:

1. Place a source of light O in front of a plane mirror PQ such that two rays OA and OC are incident on it.
2. Draw normal to the surface of the mirror PQ at points A and C.
3. Now, draw the reflected rays at points A and C. Label the reflected rays as AB and CD, respectively.
4. Extend them further. The reflected ray on the extension will appear to meet at point I.

Observation: Since the reflected rays do not actually meet at I, but only appear to do so, we say that a virtual
image of the point O is formed at I. Such an image cannot be obtained on a screen.
Conclusion: The image formed by a plane mirror is virtual.

Light Objective Type Questions

A. State whether the following statements are true or false.

1. It is only when light from an object enters our eyes that we see the object.
2. The angle of reflection is the angle between the normal and the reflected ray.
3. The angle of incidence is always greater than the angle of reflection.

Answers:

1. ITrue
2. True
3. False

B. Multiple Choice Questions

Question 1. If the angle of incidence is 45°, what will be the angle of reflection?

1. 30°
2. 45°
3. 55⁰
4. 90°

Answers: 2) 45°

Question 2. Which of these phenomena enables us to see objects?

1. Circulation of light
2. Absorption of light
3. Reflection of light
4. None of these

Answers: 3) Reflection of light

C. Fill in the blanks.

1. The light ray, which strikes any surface, is called the_____ray.
2. The angle of incidence is the angle between_______and the____ray.

Answers:

1. Incident
2. Normal, incident

Light Short Answer Type Questions

A. How do we see objects?

We see various objects due to the reflection of light. All the objects reflect light falling on them. The reflected light reaches our eyes and we can see the object.

B. State the characteristics of the image formed by a plane mirror.

The image formed by a plane mirror is always:

1.  virtual and erect,
2. of the same size as the object, and
3. laterally inverted

C. What is meant by lateral inversion?

In the image formed by a plane mirror, the left side of the object appears on the right and the right side appears on the left. This is known as lateral inversion.

Types of Reflection: Regular and Diffused

Objects which shine in the light of the other objects are called illuminated objects, for example, the Moon. Objects which emit their light are known as luminous objects, for example, the Sun.
Reflection from a smooth surface such as a mirror is called regular reflection. When the rays reflected from a surface are not parallel, the reflection is known as diffused or irregular reflection. When two mirrors are inclined to each other, they give multiple images. Beautiful patterns are formed in a kaleidoscope because of multiple reflections.

Light Activity 4

Aim: To show the reflection of light rays by an irregular surface
Procedure:

1. Imagine that parallel rays are incident on an irregular surface as shown in the figure.
2. Use the laws of reflection to construct the reflected rays at various points.
3. Observe whether the reflected rays are parallel to each other or not.

(a) Parallel rays incident on an irregular surface

(b) Rays reflected from irregular surface

Observation: The rays are reflected in different directions.
Conclusion: When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection. Reflection from a smooth surface such as a mirror is called a regular reflection.

Light Activity 5

Aim: To study the formation of multiple images
Procedure:

1. 1Take two plane mirrors. Set them at right angles to each other with their edges touching and hinge them using adhesive tape.
2. Place a coin in between the mirrors and notice the number of images of the coin formed.
3. Now, hinge the mirrors using the adhesive tape at different angles, say 45°, 60°, 120°, 180°, etc. Place an object (say, a candle) in between them.
4. Note down the number of images of the object in each case.
5. Finally, set the two mirrors parallel to each other and then find out how many images of the candle placed between them are formed.

Observations:

1. Three images are formed by the two mirrors placed at right angles to each other.
2. The number of images varies depending on the angle of inclination between the two mirrors.
3. When the two mirrors are placed parallel to each other and an object is placed between them, infinite images are formed.

Conclusion: Multiple images are formed when two or more mirrors are used in combination.

Light Activity 6

Aim: To make a kaleidoscope
Procedure:

1. Take three rectangular mirror strips, each about 15 cm long and 4 cm wide.
2. Join them together to form a prism as shown in the figure ‘a’. Fix them in a circular cardboard tube or the tube of a thick chart paper. Make sure that the tube is slightly longer than the mirror strips.
3. Close one end of the tube with a cardboard disc having a hole in the center, through which you can see as shown in Figure ‘b’. To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc.
4. At the other end, touching the mirrors, fix a circular plane glass plate as shown in Figure ‘c’. Place on this glass plate several small pieces of colored glass (broken pieces of colored bangles). Close this end of the tube with a ground glass plate.
5. Allow enough space for the color pieces to move around.

Making a kaleidoscope

Observation: When you peep through the hole of a kaleidoscope, you will be able to see a variety of patterns in the tube.
Conclusion: A kaleidoscope is based on the idea of multiple images formed by using two or more mirrors placed at certain angles to one another.

Light Activity 7

Aim: To show the dispersion of light
Procedure:

1. Take a plane mirror of suitable size and place it in a bowl (Katori).
2. Fill the bowl with water and place this arrangement near a window such that direct sunlight falls on the mirror.
3. Adjust the position of the bowl such that the reflected light from the mirror falls on a wall. If the wall is not white, fix a sheet of white paper on it.
4. Observe the reflected light.

Observation: The reflected light splits into its constituent colors.

Conclusion: The sunlight, also called white light, consists of seven colors. The mirror and water form a prism. This breaks up the light into its constituent colors. Such splitting of white light into its constituent colors is known as dispersion of light.

Light Objective Type Questions

A. State whether the following statements are true or false.

1. A reflected ray of light cannot be reflected again.
2. Regular reflection takes place from uneven or rough surfaces.
3. When two mirrors are placed parallel to each other and an object is placed between them, an infinite number of images of the object are formed.

Answers:

1. False
2. False
3. True

B. Fill in the blanks.

1. The sunlight, also known as white light, consists of_____colours.
2. Working of a kaleidoscope is based on the concept of_____

Answers:

1. Seven
2. Multiple reflections

Light Short Answer Type Questions

A. ‘Reflection from two mirrors enables you to see objects which are not visible directly. Give two examples of this.

Example 1: After a haircut, a hairdresser places a mirror at the back of the head to show the haircut.
Example 2: Periscopes are used in submarines or tanks and also by soldiers in bunkers to see the things that are outside or above the level of observation.

B. Reflection depends on the nature of the surface on which the light falls. Explain.

When parallel rays fall on a plane surface, all the reflected rays are parallel. But if the reflecting surface is irregular (uneven), all the rays reflected from the surface are not parallel. So, the reflection depends on the nature of the surface on which the light falls.

The Human Eye: Structure, Working, Defects, and Care

The important parts of the eye are the cornea, iris, pupil, lens, retina, and optic nerve.

Human eye

The transparent, front part of the white outer coat of the eye is called the cornea. The dark muscular structure behind the cornea is called the iris. The small opening in the iris is called the pupil. Its size is controlled by the iris. Behind the pupil, there is a lens that is thicker in the middle. The muscles that hold the lens in place are called ciliary muscles. The lens focuses light on a layer at the back of the eye called the retina. It contains several nerve cells.

Sensations felt by the nerve cells are transmitted to the brain through the optic nerve. The nerve cells which are sensitive to the bright light are called cones. The nerve cells which are sensitive to the dim light are called rods.

Light Activity 8

Aim: To observe the change in the size of the pupil with a change in the amount of light falling on the pupil.
Precaution: Never use a laser torch for this activity.
Procedure:

1. Look into your friend’s eye and observe the size of the pupil.
2. Throw light on his/her eye with a torch and observe the pupil again.
3. Switch off the torch and observe his/her pupil once again.

Observation: When the light from the torch falls on the pupil, its size becomes smaller. When the light falling on the pupil is dim, the size of the pupil gets bigger.
Conclusion: The iris controls the amount of light entering the eye by changing the size of the pupil. When the light from the torch falls on the pupil, its size becomes smaller so that less amount of light can enter into the retina. When the light is dim, its size gets bigger so that more amount of light can enter into the retina.

Light Activity 9

Aim: To demonstrate the presence of blind spot
Procedure:

1. Make a round mark and a cross on a sheet of white paper about 6 – 8 cm apart.
2. Hold the sheet of paper at arm’s length from the eye.
3. Close your left eye and look continuously at the cross.
4. Move the sheet slowly towards you, keeping your eye on the cross.
5. Now, close your right eye look at the round mark, and repeat the activity.

Observation: The round mark or the cross disappears at some point. The disappearance of the mark shows that there is a point on the retina that cannot send messages to the brain when light falls on it.
Conclusion: The blind spot is a region on the retina that does not have any rods and cones. Hence, the images falling on this part of the retina cannot be seen.

Light Activity 10

Aim: To show that the impression of an image persists for about 1/16th of a second on the retina even after we stop looking at the object
Procedure:

1. Take a square piece of cardboard with sides 6 – 8 cm.
2. Make two holes on it as shown in the figure.
3. Thread a string through the two holes.
4. Draw or paste a cage on one side of the cardboard and a bird on the other side.
5. Twist the string and make the card twirl rapidly.

 

Observation: When the card twirls rapidly, the bird seems to be inside the cage.
Conclusion: When still images of a moving object are flashed at a rate faster than 1/16th of a second, the eye perceives this object as moving.

Light Objective Type Questions

A. State whether the following statements are true or false.

1. Cones and rods are the light-sensitive cells.
2. The color of the human eye is determined by the cornea.
3. A normal eye can see nearby and distant objects.

Answers:

1. True
2. False
3. True

B. Multiple Choice Questions

Question 1. Which part of the human eye controls the size of the pupil?

1. Cornea
2. Lens
3. Retina
4. IrisAnswers: 4) Iris

Question 2. Which of the following holds the lens of the eye in its place?

1. Cones
2. Optic muscles
3. Ciliary muscles
4. Rods

Answers: 2) Ciliary muscles

C. Fill in the blanks.

1. Sensations felt by the nerve cells are transmitted to the brain through the_____
2. Short-sightedness is also known as_____

Answers:

1. Optic nerve
2. Myopia

Light Short Answer Type Questions

A. What is cataract? Can it be corrected?

Answers: Cataract is a common defect of the eye wherein vision becomes foggy. It occurs particularly in old age. However, it is possible to treat this defect. The opaque lens is removed and a new artificial lens is inserted. Modern technology has made this procedure simpler and safer.

B. What is the range of normal vision of the human eye?

Answers: The minimum distance at which the human eye can see objects distinctly varies with age.
However, the range of vision of a normal human eye is from infinity to about 25 cm.

C. Explain how the human eye works.

Answers: Light enters our eye through the cornea. The amount of light entering the eye is controlled by the iris. The lens focuses the image of the object on the retina, which converts these optical images into optical pulses. These pulses are sent to the brain through the optical nerves.

Light Textbook Exercises

Question 1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.

Answers: We see things only when light reflected by them enters our eyes. Since no light is reflected from the surface of the objects in a dark room, we are not able to see them. However, outside the room, where light is available, objects can reflect light and thus, we can see them.

Question 2. Differentiate between regular and diffused reflection. Does diffuse reflection mean the failure of the laws of reflection?

The key differences between regular and diffused reflection are:

No. Diffused reflection does not mean the failure of the laws of reflection because the irregularities in the reflecting surface cause it.

Question 3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

1. Polished wooden table
2. Chalk powder
3. Cardboard surface
4. Marble floor with water spread over it
5. Mirror
6. Piece of paper

Answers:

1. A polished wooden table will have regular reflection because its surface is smooth.
2. A chalk powder will give a diffused reflection because the chalk powder particles are not smooth.
3. The cardboard surface has minute irregularities on it, so it will have a diffused reflection.
4. Marble stone with water spread over it will have regular reflection if the water is stable. However, if waves are formed, diffused reflection will take place.
5. Any mirror has a shiny and smooth surface. Therefore, regular reflection will be observed.
6. A piece of paper has a rough surface. Therefore, the diffused reflection will be observed.

Question 4. State the laws of reflection.

The laws of reflection are as follows:

(a) The angle of incidence is always equal to the angle of reflection.
(b) The incident ray, the normal at the point of incidence, and the reflected ray, all lie in the same plane.

Question 5. Describe an activity to show that the incident ray, the reflected ray, and the normal at the point of incidence lie in the same plane.

Procedure:

1. Fix a white sheet of paper on a drawing board in such a way that a small portion of it projects a little beyond the edge of the drawing board.
2. Place a plane mirror strip on the sheet of paper and hold it vertically with a mirror stand.
3. Throw a ray of light on the mirror using a torch. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper.
4. Bend the part of the projected portion on which the reflected ray falls. Now, bring the projected portion of the paper back to its original position and observe.

Observation: The entire sheet fixed onto the drawing board represents a plane. When the paper is
bent, a new plane is formed which is different from the plane in which the incident ray and the normal lie. So the reflected ray is not seen.
Conclusion: The incident ray, the normal at the point of incidence, and the reflected ray lie in the same plane.

Question 6. Fill in the blanks in the following:

1. A person I’m in front of a plane mirror seems to be from his image.
2. If you touch your_____ ear with the right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with the_____ hand.
3. The size of the pupil becomes______when you see in dim light
4. Night birds have______ cones than rods in their eyes.

Answers:

1. I
2. Left, left
3. Larger
4. Lesser

Choose the correct option in Questions

Question 7. The angle of incidence is

1. Always
2. Sometimes
3. Under special conditions
4. Never

Answer: (1) Always

Question 8. An image formed by a plane mirror is

1. (virtual, behind the mirror, and enlarged
2. virtual, behind the mirror, and of the same size as the object
3. real at the surface of the mirror and enlarged
   
4. real, behind the mirror, and of the same size as the object.

Answer: (2) virtual, behind the mirror, and of the same size as the object

Question 9. Describe the construction of a kaleidoscope.
Answer: A kaleidoscope is made up of three narrow strips of plane mirrors inclined at 60° to each other and enclosed in a tube. A ground glass plate on its inner side closes one end of the tube. The space between the mirrors is filled with colored pieces of glass or broken bangles. Cardboard with a hole in the center is fixed on the other end of the tube through which different patterns formed by the mirrors are seen.

Question 10. Draw a labeled sketch of the human eye.

 

Question 11. Gurmit wanted to perform an activity to see the change in the size of the pupil by throwing light into the eyes using a laser torch. Her teacher advised her not to do so. Can you explain the basis of her teacher’s advice?

Answers: Gurmit’s teacher advised her not to do so because laser light is harmful to the eyes. It can cause permanent damage when flashed directly in the eyes.

Question 12. Explain how you can take care of your eyes.

The following precautionary measures should be taken to care for eyes:

1. We should wash our eyes frequently with cold water.
2. We should not rub our eyes or touch them with dirty hands. If dust particles enter our eyes, we should wash our eyes with clean water.
3. We must not look at the Sun or a powerful light directly.
4. We should eat a sufficient quantity of vitamin A-enriched food like raw carrots, green vegetables, papaya, mango, eggs, milk, etc.
5. We should maintain a suitable distance while reading or watching TV.
6. An eye specialist should be consulted in case of any irritation or redness in the eyes.

Question 13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

If Zi= angle of incidence, and Zr= angle of reflection
Then, Zi + <r = 90° (given)
But Zi = Zr (according to the laws of reflection)
:: Zi+Zi= 90°
(186
or, 22i = 90°
or, <r = Zi = 45°
Therefore, the angle of incidence of the ray will be 45°.

Question 14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Infinite images of the candle will be formed if it is placed between two parallel plane mirrors.

Question 15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in the figure below. Draw the reflected ray from the second mirror.
Answer:

Question 16. Bhoojo stands at A just on the side of a plane mirror as shown in the figure below. Can he see himself in the mirror? Also, can he see the image of objects situated at P, Q, and R?
Answer: Bhoojo cannot see his image. However, he can see images of objects P but not Q and R.

Question 17. (a) Find out the position of the image of an object situated at A in the plane mirror (Figure given below).

1. (Can Paheli at B see this image?
2. Can Bhoojo at C see this image?
3. When Paheli moves from B to C, where does the image of A move?

Answer:

1. The image of A in the mirror will be as far behind as the object in front of the mirror.
2. Yes, Paheli can see the image.
3. Yes, Bhoojho can see this image.
4. When Paheli moves from B to C, the image of A will not move.

Light Hots corner

A. If all the objects around us were to reflect light in a regular way, what possible problems might we face?

Answers: When the rays of light fall on an object with a smooth surface, they are reflected in one direction only. On the other hand, when the rays of light fall on an object with an uneven surface, they get reflected in different directions. It is due to the irregular reflection that we can see objects from every direction. Further, irregular reflection of the sunlight by the objects around us enables our room to have light during the day even if sunlight does not fall directly.
However, if all the objects around us were to reflect light regularly, we would not be able to see the objects (1) from different directions and (2) during the daytime even when light is not falling on them directly.

Light Practice Exercise Objective Type Questions

A. Circle the odd one out.

1. Cornea, Iris, Retina, Cataract
2. Kaleidoscope, Telescope, Microscope, Binoculars
3. Crow, Kite, Eagle, Owl

Answers:

1. Cataract
2. Kaleidoscope
3. Owl

B. Give one word for the following.

1. The splitting of light into its constituent colors
2. The dark muscular structure behind the cornea gives the eye its distinct color
3. The ray of light which returns after striking the surface

Answers:

1. Dispersion
2. Iris
3. Reflected ray

C. State whether the following statements are true or false.

1. An owl has a larger number of rods and a lesser number of cones.
2. Rods are sensitive to bright light.
3. A plane mirror forms an image of an object as far behind the mirror as the object in front of it.

Answers:

1. True
2. False
3. True

D. Fill in the blanks.

1. The image formed by a plane mirror is called_______image.
2. In cataracts, the lens of the eye becomes______.
3. There are______types of nerve cells found in the retina

Answers:

1. Virtual
2. Foggy
3. Two

Light Short Answer Type Questions

A. What is a blind spot?

Answers: At the junction of the optic nerve and the retina, there are no sensory cells. Hence, no vision is possible at that spot. This is called the blind spot.

B. Why is it difficult to see objects in a room when you come from outside on a bright sunny day?

Answers: In bright sunlight, the iris causes the pupil to become smaller so that only a small amount of light enters the eye. However, when a person enters into a room, it takes time to increase the diameter of the pupil so that more amount of light can enter into the eyes to see the object. This is why, it is difficult to see objects in a room when we come from outside on a bright sunny day.

C. Explain how the Braille system enables visually challenged people to read and write.

Answers: The Braille system has 63 dot patterns or characters. Each character represents a letter, a combination of letters, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each. The patterns of dots when embossed on Braille sheets help visually challenged people to recognize words by touching them.

Also Read

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 11 Force and Pressure
• Chapter 12 Friction
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 17 Stars and the Solar System
• Chapter 18 Pollution of Air and Water

Stars And The Solar System Abstract

• The stars, the planets, the Moon, and many other objects in the sky are called celestial objects. They all form part of the universe.
• The Moon revolves around the Earth, and the Earth along with the Moon revolves around the Sun.
• Stars and the Solar System
• Stars are the celestial bodies which emit light of their own. The Sun is also a star.
• The distance between the stars and the Earth is expressed in a unit called a light year.
• A light year is a distance that light travels in one year.
• When seen from the Earth, stars appear to move from east to west.
• Pole Star is situated close to the direction of the axis of the Earth’s rotation; hence, it appears to be stationary when seen from the Earth.
• Groups of stars that appear to form recognizable shapes/patterns in the sky are called
  constellations.
• The most common constellations are Ursa Major, Orion, and Cassiopeia.
• The Sun, the planets revolving around it and many other celestial bodies comprise the solar system.
• A celestial body revolving around another celestial body is called a satellite. There are two types of satellites: natural and artificial.
• The shape of the Moon changes every day. Different shapes of the moon visible from the Earth are known as the phases of the moon.
• The phases of the Moon occur because only the part of the Moon which reflects sunlight is visible to us. The back of the moon is never visible.
• Moon is the only natural satellite of the Earth.
• Artificial satellites are man-made objects launched into the space. Artificial satellites are used for weather forecasting, communication, navigation, transmitting radio and television signals, and military.

Read and Learn More UP Board Notes for Class 8 Science

Stars and The Solar System Important Terms and Definitions

Asteroids: Numerous small bodies (mainly rocks, metals, etc.) that lie in the large gap existing between the orbits of Mars and Jupiter are known as asteroids.
Comets: The celestial bodies that revolve around the Sun and have very long periods of revolution are known as comets.
Meteors: Small objects that occasionally enter Earth’s atmosphere are known as meteors.
Meteorites: Meteors which reach the Earth are called meteorites.

The Moon

Moon is the natural satellite of the Earth. The surface of the Moon is dusty and infertile. There is complete absence of water and atmosphere on the Moon. Hence, there is absolutely no life on the Moon. The day when the entire Moon is visible, it is known as a full Moon day. The day when the Moon is not visible is known as the new Moon day. The day when only a small portion of the Moon appears in the sky is known as the crescent Moon day. The time period between one full moon day to the next full moon day is slightly longer than 29 days. This period is called a month.

Stars and The Solar System Activity 1

Aim: To observe the shape of the Moon for a few nights
Procedure:

1. Observe the Moon continuously for several nights, preferably from one full Moon to the next.
2. Make a sketch of the Moon every night in your notebook from the day of the full Moon.
3. Also, note the part of the sky (east or west) in which the Moon is seen.

Observations:

• The Moon appears to be perfectly round on the full Moon day.
• After the full Moon day, the size of the Moon decreases every day.
• On the new Moon day, the Moon is not visible even if the sky is clear.
• After the new Moon day, the size the Moon starts increasing each day and the full Moon appears again in another fifteen days.

Conclusion: The crescent-shaped Moon keeps on increasing every day until the full face of the Moon is visible on the fifteenth day. After the full Moon, the size and brightness of the Moon goes on decreasing each night. Subsequently, in another fifteen days, the Moon becomes invisible. This is known as new Moon day.

Stars and The Solar System Activity 2

Aim: To show the positions of the Moon on its orbit and its corresponding phases
Precaution: If you perform this activity in the morning, the white portion of the ball should face east. If this activity is performed in the afternoon, the white portion of the ball should face west. In each case, the line dividing the white and black portions should be kept vertical.

Procedure:

1. Take a big ball or a pitcher. Paint half of it white and the other half black.2. Go out into
2. The playground with two of your friends. Draw a circle of radius about 2 m on the ground. Divide the circle into eight equal parts as shown in the figure given here.

1. Stand at the center of the circle. Ask your friend to hold the ball at different points of the circle. Ask her to keep the white portion of the ball towards the Sun.
2. Standing at the center of the circle, observe the visible white portion of the ball while your friend stands at the points on the circle marked earlier. Draw the shape of the white portion as you see it.
3. Compare your drawings with the different phases of the Moon as shown in the figure given below.

Observation:
On the new Moon day, the Moon lies between the Sun and the Earth. However, on the full Moon day, the
Earth comes between the Moon and the Sun.

Conclusion:
After the new Moon day, the size of the illuminated part of the Moon, visible from the Earth, increases each day. However, the Sun-lit part of the Moon visible from the Earth decreases in size every day after the full Moon day.

Stars and The Solar System Objective Type Questions

A. State whether the following statements are true or false.

1. The shape of the Moon changes every month rather than daily.
2. Sound cannot be heard on the Moon.
3. There is no human life existing on the Moon; however, the growth of plants and other microorganisms is possible on its surface.The Moon produces light on its own.
4. After the full moon day, the visible part of the Moon starts decreasing.

Answers:

1. False
2. True
3. False
4. False
5. True

B. Multiple-Choice Questions

Question 1. Who was the first person to land on the Moon?

1. Galileo
2. Neil Armstrong
3. Isaac Newton
4. Edwin Aldrin

Question 2. When the Moon is not visible at night on a clear sky, what is the day called?

1. Full Moon day
2. Eclipsed Moon day
3. Crescent Moon day
4. New Moon day

Question 3.What is the reason that it is possible to observe phases of the Moon from the Earth?

1. The Moon revolves around the Earth.
2. The Moon’s axis is tilted.
3. The Moon spins on its axis.
4. The Moon’s distance from Earth changes at a particular rate.

Answers:

1. Neil Armstrong
2. New Moon
3. The Moon revolves around the Earth

C. Fill in the blanks.

1. The day when only a small portion of the Moon appears in the sky is called the_____ Moon day.
2. The day of the full Moon is popularly known as_____while the day of New Moon is popularly called____
3. The surface of the Moon is dry, rocky and _____

Answers:

1. Crescent
2. Purnima, Amavasya
3. Infertile/Barren

Stars and The Solar System Short Answer Type Questions

A. Why does the shape of the Moon change every day?

Answers: The Moon appears bright and shiny because it reflects the sunlight that falls on its surface. The Moon along with the Earth revolves around the Sun. As a result, the relative position of the moon keeps changing every day.

B. Why there is no life on the Moon?

Answers: The two essential factors, which are required for the survival of living beings are air and water. These are not available on the Moon’s surface. Hence, no life exists on the Moon.

Stars and Constellations

Answers: Stars emit their own light. Generally, stars are very far (many millions of kilometers) away from the Earth. This is why they appear too small in the sky. The Sun is the star closest to the Earth. This is why it appears to be very big while other stars seem so small, almost like dots or points in the sky. A group of stars forming a certain pattern with a recognizable shape is called a constellation. The shape of a constellation never changes. Like stars, constellations also appear to move from east to west.

Stars and The Solar System Activity 4

Aim: To show that if the stars appear to move from east to west, the Earth rotates from the west to the east
Procedure:

1. Stand in the center of a big room.
2. Now, start rotating and observe the direction in which the objects in the room appear to move.

Observation: The objects in the room appear to be moving in the direction opposite to our motion.
Conclusion: If the stars appear to move from east to west, the Earth rotates from west to east.

Stars and The Solar System Activity 5

Aim: To show that the Pole Star lies close to the axis of the rotation of the Earth
Procedure:

1. Take an umbrella and open it. Make about 10-15 stars out of white paper. Paste one star at the position of the central rod of the umbrella and others at different places on the cloth near the end of each spoke as shown in the figure given below.
2. Now rotate the umbrella by holding its central rod in your hand. Observe the stars on the umbrella.

Observation: The star situated at the central rod of the umbrella does not appear to move.
Conclusion: The Pole Star lies close to the axis of the rotation of the Earth, thus, it does not appear to move.

Stars and The Solar System Activity 6

Aim: To show that a constellation appears to move in the sky from east to west
Procedure:

1. Locate a constellation (e.g. Ursa Major) in the sky.
2. Observe this constellation for a few hours.

Observation: The shape of the constellation remains the same. However, the constellation appears to be moving.
Conclusion: The constellation appears to move in the sky from east to west.

Stars And The Solar System Activity 7

Aim: To locate the Pole Star in the sky
Precaution: This activity should be performed on a clear Moonless night
during summer at around 9 pm.

Procedure:

1. Look towards the northern part of the sky and identify Ursa
   Major.
2. Look at the two stars at the end of Ursa Major.
3. Imagine a straight line passing through these stars as shown in the
   figure given below.
4. Extend this imaginary line towards the north direction. (About
   five times the distance between the two stars).
5. This line will lead to a star.

Observation: The imaginary line will lead to a star, that is not too bright. This is the pole star.

Stars And The Solar System Activity 8

Aim: To show that the Ursa Major constellation moves around
the Pole Star
Procedure:

1. 1. During a summer night, observe Ursa Major 3-4 times at an interval of 2 to 3 hours.
2. Locate the Pole Star each time.
3. Compare your observations with those in the figure given below.

Observation Ursa Major appears to move from east to west.
Conclusion: The Ursa Major constellation moves around the Pole Star.

Stars and The Solar System Objective Type Questions

A. State whether the following statements are true or false.

1. The North Star which indicates the north direction is also called the Pole Star.
2. Stars always seem to move in the same direction as the axis of Earth’s rotation.
3. Stars are present in the sky at night only.

Answers:

1. True
2. False
3. False

B. Multiple Choice Questions.

Question 1. Which of these constellations resembles the shape of a hunter’s body?

1. Orion
2. Ursa Major
3. Leo Major
4. CassiopeiaAnswers: 1) Orion

Question 2. Which of these stars is located closest to the Earth?

1. Pole Star
2. Alpha Centauri
3. Sirius
4. Sun
   Answers: 4) Sun

Question 3. Which is the brightest star in the sky?

1. Alpha Centauri
2. Sun
3. Sirius
4. Pole Star
   Answers: 1) Alpha Centauri

C. Fill in the blanks.

1. The distance between the Earth and the Sun is measured in_____.
2. There are stars present in the sky.

Answers:

1. Light years
2. Infinite/Countless

Stars and The Solar System Short Answer Type Questions

A. Do all the stars of a constellation lie really close together?

Answers: The different stars that form a constellation are not related and lie at widely varying distances from the Earth. However, they all appear to our eye as close together but are not so.

B. Why are stars not visible during the day?

Answers: During the day, the light of the Sun is very bright and the brightness of stars becomes relatively dim in comparison to that of the Sun. Hence, they are not visible during the day.

C. How can you identify Ursa Major?

Answers: The constellation Ursa Major is seen during summer. It is also called the Great Bear or the Saptarishi. There are seven prominently shining stars in the constellation which are arranged in the shape of a question mark. The Pole Star lies at the tail of this constellation.

Solar System: Planets and Other Members

The solar system comprises the Sun, the planets, and various other members such as asteroids, comets, meteors/meteorites, and artificial satellites. There are eight planets in the solar system. These are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. All planets revolve around the Sun in a fixed path called orbit. The time taken by a planet to complete one revolution is called its period of revolution. In addition to the Sun and the eight planets, many other celestial bodies form the part of the solar system.

Stars and The Solar System Activity 9

Aim: To confirm that planets move in their own orbits
Procedure:

1. 1. Go out into the playground with four or five friends. Draw four circles of radii 1 m, 1.8 m, 2.5 m, and 3.8 m, all having a common center as shown in the given figure.

1. Ask one of your friends to stand in the centre and represent the Sun. Your other four friends may represent Mercury, Venus, Earth and Mars.
2. Ask your friends to move around the Sun in anti-clockwise direction in their own orbits and observe whether they collide with one another or not.

Observation: They do not collide because they are moving in their own fixed orbits.

Conclusion: Planets move in their own fixed path called orbits.

Stars and The Solar System Activity 10

Aim: To observe the planet Venus
Procedure:

1. Find out from a newspaper or from an almanac the time when Venus is visible in the sky.
2. Observe Venus either 1-3 hours before sunrise or 1-3 hours after sunset.

Observation: Venus can be easily recognized by its brightness. However, it cannot be seen very high in the sky.
Conclusion: Venus is the brightest planet in the night sky. It rotates from the east to the west. Since Venus is close to the Sun, it can also be seen just before sunrise and just after sunset.

Stars and The Solar System Objective Type Questions

A. State whether the following statements are true or false.

1. Until 2006, Pluto was also a planet (ninth planet) in the solar system.
2. Mercury and Venus have rings around them, which can be seen through a telescope.
3. The outer planets have very few moons.
4. Saturn is the least dense among all the planets.
5. The appearance of a comet is a natural phenomenon and it is not an indicator of any forthcoming disaster.

Answers:

1. True
2. False
3. False
4. True
5. True

B. Multiple-Choice Questions

Question 1. Which of these planets is also called the morning star or the evening star?

1. Mercury
2. Venus
3. Mars
4. NeptuneAnswers: 2) Venus

Question 2. Which planet in the solar system appears yellowish in color?

1. Jupiter
2. Uranus
3. Satum
4. MercuryAnswers:  3) Saturn

Question 3. The axis of the Earth is inclined to its orbital plane at what angle?

1. 23.5°
2. 25.5°
3. 60.5°
4. 66.5°Answers: 4) 66.5°

Question 4. Which of these celestial bodies is visible only through large telescopes?

1. Meteors
2. Meteorites
3. Asteroids
4. CometsAnswers:  3) Asteroids

Question 5. Which of these is the first Indian satellite launched from the Earth?

1. INSET
2. Aryabhatta
3. Kalpana-I
4. EDUSAT

Answers:  2) Aryabhatta

C. Fill in the blanks.

1. _______ is commonly known as Dhruv Tara.
2. The reflection of light from______ and_____ on Earth’s surface makes it appears blue-green when seen from space.
3. Mars has_____ small natural satellites.
4. The mass of Jupiter is nearly______times that of the Earth.
5. _____ are swarms of meteors seen at the time when the Earth crosses the tail of a comet.

Answers:

1. The Pole Star
2. Water, landmass
3. Two
4. 318
5. Meteor showers

Stars and The Solar System Short Answer Type Questions

A. Why is life possible on the Earth?

Answers: Environmental conditions such as appropriate distance from the Sun, the correct level of temperature, the presence of atmosphere and water, together make the existence and continuation of life possible on the Earth. Moreover, the presence of the ozone layer around the Earth prevents organisms from harmful ultraviolet radiation of the Sun.

B. What do you mean by inner planets and outer planets?

Answers: The four planets, Mercury, Venus, Earth, and Mars, which are nearer to the Sun, are called the inner planets. However, the planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus, and Neptune are much farther off from the Sun. These planets are called the outer planets.

Stars and The Solar System Textbook Exercises

Choose the correct answer in Questions

Question 1. Which of the following is NOT a member of the solar system?

1. An asteroid
2. A satellite
3. A constellation
4. A Comet

Answer: 3) A constellation

Question 2. Which of the following is NOT a planet of the Sun?

1. Sirius
2. Mercury
3. Saturn
4. Earth

Answer: 1) Sirius

Question 3. Phases of the Moon occur because

1. We can see only that part of the Moon which reflects light toward us.
2. Our distance from the Moon keeps changing.
3. The shadow of the Earth covers only a part of the Moon’s surface.
4. The thickness of the Moon’s atmosphere is not constant.

Answer: 1) We can see only that part of the Moon which reflects light toward us.

Question 4. Fill in the blanks.

1. The planet which is farthest from the Sun is______.
2. The planet which appears reddish in color is______.
3. A group of stars that appear to form a pattern in the sky is known as a______.
4. A celestial body that revolves around a planet is known as_______.
5. Shooting stars are actually not_______.
6. Asteroids are found between the orbits of_______and_____.

Answer:

1. Neptune
2. Mars
3. Constellation
4. Satellite
5. Stars
6. Mars, Jupiter

Question 5. Mark the following statements as true (T) or false (F):

1. The Pole Star is a member of our solar system.
2. Mercury is the smallest planet of the solar system.
3. Uranus is the farthest planet in the solar system.
4. INSAT is an artificial satellite.
5. There are nine planets in the solar system.
6. Constellation Orion can be seen only with a telescope.

Answer:

1. True
2. True
3. False
4. True
5. False
6. False

Question 6. In which part of the sky can you find Venus if it is visible as an evening star?

Answers: Venus is visible as an evening star in the western part of the sky

Question 7. Name the largest planet of the solar system.

Answers: Jupiter

Question 8. What is a constellation? Name any two constellations.

Answers: A groups of stars that appear to form recognisable shapes/patterns are called constellations. The Great Bear and Orion are two constellations.

Question 9. Draw sketches to show the relative positions of prominent stars in

(a) Ursa Major and (b) Orion

Question 10. Name two objects other than planets which are members of the solar system.

Answers: Asteroids and Meteors

Question 11. Explain how you can locate the Pole Star with the help of Ursa Major.

Answers: The Pole Star is located at the end of Ursa Major. A straight line from the last two stars when extended towards the north leads to the Pole Star.

Question 12. Do all the stars in the sky move? Explain.

Answers: No, all stars in the sky do not move. However, they appear to move from east to west due to the rotation of the Earth about its axis.

Question 13. Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight light years away from the Earth?

Answers: Stars are situated very far (many millions of kilometers) away from the Earth. Such large distance cannot be conveniently expressed in kilometers. Hence, it is expressed in a unit called a light year. One light year is the distance traveled by light in one year. One light year is equivalent to 9.46 x 10¹2 km. If we say that a star is eight light years away from earth, it means that the light from the star will reach the earth in eight years.

Question 14. The radius of Jupiter is II times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate?

Answers:

Let the radius of Earth = R units
200
Volume of Earth = 4/3xpx R³ cubic units
Radius of Jupiter = ||R units
Thus, volume of Jupiter = 4/3 × px (11R)³ cubic units
= 4/3 × px 1331R³ cubic units
X
Now, ratio of the volume of Jupiter and Earth = Volume of Jupiter/ Volume of Earth
:. 4/3 × px 1331 = R³/4/3 × px R³
= 1331/1 or 331: 1
So, Jupiter can accommodate 1331 Earths.

Question 15. Boojho made the following sketch of the solar system. Is the sketch correct? If not, correct it.

Stars and The Solar System Hots corner

Question 1. Venus is far away from the Sun as compared to Mercury. Still Venus is hotter than Mercury. Why is it so?

Answers: Venus has a thick atmosphere rich in carbon dioxide gas. Carbon dioxide is a greenhouse which traps the infrared radiations of the Sun resulting in an increase in the temperature of the planet. In contrast, Mercury has a thin atmosphere with a variety of gases. The atmosphere does not have greenhouse gases which can trap heat. Thus, Venus is much hotter than Mercury.

Question 2. Why do stars twinkle when planets do not?

Answers: Stars are located at a distance from the Earth; so they appear as dots or points in the sky. These points vibrate due to the atmospheric air currents. Hence, when seen from the Earth, they generally seem to twinkle or vibrate. However, planets are not located as far from Earth. They are relatively bigger in size and so their vibrations, due to atmospheric currents, are apparently not visible as twinkling.

Stars and The Solar System Practice Exercise

Objective Type Questions

A. Give one word for the following.

1. The branch of science that deals with the study of the universe
2. The unit of measurement used to measure distance in space
3. The name of our galaxy

Answers:

1. Astronomy
2. Light year
3. Milky way

B. State whether the following statements are true or false.

1. Venus is also called the morning star.
2. Mercury is the smallest planet of the solar system.
3. Aryabhatta is a natural satellite of the Earth.

Answers:

1. False
2. True
3. False

C. Circle the odd one out.

1. Orion, Great Bear, Ursa Major, Big Dipper
2. Earth, Venus, Moon, Jupiter

Answers:

1. Orion
2. Moon

D. Fill in the blanks.

1. It is not possible to hear any sound on the Moon because of the absence of______.
2. Asteroids are found between the orbits of_____ and______.

Answers:

1. Atmosphere
2. Mars, Jupiter

Stars and The Solar System Short Answer Type Questions

A. What is meant by an equatorial plane and orbital plane?

Equatorial plane refers to the plane of the equator of the Earth. Orbital plane refers to the plane in which the Earth revolves around the Sun.

B. What are the differences between a star and a shooting star?

C. Name a periodic comet. Why is it so-called?

Answers: Halley’s comet is a periodic comet. It is called so because it appears at regular intervals of time. Halley’s comet is seen after every 76 years.

D. Why is the Sun classified as a star?

Answers:

The Sun is classified as a star due to the following reasons:

1. It has its own source of energy.
2. It continuously emits large amounts of light and energy.
3. It has a life period.

E. State in which month/season the following constellations are seen:

1. Ursa major
2. Orion
1. Visible in April in the northern part of the sky.
2. Visible during the winter season in the northern part of the sky.

Also Read

• Chapter 1 Crop Production and Management
• Chapter 2 Microorganisms: Friend and Foe.
• Chapter 3 Synthetic Fibres and Plastics
• Chapter 4 Materials: Metals and Non-Metals
• Chapter 5 Coal and Petroleum
• Chapter 6 Combustion and Flame
• Chapter 7 Conservation of Plants and Animals
• Chapter 8 Cell: Structure and Functions
• Chapter 9 Reproduction in Animals
• Chapter 10 Reaching the Age of Adolescence
• Chapter 11 Force and Pressure
• Chapter 12 Friction
• Chapter 13 Sound
• Chapter 14 Chemical Effects of Electric Current
• Chapter 15 Some Natural Phenomena
• Chapter 16 Light
• Chapter 18 Pollution of Air and Water