UP Board Notes for Class 8 Science Chapter 12 Friction

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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.

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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.

UP Board Notes For Class 8 Science Chapter 12 Friction Activity 3

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.

UP Board Notes For Class 8 Science Chapter 12 Friction Activity 4

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.

UP Board Notes For Class 8 Science Chapter 12 Friction 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.

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