UP Board Class 9 Science Chapter 9 Gravitation Multiple Choice Questions

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Gravitation Multiple-Choice Questions

Question 1. Newton’s law of gravitation is universal because it

  1. acts on all bodies and particles in the universe
  2. it acts on all the masses at all distances and is not affected by the medium
  3. is always attractive
  4. None of the above

Answer: 2. it acts on all the masses at all distances and is not affected by the medium

According to Newton’s law of gravitation, the force between two masses in the universe, F=\(\frac{G m_1 m_2}{r^2}\)

Which does not depend on medium and G is the same in the whole universe.

UP Board Class 9 Science Chapter 9 Gravitation Multiple Choice Questions

Question 2. The weakest force in the following is

  1. magnetic force
  2. nuclear force
  3. gravitational force
  4. electric force

Answer: 3. gravitational force

Gravitational force is the weakest force among given forces.

Question 3. The atmosphere is held to the earth by

  1. earth’s magnetic field
  2. earth’s rotation
  3. gravity
  4. earth’s electric field

Answer: 3. gravity

The atmosphere is held to the earth by gravity.

Question 4. Between the two gravitational constants G and g, which is called the universal gravitational constant?

  1. g
  2. G
  3. Both (1) and (2)
  4. None of these

Answer: 2. G

G is called the universal gravitational constant because its value is the same in the whole universe.

G=6.67 \(\times 10^{-11} \mathrm{~N}-\mathrm{m}^2 / \mathrm{kg}^2\)

Question 5. The value of gravitational acceleration (g) is

  1. highest at poles
  2. highest at equator
  3. lowest at poles
  4. lowest at equator

Answer: 1. highest at poles

Gravitational acceleration g is given by g = \(\frac{G m_e}{R_e^2}\)

Since the radius of the earth at the equator is greater than the poles, the value of g at the poles is greater than the equator

Question 6. The law of gravitation gives the gravitational force between

  1. The Earth and other planets only
  2. the earth and sun only
  3. two charged bodies only
  4. any two bodies having some mass

Answer: 4. any two bodies having some mass

The law of gravitation is applicable to all bodies having some mass.

Question 7. The gravitational force between two objects is 10 N. If the masses of both objects are doubled without changing the distance between them, then the gravitational force would become

  1. 2.5 N
  2. 20 N
  3. 40 N
  4. 10 N

Answer: 3. 40 N

Gravitational force, \(F_1=\frac{G m_1 m_2}{r^2}\)

When masses are doubled, then

⇒ \(F_2=\frac{G 2 m_1 2 m_2}{r^2}=4 \frac{G m_1 m_2}{r^2}\)

⇒ \(F_2=4 F_1=4 \times 10=40 \mathrm{~N}\)

Question 8. The magnitude of the weight of a body at the centre of the earth is

  1. zero
  2. equal to the mass of the body
  3. greater than g
  4. less than g

Answer: 1. zero

The value of gravitational acceleration (g) at the centre is zero, hence weight, w =mg = mx 0 = 0.

Question 9. When a ball is fully immersed in a liquid, its weight decreases. It happens due to

  1. gravitational force
  2. magnetic force
  3. buoyant force
  4. friction force

Answer: 3. buoyant force

When a ball is fully immersed in a liquid, its weight decreases due to buoyant force.

Question 10. Why the dam of the water reservoir is thick at the bottom?

  1. The quantity of water increases with the depth
  2. The density of water increases with the depth
  3. The pressure of water increases with the depth
  4. Temperature of water increases with depth

Answer: 3. Pressure of water increases with depth

The dam of water is made thick at the bottom, because due to maximum depth, the pressure of water is maximum at the bottom and to bear this maximum pressure dam should be thick at the bottom.

Question 11. Three spheres have radii of 1 cm, 2 cm and 3 cm, respectively. Which sphere exerts maximum pressure on earth?

  1. First
  2. Second
  3. Third
  4. All Equal

Answer: 1. First

We know that, pressure =\(\frac{\text { force }}{\text { area }}=\frac{F}{A}\)

or p=\(\frac{F}{A} \Rightarrow \) p \(\propto \frac{1}{A}\)

p \(\propto \frac{1}{r^2}\)

Therefore, the sphere which has a lesser area will exert maximum pressure. Since the first sphere has a small radius, its area will be small. Hence, it will exert maximum pressure.

Question 12. A petrol cane of mass 5 kg has a volume of 2880. What is the density of the cane?

  1. 17.3 \(\mathrm{~g} / \mathrm{cm}^3\)
  2. 16.5 \(\mathrm{~g} / \mathrm{cm}^3\)
  3. 1.65 \(\mathrm{~g} / \mathrm{cm}^3\)
  4. 1.73 \(\mathrm{~g} / \mathrm{cm}^3\)

Answer: 4. 1.73 \(\mathrm{~g} / \mathrm{cm}^3\)

Given, mass of cane, m=5 kg =5000 g, volume of cane, V=2880 \(\mathrm{~cm}^3\)

Density of cane, \(\rho=\frac{m}{V}\)

⇒ \(\rho=\frac{5000}{2880}=1.73 \mathrm{~g} / \mathrm{cm}^3\)

Question 13. The pressure exerted by the weight of a cubical box of side 2 cm on the surface of table is 4 Pa. What is the weight of the box?

  1. 1.6 \(\times 10^{-4} \mathrm{~N}\)
  2. 1.6 \(\times 10^{-5} \mathrm{~N}\)
  3. 16 \(\times 10^{-4} \mathrm{~N}\)
  4. 15 \(\times 10^{-4} \mathrm{~N}\)

Answer: 3. 16 \(\times 10^{-4} \mathrm{~N}\)

Given, pressure, p = 4 Pa and side of cube,

a = 2 cm = 0.02 m

Area of cubical box, A =\(a^2=(0.02)^2\)

=4 \(\times 10^{-4} \mathrm{~m}^2\)

Since, pressure, p =\(\frac{F}{A}\)

F =p \(\times A=4 \times 4 \times 10^{-4}\)

=16 \(\times 10^{-4} \mathrm{~N}\)

So, the weight of the block is \(16 \times 10^{-4} \mathrm{~N}\).

Question 14. A large ship can float, but a steel needle sinks, because of

  1. mass
  2. volume
  3. density
  4. None of these

Answer: 4. None of these

This concept is explained by Archimedes’ principle which states that when a body is fully or partly submerged in a fluid, a buoyant force from the surrounding fluid, acts on the body.

Question 15. A wooden object of mass 700 g is thrown into the water tub. What is the buoyant force acting on the object, if it displaces 700 g of water?

  1. 5 N
  2. 8 N
  3. 7 N
  4. 10 N

Answer: 3. 7 N

Given, the mass of the object (m) = 700 g = 0.7 kg

Acceleration due to gravity (g) =10 m/s²

According to Archimedes’ principle, the buoyant force is equal to the weight of displaced water,

i.e. weight of displaced water = mg = 0.7 x 10 = 7 N So, buoyant force is 7 N.

Question 16. A ball weighs 90 g in air, 60 g in water and 40 g in a liquid. If the density of water is 1 g cm-3, then what is the density of this liquid?

  1. 3 \(\mathrm{~g} \mathrm{~cm}^{-3}\)
  2. 1 \(\mathrm{~g} \mathrm{~cm}^{-3}\)
  3. 1.66 \(\mathrm{~g} \mathrm{~cm}^{-3}\)
  4. 2.66 \(\mathrm{~g} \mathrm{~cm}^{-3}\)

Answer: 3. 1.66 \(\mathrm{~g} \mathrm{~cm}^{-3}\)

  1. As given, when immersed in water, the ball displaces 90 – 60 = 30 g of water.
  2. So, it’s volume = volume of water displaced = \(\frac{30}{1 \mathrm{~g} \mathrm{~cm}^{-3}}=30 \mathrm{~cm}^3\)
  3. Density of ball = \(\frac{90}{30}=3 \mathrm{~g} \mathrm{~cm}^{-3}\)
  4. So, the ball displaces (90-40)=50 \(\mathrm{~g} \) of liquid with density =  \(\frac{50 \mathrm{~g}}{30 \mathrm{~cm}^3}=1.66 \mathrm{~g} \mathrm{~cm}^{-3} \)

Question 17. If two forces in the ratio 3 :8 act on two surface areas in ratio 2 :5. What is the ratio of pressure exerted?

  1. 3: 8
  2. 6: 40
  3. 15: 16
  4. 4: 8

Answer: 3. 15: 16

Ratio of areas =2: 5, i.e. \(\frac{A_1}{A_2}=\frac{2}{5}\)

Ratio of forces =3: 8, i.e. \(\frac{F_1}{F_2}=\frac{3}{8}\)

Ratio of pressure =\(\frac{p_1}{p_2}=\frac{F_1}{F_2} \times \frac{A_2}{A_1}\)

=\(\frac{3}{8} \times \frac{5}{2}=\frac{15}{16}\)

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