# Class 9 Science Chapter 10 Gravitation

NCERT Class 9 Science Chapter 10 Gravitation Question Answer to each chapter is provided in the list so that you can easily browse throughout different chapters Class 9 Science Chapter 10 Gravitation and select need one.

## NCERT Class 9 Science Chapter 10 Gravitation

Also, you can read the SCERT book online in these sections Solutions by Expert Teachers as per SCERT (CBSE) Book guidelines. These solutions are part of SCERT All Subject Solutions. Here we have given SCERT Class 9 Science Chapter 10 Gravitation Solutions for All Subjects, You can practice these here.

### Gravitation

Chapter – 10

GENERAL SCIENCE

Textbook Page No. 134

1. State the universal law of gravitation.

Ans. Gravitation states that every object of the universe attracts every other object. The force of attraction between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them.

2. Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.

Ans. The magnitude of the gravitational force between the earth and an object on the surface of the earth is given by the formula:

Where,

F = gravitational force

M = Mass of the earth

m = Mass of the object

d = Distance between the earth and the object

G = Universal gravitational constant

= 6.67×10⁻¹¹ N m² kg⁻²

Textbook Page No. 136

1. What do you mean by free-fall?

Ans. Whenever an object falls under the effect of only gravitational force towards the earth, it is said to be under free fall. The direction of motion of the object does not change during free fall. But due to the acceleration due to gravity, the velocity of object changes.

2. What do you mean by acceleration due to gravity?

Ans. The speed of a body falling freely from a height increases as time passes. This means that the body accelerates. This acceleration is produced due to earth’s force of gravity and known as acceleration due to gravity.

Textbook Page No. 138

1. What are the differences between the mass of an object and its weight?

Ans. The differences between the mass of an object and object and its weight mentioned below:

2. Why is the weight of an object on the moon ⅙ its weight on the earth?

Ans. The value of acceleration due to gravity on the moon is about one-sixth ⅙ of the value of acceleration due to gravity on the earth.

Now, the weight of a body is given by, W = mg

Since, the mass of a body is constant,

Therefore, W ∝ g

Hence the weight of an object on the moon is ⅙ its weight on the earth.

Textbook Page No. 141

1. Why is it difficult to hold a school bag having a strap made of a thin and strong string?

Ans. The smaller the surface area, larger is the pressure exerted. Because the pressure on the shoulders is relatively large, it is difficult to handle a school bag with a thin strap.

2. What do you mean by buoyancy?

Ans. When a body is immersed partly or completely in a fluid, the fluid exerts an upward force on the body. This tendency of the fluid to exert an upward force is called buoyancy.

3. Why does an object float or sink when placed on the surface of water?

Ans. When an object is immersed partially or fully in a liquid, it is under the action of two forces are mentioned below:

(i) The weight of the body acting downward.

(ii) The upthrust or buoyant force of the liquid acting on the body upward. If a body having density less than that of water is placed on the surface of water, the upthrust of water on the body is greater than the weight of the body. As a result, the body floats. On the other hand, if a body having density more than that of water is placed on the surface of water, the upthrust of water on the body is less than the weight of the body. As a result, the body sinks.

Textbook Page No. 142

1. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

Ans. The air exerts an upthrust on our body. Due to this upthrust, the reading of the weighing machine is slightly less than the exact value. Thus, the mass is more than 42 kg.

2. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?

Ans. The volume of the cotton bag is much more than the volume of the iron bar. As a result, the upthrust exerted by air on the cotton bag is more than that exerted on the iron bar. So, the weighing machine reads a smaller value for cotton bag than its actual value. Therefore, the cotton bag is heavier than the iron bar.

EXERCISE

Textbook Page No. 143

1. How does the force of gravitation between two objects change when the distance between them is reduced to half?

Ans. According to the universal law of gravitation, the force of attraction between two bodies is given by,

Where, m₁ and m₂ are the masses of the two bodies, r is the distance between them and G is the gravitational constant.

When the distance is reduced to half,

Hence, if the distance between the two objects is reduced to half, the force of gravitation increases by 4 times the original force.

2. Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than a light object?

Ans. Let M be the mass of the earth. Let us consider an object of mass ‘m’ situated at a distance ‘R’ from the centre of the earth.

According to the universal law of gravitational, the force of attraction between an object and the earth is

[Where, ‘G’ is the universal gravitational constant.]

Again, from Newton’s second law of motion,

Force = mass×acceleration

⇒ F = mg…..(ii) [Where, ‘g’ is the acceleration due to gravity.]

Thus, even if the gravitational force acts on all objects in proportion to their masses, the acceleration experienced by an object falling freely towards the earth does not depend on the mass of the object. Therefore, a heavy object does not fall faster than a light object.

3. What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? (Mass of the earth is 16×10²⁴ kg and radius of the earth is 6.4×10⁶ m.)

Ans.

Therefore, the required magnitude of the gravitational force is 9.8 N.

4. The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why?

Ans. The earth attracts the moon with a force that is same as the force with which the moon attracts the earth.

Newton’s third law of motion is applicable to the gravitational force also. Thus, the gravitational force exerted by the earth on the moon is equal and opposite to that exerted by the moon on the earth.

5. If the moon attracts the earth, why does the earth not move towards the moon?

Ans. The earth and the moon experience equal gravitational forces from each other. However, the mass of the earth is much larger than the mass of the moon. Hence, its acceleration towards the moon is much smaller than the acceleration of the moon towards the Earth. For this reason, the Earth does not move towards the moon.

6. What happens to the force between two objects, if

(i) The mass of one object is doubled?

(ii) The distance between the objects is doubled and tripled?

(iii) The masses of both objects are doubled?

Ans. Let the masses of the two objects A and B be M and m respectively and the distance between them be d.

The gravitational force of attraction between A and B is given by

Hence, the force of attraction increases by 4 times the original force of attraction.

7. What is the importance of universal law of gravitation?

Ans. The important of universal law osf gravitation are mentioned below:

(i) The motion of the planets moving around the sun.

(ii) The motion of the moon moving around the sun.

(iii) The force of attraction that binds us to the earth.

(iv) The occurrence of tides due to the sun and the moon.

8. What is the acceleration of free-fall?

Ans. The speed of a body falling freely from a height increases as time passes. This means that the body accelerates. This acceleration is produced due to earth’s gravitational force and is known as acceleration of free-fall or acceleration due to gravity.

Its value on earth’s surface is 9.8 m s⁻².

9. What do we call the gravitational force between the earth and an object?

Ans. The gravitationalal force between the earth and an object is called the weight of the object. It is also equal to the product of acceleration due to gravity and mass of the object.

10. Amit buys few grams of gold at the poles as per the instruction of one of his friends. He hands over the same when he meets him at the equator. Will the friend agree with the weight of gold bought ? If not, why?

Ans. The friend will not agree with the weight of gold bought.

The weight, W, of a body is given by W = mg, where m is the mass of the body and g is the acceleration due to gravity.

The value of mass remains constant at all places, but the value of g changes from one place to another. The value of g is greater at the poles than at the equator. Therefore, the weight of the gold bought at the poles is more than the weight of the gold at the equator.

11. Why does a sheet of paper fall slower than one that is crumpled into a bail?

Ans. As a sheet of paper has more surface area than a crumpled ball, it faces more air resistance. Because it faces more air resistance, its speed decreases. Therefore, it will fall slower than a crumpled ball.

12. Gravitational force on the surface of the moon is only ⅙ as strong as gravitational force on the earth. What is the weight in Newtons of a 10 kg object on the moon and on the earth?

Ans.

13. A ball is thrown vertically upwards with a velocity of 49 m/s. Calculate

(i) The maximum height to which it rises,

(ii) The total time it takes to return to the surface of the earth.

Ans.

∴ Total time taken by the ball to return to the surface of the earth

= Time taken by the ball to reach the maximum height + Time taken by the ball to travel from the maximum height to the surface of the earth.

= 5s+5s

= 10 s

14. A stone is released from the top of a tower of height 19.6 m. Calculate its final velocity just before touching the ground.

Ans.

15. A stone is thrown vertically upward with an initial velocity of 40 m s⁻¹. Taking g = 10 m s⁻², find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone?

Ans.

16. Calculate the force of gravitation between the earth and the sun, given that the mass of the earth = 6 x 10²⁴ kg and of the sun = 2 x 10³⁰ kg. The average distance between the two is 1.5 x 10 m.

Ans.

17. A stone is allowed to fall from the top of a tower 100 m high and at the same time another stone is projected vertically upwards from the ground with a velocity of 25 m/s. Calculate when and where the two stones will meet.

Ans.

Hence the two stones will meet after 4 s at a distance of 78.4 m from the top.

18. A bail thrown up vertically returns to the thrower after 6 s. Find

(a) The velocity with which it was thrown up.

(b) The maximum height it reaches. and

(c) Its position after 4 s.

Ans. The time taken by the ball to reach the maximum height (t) = The time taken by the ball to return to the ground from the maximum height (t₁).

19. In what direction does the buoyant force on an object immersed in a liquid act?

Ans. The buoyant force on an object immersed in a liquid act in vertically upward direction.

20. Why does a block of plastic released under water come up to the surface of water?

Ans. The upward force or the buoyant force exerted by the water is greater than the weight of the block of plastic immersed in it. Therefore, the block of plastic released under water comes up to the surface of water.

21. The volume of 50 g of a substance is 20 cm³. If the density of water is 1 g cm³, will the substance float or sink?

Ans.

Since, the density of the substance is more than that of water, the substance will silk.

22. The volume of a 500 g sealed packet is 350 cm³. Will the packet float or sink in water if the density of water is 1 g cm⁻³? What will be the mass of the water displaced by this packet?

Ans.

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