10. Gravitation Class 9 Science Chapter 10 NCERT Solutions (CBSE)
CBSE Class 9 Science Chapter 10: Gravitation – Full NCERT Textbook Solutions
Chapter 10: Gravitation
Internal Question and Answer :
Example 10.1: The mass of the earth is 
kg and that of the moon is 
. If the distance between the earth and the moon is 
km, calculate the force exerted by the earth on the moon. (Take G = 
)
Solution: The mass of the earth, M 
kg , The mass of the moon, m 
kg , The distance between the earth and the moon, d 
km 
m 
m and G 
The force exerted by the earth on the moon is 
N
Thus, the force exerted by the earth on the moon is 
N .
1. State the universal law of gravitation.
Answer: The law of gravitation states that the force of attraction between any 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.
Answer: The formula of the magnitude of the gravitational force between the earth and an object on the surface of the earth is
Example 10.2: A car falls off a ledge and drops to the ground in 0.5 s. Let g = 10 
(for simplifying the calculations).
(i) What is its speed on striking the ground?
(ii) What is its average speed during the 0.5 s?
(iii) How high is the ledge from the ground?
Solution: Time 
second, Initial velocity 
Acceleration due to gravity, 
Acceleration of the car, 
(downward)
(i) speed 
m/s
(ii) Average speed 
(iii) Distance travelled, 
(i) its speed on striking the ground 
(ii) its average speed during the 
(iii) height of the ledge from the ground = 1.25 m.
Example 10.3: An object is thrown vertically upwards and rises to a height of 10 m. Calculate (i) the velocity with which the object was thrown upwards and (ii) the time taken by the object to reach the highest point.
Solution: Distance travelled 
, Final velocity 
Acceleration due to gravity, 
Acceleration of the object,
(upward motion)
(i) We have, 
(ii) We have, 
Thus, (i) Initial velocity, u = 14 
, and (ii) Time taken, t = 1.43 s.
Internal Question and Answer :
1. What do you mean by free fall?
Answer: free fall is the motion of an object falling solely under the influence of gravity . During free fall, the object accelerates towards the Earth at a constant rate, known as acceleration due to gravity (g)
2. What do you mean by acceleration due to gravity?
Answer: Acceleration due to gravity is the acceleration produced in a body when it falls freely under the influence of Earth's gravity alone. It is denoted by 'g' and its standard value is approximately 9.8 m/s². It acts towards the center of the Earth.
Example 10.4: Mass of an object is 10 kg. What is its weight on the earth?
Solution: Here, Mass, m = 10 kg
Acceleration due to gravity, g = 9.8 
W = m × g
W = 10 kg × 9.8 m/s² = 98 N
Thus, the weight of the object is 98 N.
Example 10.5: An object weighs 10 N when measured on the surface of the earth. What would be its weight when measured on the surface of the moon .
Solution: We know, Weight of object on the moon 
= 1.67 N .
Thus, the weight of object on the surface of the moon would be 1.67 N.
Internal Question and Answer :
1. What are the differences between the mass of an object and its weight?
Answer : The differences between the mass of an object and weight are :
|
Mass |
Weight |
|
Mass is the amount of matter present in an object. |
Weight is the force with which gravity pulls an object towards the Earth or another planet. |
|
SI unit of mass is kilogram (kg). |
SI unit of weight is newton (N). |
|
Mass remains constant everywhere. |
Weight changes from place to place depending on gravity. |
|
Mass is measured using a beam balance. |
Weight is measured using a spring balance. |
|
Mass is a scalar quantity. |
Weight is a vector quantity because it has both magnitude and direction. |
2. Why is the weight of an object on the moon 
its weight on the earth?
Answer: The moon’s gravitational force is much weaker than the Earth’s. Since weight depends on gravity, an object weighs less on the moon. The moon’s gravity is about of Earth’s gravity, so the object’s weight becomes of its weight on Earth.
Example 10.6: A block of wood is kept on a tabletop. The mass of wooden block is 5 kg and its dimensions are 40 cm × 20 cm × 10 cm. Find the pressure exerted by the wooden block on the table top if it is made to lie on the table top with its sides of dimensions (a) 20 cm × 10 cm and (b) 40 cm × 20 cm.
Solution: The mass of the wooden block (m) = 5 kg
The dimensions of the wooden block = 40 cm × 20 cm × 10 cm
If the weight of the wooden block applies a thrust on the table top, then
Thrust Thrust 
= 49 N
Area of a side = length × breadth 
Pressure 
When the block lies on its side of dimensions 40 cm × 20 cm, it exerts the same thrust.
Area of the side 
Pressure 
The pressure exerted by the side 20 cm × 10 cm is 2450
and by the side 40 cm × 20 cm is 612.5 
.
Internal Question and Answer :
1. Why is it difficult to hold a school bag having a strap made of a thin and strong string?
Answer: A thin strap exerts a small contact area, which increases pressure on the shoulder. This pressure causes more pain and discomfort, making the bag feel heavier and difficult to hold. A wider strap would spread the force over a larger area, reducing pressure.
2. What do you mean by buoyancy?
Answer: Buoyancy is the upward force exerted by a fluid (liquid or gas) on an object placed in it. It acts opposite to gravity, making objects feel lighter.
3. Why does an object float or sink when placed on the surface of water?
Answer: Whether an object floats or sinks depends on its density relative to water. If the object's density is less than water, it floats because the upward buoyant force is greater than its weight. If its density is more than water, it sinks because its weight exceeds the buoyant force.
Internal Question and Answer :
1. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
Answer: Your actual mass is slightly less than 42 kg. A weighing machine measures your weight due to gravity and converts it into mass assuming standard gravity. Since Earth’s gravity varies slightly from place to place, the displayed value is approximate.
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?
Answer: The iron bar is actually heavier. The bag of cotton occupies more space and experiences greater upthrust (buoyant force) from air. Because of this, the weighing machine shows a slightly lower value for cotton than its true weight.
Example 10.7 Relative density of silver is 10.8. The density of water is 
. What is the density of silver in SI unit?
Solution: Relative density of silver = 10.8
Relative density 
Density of silver = Relative density of silver × density of water 
Exercises of Chapter 10: Gravitation
1. How does the force of gravitation between two objects change when the distance between them is reduced to half ?
Solution: According to the universal law of gravitation, the force is inversely proportional to the square of the distance between the two objects. i.e., 
If the distance is reduced to half ( 
) , then 
Thus, the gravitational force becomes four 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?
Answer : A heavy object does not fall faster than a light object because the acceleration due to gravity (gg) is independent of the object's mass. While gravitational force is greater for a heavier object, its inertia (resistance to motion) is also greater by the same proportion. Hence, both objects experience the same acceleration and fall together in the absence of air resistance.
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 kg and radius of the earth is 
m.)
Answer: Here, Mass of the object 
kg
Mass of the earth 
kg
and radius of the earth 
m
Using Newton's law of universal gravitation, 
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?
Answer: The Earth attracts the Moon with the same force as the Moon attracts the Earth. This is explained by Newton's third law of motion, which states that every action has an equal and opposite reaction. The gravitational force is mutual and acts equally on both bodies, regardless of their different masses.
5. If the moon attracts the earth, why does the earth not move towards the moon ?
Answer: The Earth does move towards the Moon due to the gravitational force, but the movement is very small. According to Newton's third law, the Moon attracts the Earth with the same force as the Earth attracts the Moon. However, because the Earth has a much larger mass than the Moon, its acceleration ( 
) is very tiny. Hence, the Earth's motion toward the Moon is not noticeable.
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?
Answer: (i) If the mass of one object is doubled, the force between the objects will also double.
(ii) If the distance between the objects is doubled, the force between them becomes one-fourth (1/4) of its original value. If the distance is tripled, the force becomes one-ninth (1/9) of its original value.
(iii) If the masses of both objects are doubled, the force between them will quadruple (become four times stronger).
7. What is the importance of universal law of gravitation?
Answer: The importance of universal law of gravitation are :
i) The force that binds us to the earth;
(ii) The motion of the moon around the earth;
(iii) The motion of planets around the Sun; and
(iv) The tides due to the moon and the Sun.
8. What is the acceleration of free fall?
Answer: The acceleration of free fall is the acceleration experienced by an object when it falls freely under the influence of Earth's gravity alone. It is denoted by 'g' and its value is 9.8 m/s². This acceleration is constant near the Earth's surface and acts toward the center of the Earth.
The acceleration of free fall is the rate at which an object speeds up when it falls towards the ground due to gravity.
It is approximately 9.8 meters per second squared, which means that every second an object falls, its speed increases by 9.8 meters per second.
9. What do we call the gravitational force between the earth and an object?
Answer: The gravitational force between the Earth and an object is called weight. More precisely, weight is the force with which the Earth attracts an object towards its center. It is given by the formula W=m×gW=m×g, where mm is the mass of the object and gg is the acceleration due to gravity.
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? [Hint: The value of g is greater at the poles than at the equator.]
Answer: No, the friend will not agree with the weight of the gold. The weight of an object depends on the value of 'g'. Since gg is greater at the poles than at the equator, the same mass of gold will weigh less at the equator. Therefore, when Amit hands over the gold at the equator, its weight will be slightly less than what he bought at the poles. However, the mass remains the same.
11. Why will a sheet of paper fall slower than one that is crumpled into a ball?
Answer: A flat sheet of paper falls slower than a crumpled paper ball because it experiences greater air resistance due to its larger surface area. This upward air resistance opposes gravity more effectively, reducing its net downward acceleration. The crumpled ball has less surface area, so less air resistance and falls faster.
12. Gravitational force on the surface of the moon is only 
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?
Solution: The acceleration due to gravity on Earth = 9.8 m/s²,
The acceleration due to gravity on the moon 
1.63 m/s².
Weight on Earth: Weight = mass × acceleration due to gravity
Weight = 10 kg × 9.8 m/s² = 98 N
Weight on the Moon:
Weight = mass × acceleration due to gravity
Weight = 10 kg × 1.63 m/s² = 16.3 N
Therefore, the weight of a 10 kg object would be 98 N on Earth and 16.3 N on the moon
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.
Solution: Given, 
m/s , 
m/s , 
(i) Maximum Height: We have, 
Therefore, the maximum height to which the ball rises is 122.5 meters.
(ii) Total Time of Flight: We have, 
Therefore, the total time it takes for the ball to return to the surface of the Earth is 5 seconds.
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.
Solution: Given, 
m/s , 
,
We have,
m/s
Therefore, the final velocity of the stone just before touching the ground is 19.6 m/s.
15. A stone is thrown vertically upward with an initial velocity of 40 m/s. Taking g = 10 m/s² , find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone?
Solution: Given, 
m/s , 
m/s and
We have, 
Therefore, the maximum height reached by the stone is 80 meters.
The stone returns to its starting point .So, net displacement = 0 m .
The stone goes up to the maximum height and then returns back to the starting point.
Total distance covered 
meters.
16. Calculate the force of gravitation between the earth and the Sun, given that the mass of the earth = kg and of the Sun = 
kg. The average distance between the two is 
m.
Solution: Given, the mass of the earth 
kg
The mass of the Sun 
kg.
The average distance between the earth and Sun 
m.
The gravitational constant 
Using Newton's law of universal gravitation, 
Therefore, the force of gravitation between the Earth and the Sun is 
N .
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.
Solution: Given, 
For the stone falling from the top of the tower : Here, 
m/s
We have, 
For the stone projected vertically upwards from the ground: Here, 
m/s ,
,
We have, 
Adding (i) and (ii) , we get 
From (i), We get 
Therefore, the two stones will meet after 4 seconds, and they will meet at a height of 80 meters above the ground .
18. A ball 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.
Solution: (a) Given, m/s , 
, 
, 
s
We have,
m/s
Therefore, the ball was thrown up with an initial velocity of 29.4 m/s.
(b) We have, v 
, 
m/s , 
m/s²
Therefore, the maximum height reached by the ball is 44.1 meters.
(c) Here, 
m/s , 
seconds , 
m/s²
We have, 
Therefore, after 4 seconds, the ball is 39.2 meters below its starting point.
(a) The velocity with which the ball was thrown up is 29.8 m/s.
(b) The maximum height reached by the ball is 44.1 meters.
(c) After 4 seconds, the ball is 39.2 meters below its starting point.
19. In what direction does the buoyant force on an object immersed in a liquid act?
Answer: The buoyant force on an object immersed in a liquid acts in the vertically upward direction. It always acts opposite to the force of gravity (weight), which pulls the object downward. This upward force is exerted by the liquid on the object, making it appear lighter.
20. Why does a block of plastic released under water come up to the surface of water?
Answer: A block of plastic released under water comes up to the surface because the upward buoyant force exerted by water on the block is greater than the downward weight of the block. This happens because plastic has a lower density than water. The net upward force pushes the block to the surface, where it floats.
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?
Solution: Given, the mass of the substance = 50 g
The volume of the substance = 20 cm³
Density 
Density = 2.5 g/cm³
Since the substance's density is greater than that of water, it will sink when placed in water.
22. The volume of a 500 g sealed packet is 350 
. Will the packet float or sink in water if the density of water is 1 
? What will be the mass of the water displaced by this packet?
Solution: Given, the mass of the packet = 500 g
The volume of the packet = 350 cm³
We know that ,
Density 
Density = 1.43 g/cm³
Since the packet's density is greater than that of water, it will sink when placed in water.
Therefore, the mass of the water displaced by the packet is 500 g.
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