In Class 9 Science Ch 10 Gravitation, you will learn about the force that pulls everything towards the Earth – gravity. This chapter explains Newton’s Universal Law of Gravitation, which tells us how all objects in the universe attract each other. You will also learn the difference between mass and weight, and how gravity affects falling objects.
The chapter covers acceleration due to gravity, buoyancy, thrust, and pressure in liquids and gases. Concepts like Archimedes’ Principle and density are also explained with real-life examples. This chapter helps students understand how objects float, why things fall, and how gravity works in our daily life. It builds a strong base for future topics in Physics.
Class 9 Science Ch 10 Gravitation – Textbook Solutions
Q1. State the universal law of gravitation.
Ans. Every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them. The force is along the line joining the centres of two objects
Q2. 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 formula to find the magnitude of the gravitational force between the Earth and an object on its surface is:
F = G × (M × m) / R²
Where:
- F = gravitational force
- G = universal gravitational constant (6.674 × 10⁻¹¹ N·m²/kg²)
- M = mass of the Earth
- m = mass of the object
- R = radius of the Earth
This formula is derived from Newton’s Law of Universal Gravitation.
Q1. What do you mean by free fall?
Ans. Free fall is the motion of an object under the influence of gravitational force only, without any air resistance. In free fall, the object accelerates towards the Earth with a constant acceleration called acceleration due to gravity (g), which is approximately 9.8 m/s².
In simple words, when an object falls freely towards the Earth, only gravity is acting on it.
Q2. What do you mean by acceleration due to gravity?
Ans. Acceleration due to gravity is the acceleration produced in a body due to the gravitational force of the Earth. It is denoted by g.
Its value near the surface of the Earth is approximately 9.8 m/s², which means that the velocity of a freely falling object increases by 9.8 meters per second every second.
In simple words, it is the rate at which objects speed up when they fall freely towards the Earth.
NCERT TEXTBOOK PAGE 138
Q1. What are the differences between the mass of an object and its weight?
Ans.
| Mass | Weight |
| 1. Mass of a body is the measure of its inertia. | 1. Weight of the body is the force with which it is attracted towards the earth (W = m x g). |
| 2. Its S.I. unit is kg. | 2. Its S.I unit is Newton. |
| 3. It remains constant everywhere and it cannot be zero. | 3. Its value changes from place to place and it can be zero. |
| 4. It can be measured by beam-balance. | 4. It can be measured by spring balance. |
| 5. It has only magnitude i.e. it is a scalar quantity. | 5. It has both magnitude and direction i.e. |
Q2. Why is the weight of an object on the moon 1/6th its weight on the earth?
Ans.The weight of an object depends on the gravitational force acting on it. The moon has much weaker gravity than the Earth.
The acceleration due to gravity on the moon is about 1/6th that of the Earth. So, the gravitational force on the moon is also 1/6th.
Since weight = mass × gravity, the weight of an object on the moon is 1/6th of its weight on the Earth.
Therefore, the weight of an object on the moon is 1/6th because the moon’s gravity is 6 times weaker than the Earth’s more than the weight of a same body on moon.
QUESTIONS FROM NCERT TEXTBOOK
Q1. How does the force of gravitation between two objects change when the distance between them is reduced to half?
Ans: According to Newton’s law of gravitation, the gravitational force between two objects is inversely proportional to the square of the distance between them.
The formula is:
F = G × (m₁ × m₂) / r²
If the distance r is reduced to half, then the new distance becomes r/2.
Now, the new force:
F’ = G × (m₁ × m₂) / (r/2)²
= G × (m₁ × m₂) / (r²/4)
= 4 × [G × (m₁ × m₂) / r²]
= 4F
Therefore, when the distance between two objects is reduced to half, the gravitational force becomes 4 times stronger.
Q2. 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: Gravitational force is proportional to mass, so a heavy object experiences a greater force than a light object.
However, heavier objects also have more inertia (resistance to acceleration), which means they require more force to accelerate.
These two effects cancel each other out, so all objects fall with the same acceleration due to gravity (g), regardless of their mass.
Therefore, a heavy object does not fall faster than a light object in free fall. Both fall at the same rate when there is no air resistance.
Q3. What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface?
[Mass of the earth is 6 x 1024 kg and radius of the earth is 6.4 x 106 m].
Ans. The magnitude of the gravitational force between earth and an object is given by the formula.
Q4. 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 value of F is same for earth and the moon. Both bodies will exert the same amount of force on each other.
As per universal law of gravitation, every body attracts the other body with some force and this force is same for both the bodies called gravitational force.
Q5. If the moon attracts the earth, why does the earth not move towards; the moon?
Ans. Actually, the Earth does move towards the Moon. Both the Earth and the Moon exert gravitational forces on each other, so they both attract each other.
However, because the Earth is much more massive than the Moon, its acceleration towards the Moon is much smaller (Newton’s second law: a=F/m
As a result, the Earth moves very slightly, while the Moon moves more noticeably around the Earth. This mutual attraction causes the Moon to orbit the Earth, and the Earth and Moon both orbit their common center of mass.
Q6. 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. (i) If the mass of one object is doubled, the force between two objects will be doubled (increases)
(ii) If the distance been the objects is doubled the force between two objects will be one-fourth and if the distance will be tripled, the force will be one-ninth (1/9).
(iii) If the masses of both objects are doubled the force will be 4 times.
Q7. What is the importance of universal law of gravitation?
Ans. The universal law of gravitation explains several phenomena:
(i) it explains about the force that binds 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.
Q8. What is the acceleration-of free fall?
Ans. The acceleration of free fall is; when the Body falls due to earth’s gravitational pull, its velocity changes and is said to be accelerated due to .the earth’s gravity and it falls freely called as free fall. This acceleration is calculated to be 9.8 m/s2.
Q9. What do we call the gravitational force between the earth and an object?
Ans. The gravitational force between the earth and an object is called force due to gravity.
Q10. Amit buys few grams of gold at the poles 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?
[Hints: The value of g is greater at the poles than at the equator.]
Ans. Weight of the body is given by the formula
W = mg
It depends ors the value of ‘g’ i.e., acceleration due to gravity.
The weight of gold at poles = Wp = m × g (poles)
Value of g at poles is more than the value of g at equator.
The weight of gold at equator = We = m × g (equator)
∴ Wp > We.
The weight at pole of the same gold is found to be more as compared to the weight at the equator.
Q11. Why will a sheet of paper fall slower than one that is crumpled into a ball?
Ans. A sheet of paper has larger surface area and while falling down it has to overcome the force exerted by air/wind. current, called as air resistance.
The crumpled paper has smaller surface area and it has to overcome very less amount of air current.
Q12. Gravitational force on the surface of the moon is only 1/6 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. Mass of the object = 10 kg
Weight of the object on earth = W = m × g
∴ W = 10 × 9.8
W = 98 N
Weight of the object on moon = 
th the weight on the earth.
As the gravitational force on the surface of the moon is only th as strong as gravitational force on the surface of the earth.
∴ Weight of the object on moon 
Weight on earth = 98 N
Weight on moon = 16.3 N
Q13. 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. (i) Initial velocity = 49 m/s
Final velocity = 0 m/s
a = g = –9.8 m/s2
Height = Distance = s = ?
∴ v2 – u2 = 2gs
02 – (49)2 = 2 (–9.8) × s
(ii) Time take t = ?
v = u + gt
∴ 0 = 49 + (–9.8) × t
Total time taken to return the surface of the earth by the ball is 5 s + 5 s = 10 s.
Q14. 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. Data u = 0 m/s
v = ?
h = s = 19.6 m
g = 9.8 m/s2 (falling down)
v2 – u2 = 2gs
v2 – (0)2 = 2 x 9.8 x 19.6
v = 19.6 m/s
The final velocity just before touching the ground is 19.6 m/s.
Q15. 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?
Ans. u = 40 m/s
g = –10 m/s2 (going against gravity)
h = s = ?
v = 0
v2 – u2 = 2gs
(0)2 – (40)2 = 2 (–10) x s
Net displacement of the stone = 0 (As the stone falls, back to the same point.)
Total distance covered by stone = 80 m + 80 m
(up) (down)
= 160 m
Q16. 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 1011m.
Ans. Me = 6 x 1024 kg G = 6.67 x 10–11 Nm2/kg2
Ms = 2 x 1030 kg
d = 1.5 x 1011 m
Q17. A stone is allowed to fall from the top of a tower 100 m high and at the same time another stone is projected vetically upwards from the ground with a velocity of 25 m/s. Calculate when and where the two stones will meet.
Ans. h = 100 m
time t = ? g = 10 m/s2
Height covered by the falling stone = s1
The distance covered by the stone thrown upward = s2
g = –10 m/s
u = 25 m/s
Total height given = 100 m
∴ s1 + s2 = 100m
5t2 + (25t – 5t2) = 100 m
∴ 25t = 100 m
Putting the value of (3) in equation (1), we get
∴ s1 = 5t2
= 5 × (4)2 = 80 m
∴ The two stones will meet after 4 seconds when the falling stone has covered a distance of 80 m.
Q18. 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.
Ans. u = ?
v = 0
g = –9.8 m/s2 (thrown upward)
Total time = 6 s (to go up and down)
∴ Time for upward journey 
(a) v = u + gt
0 = u = (–9.8) × 3
u = 29.4 m/s
(b) Maximum height h = s = ?
(c) Position after 4 s
t = 4s
∴ Position after 4 s = 39.2 m from the top.
Book Inside Questions
Q1. What do you mean by free fall?
Answer:
Free fall refers to the motion of an object under the influence of gravity only, without any other force acting on it (like air resistance). In free fall, an object accelerates towards the Earth at a constant rate of approximately 9.8 m/s2 known as the acceleration due to gravity.
Q2. If an object is thrown upwards, does its speed decrease or increase? Why?
Answer:
When an object is thrown upwards, its speed decreases. This happens because the force of gravity pulls the object downward, opposing its upward motion. As a result, the object’s speed decreases until it reaches its highest point, where the speed becomes zero before it starts coming down.
Q3. What is the acceleration of an object in free fall?
Answer:
The acceleration of an object in free fall near the surface of the Earth is approximately 9.8 m/s2. This value is called the acceleration due to gravity (g).
Exercise 10.2
Q1. What is the value of gravitational force acting on a 1 kg mass?
Answer:
The gravitational force (also known as weight) acting on an object is given by the formula: F=m×gF = m \times gF=m×g
Where:
- m=1 kg
- g=9.8 m/s2
So, the force is: F=9.8 N
Thus, the gravitational force acting on a 1 kg mass is 9.8 N.
Q2. What is the acceleration due to gravity on the surface of the Earth?
Answer:
The acceleration due to gravity on the surface of the Earth is approximately 9.8m/s2. This value can vary slightly depending on altitude and location, but 9.8 m/s2 is the standard value used.
Exercise 10.3
Q2. What happens to the gravitational force if the mass of one object is doubled?
Answer:
According to Newton’s Law of Universal Gravitation: F=G⋅m1⋅m2/sq d
If the mass of one object is doubled, the gravitational force between the two objects also doubles. This is because gravitational force is directly proportional to the product of the masses. So, doubling one mass doubles the force.
Exercise 10.4
Q1. Why do action and reaction forces not cancel each other when a rocket is launched?
Answer:
Action and reaction forces do not cancel each other during a rocket launch because they act on different objects. The action force is the expulsion of gas from the rocket’s engines, while the reaction force is the thrust pushing the rocket forward. Since these forces act on different objects (the exhaust gases and the rocket), they do not cancel each other out.
Q2. Why is the value of gravitational force greater on the surface of the Earth than on the surface of the Moon?
Answer:
The value of gravitational force depends on both the mass of the celestial body and the distance from its center. The Earth has a much larger mass than the Moon, so its gravitational force is greater. Additionally, the radius of the Earth is larger than that of the Moon, but since the Earth’s mass is much greater, the gravitational force on the Earth’s surface is still stronger than that on the Moon’s surface.
Exercise 10.6
Q1. What is the value of g at the top of Mount Everest and at the bottom of a mine? What are the reasons for this difference?
Answer:
The value of g (acceleration due to gravity) decreases with height and increases with depth.
- At the top of Mount Everest: The value of g is slightly less than the value at sea level because the distance from the Earth’s center increases as you go higher. As height increases, the gravitational force decreases.
- At the bottom of a mine: The value of ggg increases because you are closer to the Earth’s center, and gravitational force increases as you go deeper into the Earth.
The reason for this difference is that gravitational force depends on both the distance from the Earth’s center and the distribution of mass inside the Earth. At higher altitudes, the distance increases, and at greater depths, the distance decreases.
Class 9 Science Ch 10 Gravitation – 1-Mark Questions (Very Short Answer)
- What is the SI unit of gravitational force?
Ans: Newton (N) - Who discovered the law of gravitation?
Ans: Isaac Newton - What is the value of acceleration due to gravity on Earth?
Ans: 9.8 m/s² - Define free fall.
Ans: Motion of an object under gravity alone. - Name the force responsible for the revolution of planets around the sun.
Ans: Gravitational force - State the value of G.
Ans: 6.674×10−11 Nm2/kg2 - What is weightlessness?
Ans: A state when a body experiences no net gravitational force. - Do all objects fall at the same rate in vacuum?
Ans: Yes - What is the direction of gravitational force?
Ans: Towards the center of the Earth
Class 9 Science Ch 10 Gravitation – 2-Mark Questions (Short Answer)
- Differentiate between mass and weight.
Ans:
- Mass is the amount of matter; weight is the force due to gravity.
- Mass is constant; weight changes with gravity.
- Why does a feather fall slower than a stone in air?
Ans: Due to air resistance; the feather experiences more air drag. - State two applications of the Universal Law of Gravitation.
Ans:
- It explains the motion of planets.
- It explains the tides in oceans.
- Why is G called a universal constant?
Ans: Because it remains the same throughout the universe. - What happens to the weight of a body at the center of the earth?
Ans: It becomes zero. - Define acceleration due to gravity. Mention its SI unit.
Ans:
Acceleration produced in a body due to Earth’s gravity. SI unit: m/s² - What is the relation between mass and weight?
Ans:
W=mgW = mgW=mg, where W = weight, m = mass, g = acceleration due to gravity - State two differences between thrust and pressure.
Ans:
- Thrust is total force; pressure is force per unit area.
- Thrust has Newton as unit; pressure has Pascal.
- Why are objects in space said to be in free fall?
Ans: Because they move only under gravity with no air resistance. - Why do objects appear weightless in an orbiting spacecraft?
Ans: Because both the object and spacecraft are in free fall around the Earth.
3-Mark Questions (Detailed Answer)
- State and explain the Universal Law of Gravitation with formula.
Ans:
The law states that every object attracts every other object with a force proportional to the product of their masses and inversely proportional to the square of the distance. - Explain why the moon revolves around the Earth and not falls onto it.
Ans:
Due to the tangential velocity of the moon, it continuously falls towards the Earth but keeps missing it, thus revolving around it due to gravitational force. - Describe an experiment to show that objects fall at the same rate in vacuum.
Ans:
In a vacuum chamber, drop a feather and a coin. Both fall at the same rate as there’s no air resistance. - What are the factors affecting the gravitational force between two bodies?
Ans:
- Mass of both bodies (directly proportional)
- Distance between them (inversely proportional to square of distance)
An object is thrown upwards with a speed of 20 m/s. How high will it go? (g = 10 m/s²)
Ans:
Where:
v = final velocity at the highest point = 0 m/s
u = initial velocity = 20 m/s
g = acceleration due to gravity = 10 m/s²
h = maximum height
Substitute the values:
0 = 20² – 2 × 10 × h
0 = 400 – 20h
20h = 400
h = 400 / 20
h = 20 meters
Answer: The object will go up to a height of 20 meters.
Class 9 Science Ch 10 Gravitation – 1 Mark Questions (Very Short Answer)
- What is the SI unit of pressure?
Ans: Pascal (Pa) - Define thrust.
Ans: Force acting perpendicular to a surface. - What is density?
Ans: Mass per unit volume of a substance. - Does gravity act in a vacuum?
Ans: Yes - Name the quantity: force per unit area.
Ans: Pressure
Class 9 Science Ch 10 Gravitation – 2-Mark Questions (Short Answer)
- Why is the value of ‘g’ less on the moon than on Earth?
Ans: Because the moon’s mass and radius are much smaller than Earth’s, resulting in less gravitational pull. - What is buoyancy?
Ans: The upward force exerted by a fluid on a body immersed in it. - State Archimedes’ Principle.
Ans: When a body is immersed in a fluid, it experiences an upward force equal to the weight of the fluid displaced. - Why do objects float or sink in water?
Ans: Based on whether their density is less than or greater than that of water. - How does atmospheric pressure affect our body?
Ans: It is balanced by the pressure of fluids inside our body, so we don’t feel it. - What happens when an object is fully immersed in water?
Ans: It displaces a volume of water equal to its own volume and experiences an upward buoyant force. - Give two uses of Archimedes’ principle.
Ans:
- Designing ships and submarines
- Determining purity of metals
- Why do we weigh less in water?
Ans: Because the buoyant force reduces the net downward force (apparent weight). - Why do iron nails sink in water but ships float?
Ans: Nails are dense and compact; ships have a large volume and displace more water (greater buoyant force). - What is the relation between pressure, force, and area?
Ans:
P=F/A
Class 9 Science Ch 10 Gravitation – 3-Mark Questions (Detailed Answer)
- Explain why a balloon filled with helium rises in air.
Ans:
Helium is lighter than air. The buoyant force acting on the balloon is greater than its weight, so it rises. - Describe the factors affecting the pressure exerted by a liquid.
Ans:
- Depth of the liquid (pressure increases with depth)
- Density of the liquid
- Acceleration due to gravity (g)
- Why are dams constructed with a wider base?
Ans:
Water pressure increases with depth, so a wider base provides strength to withstand more pressure. - Derive the formula for the force of gravity (weight).
Ans:
Using Newton’s second law:
F=ma
When only gravity acts,
F==mg → Weight = mass × gravity - Why do astronauts feel weightless in space?
Ans:
They are in a state of continuous free fall around Earth, so they don’t experience any normal force, hence feel weightless. - What will happen to an object’s weight if taken to the moon? Why?
Ans:
Weight will decrease to 1/6th because the moon’s gravity is 1/6th that of Earth. - Give three differences between mass and weight.
Ans:
- Mass is constant; weight changes
- Mass in kg; weight in Newton
- Mass is scalar; weight is vector
- State and explain how density is related to floating.
Ans:
If an object’s density is less than that of the fluid, it floats; if more, it sinks. - An object displaces 500 mL of water and weighs 4 N in air, 3 N in water. What is the buoyant force?
Ans:
Buoyant force = Weight in air – Weight in water = 4 N – 3 N = 1 N
You can access the official NCERT Solutions for Class 10 Mathematics on the NCERT website at the following link:
NCERT Class 10 Mathematics Solutions
This page will guide you to the textbook and solutions, as provided by the National Council of Educational Research and Training (NCERT).