📅 Published on: 01.12.2025
Gravitation is a universal force that attracts every object in the universe toward every other object. This chapter explains how Newton’s universal law of gravitation governs the motion of planets, falling objects, tides, and many natural phenomena around us. It introduces free fall, acceleration due to gravity (g), mass, and weight, showing how weight changes with gravity while mass remains constant. The concept of gravitational force helps us understand why objects fall to the ground, why satellites stay in orbit, and why the moon attracts Earth’s water to create tides. Overall, gravitation connects everyday experiences with scientific principles that rule the universe.
Gravitation class 9 Questions and answers
Gravitation – Very Short Answer Questions with AnswersWhat is the value of G (i) on earth (ii) on moon?
Ans. G = 6.67 × 10-11 N m2 kg-2.
The value is the same on earth and on moon.
Which force revolves moon around earth?
Ans. Gravitational force of earth on moon.
The distance between two objects is doubled. What happens to gravitational force between them?
Ans. The gravitational force becomes one-fourth (1/4) of the original value, because F ∝ 1/r2.
Which force accelerates a body in free fall?
Ans. Gravitational force of earth.
Which force is responsible for tides in the ocean at night?
Ans. Gravitational pull of the moon on sea water.
Does gravitational force between two objects depend on the medium between them?
Ans. No.
Which force is responsible for holding the solar system together?
Ans. Gravitational pull of the sun on the planets.
Who gave three laws of planetary motion?
Ans. Kepler.
Who explained the motion of planets around the sun?
Ans. Newton.
What is the real shape of orbits of planets around the sun?
Ans. The orbits are elliptical.
Does Newton’s third law of motion apply to gravitational force?
Ans. Yes, it does.
A stone falling towards the earth also attracts the earth with the same force. Is it true?
Ans. Yes, it is true.
Then why is the earth not seen moving towards the stone?
Ans. Because the mass of the earth is extremely large, so its acceleration a = F/m is almost zero (negligible).
What is value of g on earth?
Ans. g = 9.8 m/s2.
Does value of g vary from place to place on earth?
Ans. Yes, because the earth is not a perfect sphere.
Where is g greater, at poles or at equator?
Ans. g is maximum at poles and minimum at equator.
Which force holds the atmosphere around earth?
Ans. Gravitational force of earth on the atmosphere.
Is force of gravitation ever repulsive?
Ans. No, never. It is always attractive.
What is the value of universal gravitational constant?
Ans. G = 6.67 × 10-11 N m2 kg-2.
(a) Value of gravitational constant depends neither on mass of two objects nor on distance between them. Is it true?
(b) Magnitude of gravitational force does not depend upon medium separating the two objects. Do you agree?
Ans. (a) Yes, it is true.
(b) Yes, we agree.
(a) The gravitational force between two objects depends only on their masses and distance of separation. The objects may be at rest or rotating. Is this statement correct?
(b) Force exerted by earth on an apple is same as force exerted by the apple on earth. Is it true?
Ans. (a) Yes, it is correct.
(b) Yes, it is true.
(a) What are the SI units of G?
(b) On what factors does the value of G depend?
Ans. (a) N m2 kg-2.
(b) G is an absolute constant; it does not depend on any factor.
The areal velocity of a planet around the sun is constant. Which law is this?
Ans. This is Kepler’s second law of planetary motion.
How does the period of revolution of a planet around the sun vary with its distance from the sun?
Ans. The square of the time period T2 is directly proportional to the cube of its distance r3 from the sun, i.e., T2 ∝ r3.
What is the value of g at the centre of earth?
Ans. g = 0 at the centre of the earth.
A body is projected upwards. What is its velocity at maximum height?
Ans. At maximum height, velocity = 0.
A body is just dropped from a height. What is its initial velocity?
Ans. Initial velocity u = 0.
What is the SI unit of mass?
Ans. Kilogram (kg).
What is the SI unit of weight?
Ans. Newton (N).
What is the weight of a body of mass 1 kg on earth?
Ans. W = m g = 1 × 9.8 = 9.8 N.
What is meant by gravity?
Ans. Gravity is the attraction between the earth and any other body.
Does the value of g depend upon mass of the body?
Ans. No, it does not.
All bodies lose their weight at the centre of earth. Why?
Ans. At the centre of earth, g = 0, so weight W = m g = 0.
Can mass of a body ever be zero?
Ans. No, mass of a body cannot be zero.
(a) Where is value of g maximum; at poles or at equator?
(b) Where is value of g minimum; at poles or at equator?
Ans. (a) g is maximum at poles.
(b) g is minimum at equator.
(a) Which balance is used for measuring mass of a body?
(b) Which balance is used for measuring weight of a body?
Ans. (a) Physical balance.
(b) Spring balance.
(a) A person weighs 70 kg on earth, where g = 10 m/s2. What is his mass?
(b) The mass of a child is 20 kg. If g = 9.8 m/s2, what is his weight?
Ans. (a) Mass = 70 kg.
(b) W = m g = 20 × 9.8 = 196 N.
What is the value of G (i) on earth (ii) on moon?
Ans. G = 6.67 × 10-11 N m2 kg-2. The value is the same on earth and on moon.
Which force revolves moon around earth?
Ans. Gravitational force of earth on moon.
The distance between two objects is doubled. What happens to gravitational force between them?
Ans. The gravitational force becomes one-fourth (1/4) of the original value, because F ∝ 1/r2.
Which force accelerates a body in free fall?
Ans. Gravitational force of earth.
Which force is responsible for tides in the ocean at night?
Ans. Gravitational pull of the moon on sea water.
Does gravitational force between two objects depend on the medium between them?
Ans. No.
Which force is responsible for holding the solar system together?
Ans. Gravitational pull of the sun on the planets.
Who gave three laws of planetary motion?
Ans. Kepler.
Who explained the motion of planets around the sun?
Ans. Newton.
What is the real shape of orbits of planets around the sun?
Ans. The orbits are elliptical.
Does Newton’s third law of motion apply to gravitational force?
Ans. Yes, it does.
A stone falling towards the earth also attracts the earth with the same force. Is it true?
Ans. Yes, it is true.
Then why is the earth not seen moving towards the stone?
Ans. Because the mass of the earth is extremely large, so its acceleration a = F/m is almost zero (negligible).
What is value of g on earth?
Ans. g = 9.8 m/s2.
Does value of g vary from place to place on earth?
Ans. Yes, because the earth is not a perfect sphere.
Where is g greater, at poles or at equator?
Ans. g is maximum at poles and minimum at equator.
Which force holds the atmosphere around earth?
Ans. Gravitational force of earth on the atmosphere.
Is force of gravitation ever repulsive?
Ans. No, never. It is always attractive.
What is the value of universal gravitational constant?
Ans. G = 6.67 × 10-11 N m2 kg-2.
(a) Value of gravitational constant depends neither on mass of two objects nor on distance between them. Is it true?
(b) Magnitude of gravitational force does not depend upon medium separating the two objects. Do you agree?
Ans. (a) Yes, it is true.
(b) Yes, we agree.
(a) The gravitational force between two objects depends only on their masses and distance of separation. The objects may be at rest or rotating. Is this statement correct?
(b) Force exerted by earth on an apple is same as force exerted by the apple on earth. Is it true?
Ans. (a) Yes, it is correct.
(b) Yes, it is true.
(a) What are the SI units of G?
(b) On what factors does the value of G depend?
Ans. (a) N m2 kg-2.
(b) G is an absolute constant; it does not depend on any factor.
The areal velocity of a planet around the sun is constant. Which law is this?
Ans. This is Kepler’s second law of planetary motion.
How does the period of revolution of a planet around the sun vary with its distance from the sun?
Ans. The square of the time period T2 is directly proportional to the cube of its distance r3 from the sun, i.e., T2 ∝ r3.
What is the value of g at the centre of earth?
Ans. g = 0 at the centre of the earth.
A body is projected upwards. What is its velocity at maximum height?
Ans. At maximum height, velocity = 0.
A body is just dropped from a height. What is its initial velocity?
Ans. Initial velocity u = 0.
What is the SI unit of mass?
Ans. Kilogram (kg).
What is the SI unit of weight?
Ans. Newton (N).
What is the weight of a body of mass 1 kg on earth?
Ans. W = m g = 1 × 9.8 = 9.8 N.
What is meant by gravity?
Ans. Gravity is the attraction between the earth and any other body.
Does the value of g depend upon mass of the body?
Ans. No, it does not.
All bodies lose their weight at the centre of earth. Why?
Ans. At the centre of earth, g = 0, so weight W = m g = 0.
Can mass of a body ever be zero?
Ans. No, mass of a body cannot be zero.
(a) Where is value of g maximum; at poles or at equator?
(b) Where is value of g minimum; at poles or at equator?
Ans. (a) g is maximum at poles.
(b) g is minimum at equator.
(a) Which balance is used for measuring mass of a body?
(b) Which balance is used for measuring weight of a body?
Ans. (a) Physical balance.
(b) Spring balance.
(a) A person weighs 70 kg on earth, where g = 10 m/s2. What is his mass?
(b) The mass of a child is 20 kg. If g = 9.8 m/s2, what is his weight?
Ans. (a) Mass = 70 kg.
(b) W = m g = 20 × 9.8 = 196 N.
SHORT ANSWER QUESTIONS (Carrying 2 Marks Each)1. (a) A person weighs 60 kg on earth. Will his weight increase as he moves to the top of a hill?
Answer: No, his weight will decrease on top of a hill. This is because acceleration due to gravity at the top of a hill is less than the acceleration due to gravity on the surface of the Earth.
1. (b) Will the weight of a person increase at the bottom of a mine?
Answer: No, weight of the person at the bottom of a mine will also decrease. This is because the value of acceleration due to gravity decreases inside the Earth and is less than its value on the surface.
2. What is inverse square rule?
Answer: The gravitational force of attraction (F) between any two objects is inversely proportional to the square of the distance (r) between them, that is:
F ∝ 1/r²
3. Name two applications of Newton’s law of gravitation.
Answer:
- Rotation of planets around the Sun.
- Revolution of the Moon and artificial satellites around the Earth.
4. Which of Kepler’s laws led Newton to inverse square rule for gravitational force?
Answer: Kepler’s third law of planetary motion (r³ ∝ T²) led Newton to establish the inverse square rule for gravitational force.
5. Can a body have mass but no weight?
Answer: Yes, a body has mass everywhere, but its weight can be zero. Since weight W = mg, the weight becomes zero when g = 0, for example at the centre of the Earth.
6. Can a body have weight, but no mass?
Answer: No, weight = mg. If mass (m) = 0, W = 0. When mass is zero, weight is also zero.
7. On what factors does the weight of a body depend?
Answer: Weight of a body depends on mass of the body and acceleration due to gravity.
8. How are gravitation and gravity related?
Answer: Force of gravitation exerted by Earth on any other body is called gravity of Earth.
9. What is the relation between g and G?
Answer: g = GM / R², where M is mass of Earth, R is radius of Earth, and g is acceleration due to gravity.
10. Can you calculate mass of Earth from Newton’s law of gravitation?
Answer: Yes. From g = GM / R², M = gR² / G. Using known values of g, R and G, mass of Earth can be calculated.
11. Will gravitational force between two bodies change when they are placed at same distance (i) on earth (ii) on moon?
Answer: No, it will not change.
12. How will gravitational force change between two bodies when distance between them is halved?
Answer: Since F ∝ 1/d², when distance is halved, gravitational force becomes 4 times.
13. Mass of each of two bodies are doubled. How should the distance be changed to keep gravitational force constant?
Answer: Numerator becomes 4 times, so distance must be doubled (d becomes 2d) to keep F constant.
14. Mass of one body is increased 4 times and the other 16 times. How should distance be changed to keep force constant?
Answer: F = G × (4m₁)(16m₂) / d² = 64m₁m₂ / d². To keep F constant, distance must be increased 8 times (d becomes 8d).
15. If the moon attracts the earth, why does the earth not move towards the moon?
Answer: Earth is extremely massive. The same force produces noticeable motion in the moon but negligible motion in Earth. The moon moves around Earth due to this force.
16. Calculate the gravitational force on a 1 kg body lying on Earth. Given: Mass of Earth = 6 × 10²⁴ kg, Radius = 6400 km.
Answer: m₁ = 1 kg, m₂ = 6 × 10²⁴ kg, d = 6400 km = 6.4 × 10⁶ m F = Gm₁m₂ / d² = 9.8 N.
17. Why is the speed of a planet greater when it is closer to the Sun?
Answer: According to Kepler’s 3rd law (T² ∝ R³), larger R means larger T (time). Hence speed decreases with distance and increases when closer to the Sun.
class 9 science chapter 10 question answer Pdf download
18. Can you calculate density of Earth from Newton’s law of gravitation?
Answer: Yes. Assuming Earth to be a perfect sphere of radius R and uniform density ρ,
Mass of Earth = (4/3)πR³ρ
Using g = GM/R²,
g = (G/R²) × (4πR³ρ / 3)
g = (4πGRρ) / 3
Therefore, ρ = 3g / (4πGR).
Thus, density of Earth can be calculated.
19. Where is ‘g’ maximum—on the surface of earth, above the surface or below the surface?
Answer: ‘g’ is maximum on the surface of Earth. It decreases when we go above the surface and also decreases when we go below the surface.
20. What is the ratio of weight of an object on moon to its weight on earth?
Answer: Weight on moon = 1/6 of weight on earth. Thus, ratio = 1/6.
21. Why don’t two objects move towards each other due to gravitational pull between them?
Answer: Because gravitational force between two ordinary objects is very weak compared to friction and other opposing forces. Hence they do not move noticeably.
22. Though gravitational force is the weakest force in nature, yet it is responsible for holding our solar system. Why?
Answer: Masses of celestial bodies are extremely large. Even though gravity is weak, large masses produce strong gravitational attraction, which holds the solar system together.
23. 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: Acceleration due to gravity (a = F/m) is constant for all bodies. Hence, heavy and light objects fall with the same acceleration.
24. A body weighs 10 kg on the surface of earth. What would be its mass and weight at the centre of earth?
Answer: Mass remains 10 kg. At the centre of Earth, g = 0, so weight W = mg = 10 × 0 = 0 N.
SHORT ANSWER QUESTIONS (Carrying 3 Marks Each)
1. Calculate the force of attraction between the earth and the sun. Given: Mass of earth = 6 × 10²⁴ kg, Mass of sun = 2 × 10³⁰ kg, Distance = 1.5 × 10¹¹ m.
Answer:
F = GMm / d²
= (6.67 × 10⁻¹¹) × (6 × 10²⁴) × (2 × 10³⁰) / (1.5 × 10¹¹)²
= 3.58 × 10²² N.
2. A sphere of mass 25 kg attracts another sphere of mass 24 kg with a force of 0.1 milligram weight. If the distance between their centres is 20 cm, find the value of G.
Answer:
m₁ = 25 kg, m₂ = 24 kg
Force = 0.1 mg = 9.8 × 10⁻⁷ N
Distance = 20 cm = 0.2 m
Using G = Fd² / (m₁m₂),
G = (9.8 × 10⁻⁷ × 0.2²) / (25 × 24)
= 6.53 × 10⁻¹¹ N·m²/kg².
3. If distance between two masses is quadrupled, what will be the new force of attraction between them? Initial force = 9.8 N.
Answer:
F ∝ 1/d²
If distance becomes 4 times:
F₂/F₁ = (1/4)² = 1/16
F₂ = 9.8 / 16 = 0.61 N.
4. An electron of mass 9.1 × 10⁻³¹ kg is at a distance of 10⁻⁹ m from a proton of mass 1.67 × 10⁻²⁷ kg. Calculate gravitational force between them.
Answer:
F = G m₁ m₂ / d²
= (6.67 × 10⁻¹¹)(9.1 × 10⁻³¹)(1.67 × 10⁻²⁷) / (10⁻⁹)²
= 1.01 × 10⁻⁴⁹ N.
5. Distinguish between gravitation and gravity.
Answer: Gravitation is the universal force of attraction between any two masses. Gravity is the force of attraction exerted by Earth on objects. (As per textbook reference.)
6. State and explain universal law of gravitation. What is its importance?
Answer: Every object attracts every other object with a force directly proportional to the product of their masses and inversely proportional to the square of distance between them. Importance: It explains motion of planets, satellites, tides, etc.
7. State Kepler’s laws of planetary motion.
Answer:
1. Planets move in elliptical orbits with Sun at one focus.
2. Line joining planet and Sun sweeps equal areas in equal time.
3. T² ∝ R³ (square of time period is proportional to cube of mean distance).
8. Compare gravitational force between light and heavy objects.
Answer: Heavy objects experience greater gravitational force than light objects, but acceleration remains the same for both.
9. Write the equation of motion under gravity and explain symbols.
Answer:
v = u + gt
s = ut + ½gt²
v² = u² + 2gs
Where: u = initial velocity, v = final velocity, g = acceleration due to gravity, s = displacement, t = time.
10. A body weighs 10 kg on earth. What would be its mass and weight on moon when g = 1.6 m/s²?
Answer:
Mass = 10 kg (same everywhere).
Weight on moon = mg = 10 × 1.6 = 16 N.
11. (i) Mass of a body is always constant. Why?
Answer: Mass is the quantity of matter in a body; it cannot change.
11. (ii) Weight of a body can never be zero. Why?
Answer: Weight W = mg. It becomes zero only when g = 0, such as at the centre of Earth.
11. (iii) At the centre of earth, weight of a body is zero. Why?
Answer: Because gravitational force is zero at the centre; hence g = 0 and W = 0.
12. A ball thrown up vertically returns after 8 seconds. Calculate:
- (i) Velocity with which it was thrown
- (ii) Maximum height it acquired
- (iii) Velocity with which it hits the ground
Answer:
Time of ascent = 8/2 = 4 s
(i) u = gt = 9.8 × 4 = 39.2 m/s
(ii) h = ut – ½gt² = 156.8 – 78.4 = 78.4 m
(iii) Velocity on return = 39.2 m/s.
13. Distinguish between mass and weight.
| Mass | Weight |
|---|---|
| Amount of matter in a body. | Force with which Earth attracts a body. |
| Constant everywhere. | Changes with value of g. |
| Scalar quantity. | Vector quantity. |
| SI unit: kg | SI unit: newton (N) |
| Measured by beam balance. | Measured by spring balance. |
14. Explain what is meant by ‘free fall’ and acceleration due to gravity.
Free fall: When a body falls under the influence of Earth’s gravity alone, it is said to be in free fall.
Acceleration due to gravity (g): The acceleration produced in a freely falling object due to Earth’s gravity. Its value is 9.8 m/s².
15. A ball is thrown vertically upwards with a velocity of 98 m/s. Calculate (i) maximum height it rises (ii) total time it takes to return.
Given: u = 98 m/s, v = 0, g = -9.8 m/s²
(i) Maximum height:
v² – u² = 2gh
0 – (98)² = 2(-9.8)h
h = 490 m
(ii) Time to reach top:
v = u – gt → 0 = 98 – 9.8t → t = 10 s
Total time = 2 × 10 = 20 s
16. Suppose a planet has mass and diameter both twice of Earth. Calculate acceleration due to gravity on this planet (g = 9.8 m/s² on Earth).
Mass = 2Mₑ, Radius = 2Rₑ
g = GM/R²
gₚ = G(2Mₑ)/(2Rₑ)² = (2/4)g = 4.9 m/s²
17. (a) A person weighs 60 kg on Earth. Will his weight increase as he moves to the top of a hill?
17. (b) Will the weight of a person increase at the bottom of a mine?
18. Can you calculate mass of Earth from Newton’s law of gravitation?
Yes.
g = GM/R²
M = gR²/G
Using known values of g, R, and G, mass of Earth can be calculated.
19. Masses of each of two bodies are doubled. How should the distance between them be changed to keep the gravitational force constant?
F = G(2m₁)(2m₂) / d² → Numerator becomes 4 times.
To keep F constant, d² must become 4d² → d = 2d.
Therefore, distance must be doubled.
20. The mass of a body is increased 4 fold and the other 16 fold. How should the distance be changed to keep gravitational force constant?
F = G(4m₁)(16m₂) / d² → 64 times effect.
So d² must become 64d² → d = 8d.
Therefore, distance must be increased 8 times.
21. If the moon attracts the earth, why does the earth not move towards the moon?
Earth and moon attract each other with the same force, but Earth is extremely massive.
Thus acceleration produced in Earth is negligible, so Earth does not move noticeably towards the moon.
22. Calculate the gravitational force on a body of mass 1 kg lying on the surface of Earth.
m₁ = 1 kg, m₂ = 6 × 10²⁴ kg
d = 6400 km = 6.4 × 10⁶ m
F = G m₁ m₂ / d²
F = (6.67×10⁻¹¹ × 1 × 6×10²⁴) / (6.4×10⁶)²
F = 9.8 N
23. The speed of a planet is greater when it is closer to the sun. Why?
According to Kepler’s 3rd law (T² ∝ R³):
Closer planet → smaller R → smaller T → planet must move faster.
Hence speed is greater near the Sun.
24. Can you calculate density of Earth from Newton’s law of gravitation?
Mass of Earth, M = (4/3)πR³ρ
g = GM / R²
Substituting M:
g = (4πGRρ) / 3
Therefore,
ρ = 3g / (4πGR)
Using values of g, G, R, density of Earth can be calculated.
LONG ANSWER QUESTIONS (Carrying 5 Marks Each)
1. State and explain universal law of gravitation. What is the importance of this law?
Every object attracts every other object with a force:
F ∝ m₁m₂ and F ∝ 1/r²
F = G m₁m₂ / r²
Importance:
• Explains planetary motion
• Explains motion of satellites
• Helps calculate masses of planets
• Explains tides and falling bodies
• Explains formation of orbits
2. State Kepler’s laws of planetary motion. How did Newton guess inverse square law?
1. Planets move in elliptical orbits with Sun at one focus.
2. Line joining Sun and planet sweeps equal areas in equal times.
3. T² ∝ R³ (Time period rule).
Newton guessed inverse square law:
From T² ∝ R³, Newton deduced that gravitational force must vary as 1/R².
3. Define G and g. Establish relation between them.
G = 6.67 × 10⁻¹¹ N·m²/kg²
g (Acceleration due to gravity):
g = 9.8 m/s²
Relation:
g = GM / R²
4. Comment on application of Newton’s third law of motion to gravitation. Illustrate with examples.
In gravitation:
• Earth attracts moon → Moon attracts Earth with same force.
• Sun attracts planet → Planet attracts Sun with equal force.
But due to huge masses, effects on larger bodies are not noticeable.
Conclusion
Gravitation is a key force that controls the motion of everything in the universe—from falling objects to the movement of planets and satellites. Understanding free fall, acceleration due to gravity, mass, weight, and Newton’s law of gravitation helps students build strong scientific fundamentals. This chapter explains how gravity shapes our daily life, natural events, and the structure of the universe.
