🌍 GRAVITATION

1. Introduction to Gravitation

Gravitation is one of the fundamental forces of nature. It is the force of attraction between any two objects having mass. Every object in the universe attracts every other object due to gravitational force. This force keeps planets in their orbits, holds the atmosphere around the Earth, and makes objects fall toward the ground.

The concept of gravitation was first scientifically explained by Isaac Newton in 1687. His discovery of the Universal Law of Gravitation revolutionized science.

2. Universal Law of Gravitation

According to Newton:

Every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Mathematical Expression

F = G * (m1 * m2) / r^2

Where:

F = Gravitational force
G = Universal gravitational constant
m₁, m₂ = Masses of two objects
r = Distance between their centers
G = 6.67 × 10⁻¹¹ N m²/kg²

Key Points:

Gravitational force is always attractive.
It acts along the line joining the centers of two bodies.
It is a weak force compared to nuclear and electromagnetic forces.
It is a long-range force (acts over infinite distance).

3. Importance of Universal Gravitation

Gravitation:

Keeps us bound to Earth.
Keeps planets revolving around the Sun.
Keeps the Moon revolving around Earth.
Causes tides in oceans.
Holds galaxies together.

4. Free Fall

When an object falls under the influence of gravity alone (ignoring air resistance), it is said to be in free fall.

Example:

A stone dropped from a height.
A fruit falling from a tree.

5. Acceleration Due to Gravity (g)

When an object falls freely toward Earth, it experiences acceleration called acceleration due to gravity, denoted by g.

Formula for g

g = G * M / R^2

Where:

M = Mass of Earth
R = Radius of Earth
g ≈ 9.8 m/s²

Important Points:

g is constant near Earth’s surface.
g varies with height and depth.
g is maximum at poles and minimum at equator.
g decreases with altitude.

6. Motion Under Gravity (Equations of Motion)

When an object is thrown upward or dropped, we use motion equations by replacing a with g.

v = u + gt

s = ut + (1/2)gt^2

v^2 = u^2 + 2gs

Where:

u = Initial velocity
v = Final velocity
t = Time
s = Displacement

For upward motion, g is taken as negative (−9.8 m/s²).

7. Mass

Mass is the quantity of matter in an object.

Characteristics:

SI unit: kilogram (kg)
Scalar quantity
Constant everywhere
Measure of inertia
Measured using a beam balance

Mass does not change with location.

8. Weight

Weight is the force with which Earth attracts an object.

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Where:

W = Weight
m = Mass
g = Acceleration due to gravity

Characteristics:

SI unit: Newton (N)
Vector quantity
Changes with location
Measured using spring balance

9. Difference Between Mass and Weight

Mass	Weight
Amount of matter	Force of gravity
Unit: kg	Unit: N
Constant	Variable
Scalar	Vector

10. Weight on the Moon

The Moon’s gravity is 1/6th that of Earth.

W_moon = W_earth / 6

Mass remains same, weight reduces.

Example:
If a person weighs 60 kg on Earth, weight on Moon = 10 kg (approx. in terms of force comparison).

11. Thrust and Pressure

Thrust

Force acting perpendicular to a surface.

Pressure

Thrust per unit area.

P = F / A

Where:

P = Pressure
F = Force
A = Area

Unit:

Pascal (Pa)

1 Pa = 1 N/m²

12. Why Camels Have Broad Feet?

Camels have broad feet to reduce pressure on sand. Larger area → less pressure → prevents sinking.

13. Why Sharp Knives Cut Better?

Sharp knives have smaller area → more pressure → easier cutting.

14. Pressure in Liquids

Liquids exert pressure in all directions.

Properties:

Pressure increases with depth.
Pressure depends on density.
Same depth → same pressure.

15. Buoyancy

When an object is placed in a liquid, it experiences an upward force called buoyant force.

This force is due to pressure difference in liquid.

16. Archimedes’ Principle

Discovered by Archimedes.

When a body is immersed in a fluid, it experiences an upward force equal to the weight of the fluid displaced.

This principle explains floating and sinking.

17. Applications of Archimedes’ Principle

Designing ships and submarines
Determining density of substances
Lactometer (measures milk purity)
Hydrometer (measures density)

18. Relative Density

Relative density = Density of substance / Density of water

No unit (ratio).

If:

RD > 1 → sinks
RD < 1 → floats

19. Floating of Objects

An object floats if:

Weight of object = Buoyant force

Density of object < Density of liquid

Example:

Ships float due to large volume.
Ice floats on water because density of ice is less.

20. Why Ships Made of Iron Float?

Although iron sinks, ships are hollow, increasing volume and reducing average density.

21. Tides

Tides are rise and fall of sea levels caused by gravitational pull of:

Moon
Sun

The Moon has stronger effect due to closeness.

22. Importance of Gravitation in Solar System

Planets revolve around the Sun.
Moons revolve around planets.
Artificial satellites revolve around Earth.
Atmosphere is held by gravity.

23. Kepler’s Laws (Brief)

Johannes Kepler gave three laws of planetary motion:

Planets move in elliptical orbits.
Equal areas in equal time.
Square of period ∝ cube of distance.

These were later explained using Newton’s law.

24. Universal Gravitational Constant (G)

G = 6.67 × 10⁻¹¹ N m²/kg²
Measured by Henry Cavendish
Very small value → gravitational force is weak.

25. Variation of g

With Height:

g decreases as height increases.

With Depth:

g decreases as we go inside Earth.

At Poles and Equator:

Maximum at poles
Minimum at equator

26. Gravitational Force vs Electrostatic Force

Gravitational	Electrostatic
Always attractive	Attractive/repulsive
Depends on mass	Depends on charge
Very weak	Strong

27. Key Numerical Concepts

Calculate gravitational force.
Calculate weight.
Motion under gravity.
Relative density.
Pressure calculations.

28. Summary

Gravitation is a universal force acting between all objects. Newton’s Universal Law explains planetary motion and falling bodies. Acceleration due to gravity determines motion near Earth. Mass remains constant, but weight changes with gravity. Pressure and buoyancy explain floating and sinking. Archimedes’ Principle gives scientific explanation of buoyant force. Gravitation governs the structure of the entire universe—from falling apples to orbiting planets.

🔚 Conclusion

Gravitation is one of the most important natural forces shaping our universe. From keeping us grounded to controlling planetary motion, it plays a vital role in daily life and cosmic structure. Understanding gravitation helps us explain motion, tides, satellites, and space exploration. The contributions of Newton and Archimedes laid the foundation for modern physics and astronomy. Without gravity, life on Earth would not exist. Therefore, gravitation is not just a scientific concept but the force that holds the universe together.

Here are 100 MCQs with Answers from Class 9 – Gravitation (Based on NCERT pattern):

✅ GRAVITATION – 100 MCQs WITH ANSWERS

1–10: Universal Law of Gravitation

Who discovered the law of gravitation?
A) Galileo
B) Einstein
C) Newton
D) Kepler
Answer: C
Gravitational force between two bodies depends on:
A) Volume
B) Mass
C) Shape
D) Colour
Answer: B
Gravitational force is always:
A) Repulsive
B) Attractive
C) Zero
D) Magnetic
Answer: B
The formula for gravitational force is:
A) F = ma
B) F = Gm₁m₂/r²
C) F = mv
D) F = mg
Answer: B
The SI unit of gravitational force is:
A) Joule
B) Newton
C) Pascal
D) Watt
Answer: B
The value of G is:
A) 9.8
B) 6.67 × 10⁻¹¹
C) 1
D) 10
Answer: B
Gravitational force varies inversely with:
A) r
B) r²
C) m
D) m²
Answer: B
Gravitational force is a:
A) Contact force
B) Magnetic force
C) Non-contact force
D) Frictional force
Answer: C
If distance doubles, force becomes:
A) Double
B) Half
C) One-fourth
D) Four times
Answer: C
Gravitational force acts along the:
A) Surface
B) Vertical line
C) Line joining centres
D) Horizontal line
Answer: C

11–20: Acceleration Due to Gravity

Value of g on Earth is:
A) 10 m/s
B) 9.8 m/s²
C) 8.9 m/s²
D) 1 m/s²
Answer: B
g depends on:
A) Mass of Earth
B) Radius of Earth
C) Both A and B
D) Time
Answer: C
g decreases with:
A) Depth
B) Height
C) Both
D) Temperature
Answer: C
g is maximum at:
A) Equator
B) Poles
C) Centre
D) Sea
Answer: B
g is minimum at:
A) Poles
B) Equator
C) Centre
D) Mountains
Answer: B
In free fall, acceleration equals:
A) 0
B) g
C) 1
D) Infinity
Answer: B
Unit of g is:
A) m/s
B) m/s²
C) N
D) kg
Answer: B
If object is thrown upward, g is taken as:
A) Positive
B) Negative
C) Zero
D) Infinite
Answer: B
Weight is equal to:
A) m
B) mg
C) ma
D) mr
Answer: B
If mass is 5 kg, weight on Earth ≈
A) 49 N
B) 5 N
C) 10 N
D) 98 N
Answer: A

21–30: Mass and Weight

Mass is measured by:
A) Spring balance
B) Beam balance
C) Barometer
D) Hydrometer
Answer: B
Weight is measured by:
A) Beam balance
B) Thermometer
C) Spring balance
D) Clock
Answer: C
Mass is:
A) Variable
B) Constant
C) Vector
D) Zero
Answer: B
Weight is:
A) Scalar
B) Constant
C) Vector
D) Mass
Answer: C
Weight changes with:
A) Time
B) Location
C) Shape
D) Colour
Answer: B
On Moon, weight becomes:
A) Same
B) Double
C) Half
D) One-sixth
Answer: D
Mass on Moon is:
A) Zero
B) Same
C) Double
D) Half
Answer: B
SI unit of mass is:
A) Newton
B) kg
C) Pascal
D) m
Answer: B
SI unit of weight is:
A) N
B) kg
C) m
D) J
Answer: A
Weight is a type of:
A) Energy
B) Power
C) Force
D) Pressure
Answer: C

31–40: Pressure and Thrust

Thrust is:
A) Energy
B) Force
C) Pressure
D) Power
Answer: B
Pressure =
A) F × A
B) F/A
C) A/F
D) m/A
Answer: B
SI unit of pressure is:
A) Newton
B) Pascal
C) Watt
D) Joule
Answer: B
1 Pascal =
A) 1 N/m²
B) 1 N
C) 1 m²
D) 1 J
Answer: A
Pressure increases when area:
A) Increases
B) Decreases
C) Same
D) Doubles
Answer: B
Camels have broad feet to:
A) Increase pressure
B) Decrease pressure
C) Increase force
D) Reduce mass
Answer: B
Sharp knife cuts better due to:
A) Less pressure
B) More area
C) More pressure
D) Less force
Answer: C
Liquids exert pressure in:
A) One direction
B) Upward only
C) Downward only
D) All directions
Answer: D
Pressure in liquid increases with:
A) Height
B) Depth
C) Area
D) Colour
Answer: B
Dam walls are thicker at bottom because:
A) Pressure decreases
B) Pressure increases
C) Weight decreases
D) Mass increases
Answer: B

41–50: Buoyancy & Archimedes’ Principle

Upward force by liquid is:
A) Thrust
B) Buoyancy
C) Weight
D) Friction
Answer: B
Archimedes was from:
A) India
B) Greece
C) Italy
D) France
Answer: B
Archimedes’ principle relates to:
A) Pressure
B) Buoyancy
C) Force
D) Motion
Answer: B
Object floats when:
A) Density > liquid
B) Density < liquid
C) Mass > water
D) Weight zero
Answer: B
Relative density has:
A) Unit
B) No unit
C) kg
D) N
Answer: B
If RD > 1, object:
A) Floats
B) Sinks
C) Explodes
D) Evaporates
Answer: B
Hydrometer measures:
A) Pressure
B) Density
C) Force
D) Volume
Answer: B
Lactometer measures:
A) Milk purity
B) Air pressure
C) Force
D) Height
Answer: A
Ice floats because:
A) Heavier
B) Less dense
C) More force
D) Hot
Answer: B
Ships float due to:
A) Iron
B) Shape
C) Weight
D) Colour
Answer: B

51–100: Mixed Concept-Based Questions

Gravitational force is weakest among:
A) Nuclear
B) Magnetic
C) Electric
D) Gravitational
Answer: D
Unit of G is:
A) N m²/kg²
B) N
C) m/s²
D) kg
Answer: A
If mass doubles, force becomes:
A) Same
B) Double
C) Half
D) Zero
Answer: B
If distance triples, force becomes:
A) 1/9
B) 3
C) 9
D) 1/3
Answer: A

Free fall means motion under:
A) Air resistance
B) Friction
C) Gravity only
D) Magnetic force
Answer: C
Artificial satellites revolve around Earth due to:
A) Gravity
B) Magnetism
C) Wind
D) Rotation
Answer: A
Tides in oceans are mainly caused by the gravitational pull of the:
A) Sun
B) Moon
C) Mars
D) Jupiter
Answer: B
Kepler was a:
A) Physicist
B) Chemist
C) Astronomer
D) Biologist
Answer: C
Value of g at the centre of Earth is:
A) 9.8 m/s²
B) 4.9 m/s²
C) 1 m/s²
D) Zero
Answer: D
Weightlessness is experienced during:
A) Free fall
B) Rest
C) Walking
D) Standing
Answer: A
Density is defined as:
A) Volume/mass
B) Mass/volume
C) Mass × volume
D) Force/area
Answer: B
SI unit of density is:
A) kg
B) m³
C) kg/m³
D) N/m²
Answer: C
Earth attracts objects due to:
A) Friction
B) Magnetism
C) Gravity
D) Pressure
Answer: C
Gravitational force depends on the product of:
A) Distances
B) Volumes
C) Masses
D) Areas
Answer: C
The gravity on Moon is about:
A) 1/2 of Earth
B) 1/3 of Earth
C) 1/6 of Earth
D) Same as Earth
Answer: C
A spring balance measures:
A) Mass
B) Density
C) Weight
D) Volume
Answer: C
A beam balance measures:
A) Weight
B) Mass
C) Pressure
D) Force
Answer: B
Buoyant force acts in:
A) Downward direction
B) Sideways direction
C) Upward direction
D) No direction
Answer: C
A body floats when:
A) Weight > Buoyant force
B) Weight < Buoyant force
C) Weight = Buoyant force
D) Density is zero
Answer: C
The unit of pressure is named after:
A) Newton
B) Pascal
C) Joule
D) Watt
Answer: B
An object denser than water will:
A) Float
B) Sink
C) Evaporate
D) Expand
Answer: B
The atmosphere is held to Earth due to:
A) Pressure
B) Wind
C) Gravity
D) Temperature
Answer: C
The universal gravitational constant is denoted by:
A) g
B) G
C) F
D) M
Answer: B
The shape of Earth is nearly:
A) Cubical
B) Cylindrical
C) Spherical
D) Conical
Answer: C
When a ball is thrown upward, its velocity at the highest point is:
A) Maximum
B) Zero
C) Infinite
D) Negative
Answer: B
The time taken to reach maximum height is given by:
A) g/u
B) u/g
C) u × g
D) u + g
Answer: B
Weight of an object at poles is:
A) Less than equator
B) More than equator
C) Same
D) Zero
Answer: B
Pressure in a liquid depends on:
A) Shape of vessel
B) Colour
C) Depth
D) Area only
Answer: C
Why are dams thicker at bottom?
A) Less pressure
B) No pressure
C) More pressure
D) Equal pressure
Answer: C
A hydrometer works on:
A) Newton’s law
B) Archimedes’ principle
C) Ohm’s law
D) Hooke’s law
Answer: B
Relative density of pure water is:
A) 0
B) 0.5
C) 1
D) 10
Answer: C
An iron nail sinks in water because its density is:
A) Less than water
B) Equal to water
C) Greater than water
D) Zero
Answer: C
Air exerts:
A) No pressure
B) Pressure
C) Mass only
D) Volume only
Answer: B
Gravitational force between two bodies becomes zero when:
A) Mass is zero
B) Distance is zero
C) Mass is large
D) Never (if masses exist)
Answer: D
The centre of gravity of a uniform sphere lies at its:
A) Surface
B) Bottom
C) Centre
D) Edge
Answer: C
An artificial satellite is actually in:
A) Rest
B) Free fall
C) Friction
D) Rotation only
Answer: B
The Moon revolves around the:
A) Sun
B) Earth
C) Mars
D) Jupiter
Answer: B
The Earth revolves around the:
A) Moon
B) Mars
C) Sun
D) Jupiter
Answer: C
Weight of an object at the centre of Earth is:
A) Maximum
B) Same
C) Half
D) Zero
Answer: D
Pressure is inversely proportional to:
A) Force
B) Area
C) Mass
D) Time
Answer: B
Unit of thrust is:
A) Pascal
B) Newton
C) Joule
D) Watt
Answer: B
Buoyant force depends on:
A) Colour of liquid
B) Density of liquid
C) Shape of object only
D) Temperature only
Answer: B
The value of G was experimentally determined by:
A) Newton
B) Einstein
C) Cavendish
D) Kepler
Answer: C
Density of ice is:
A) Greater than water
B) Equal to water
C) Less than water
D) Zero
Answer: C
Gravitational force increases when:
A) Mass decreases
B) Distance increases
C) Mass increases
D) Volume decreases
Answer: C
The radius of Earth affects:
A) Mass
B) g
C) Colour
D) Shape
Answer: B
If mass is doubled and distance remains same, force becomes:
A) Same
B) Double
C) Half
D) Zero
Answer: B
If distance is doubled, gravitational force becomes:
A) Double
B) Half
C) One-fourth
D) Four times
Answer: C
Gravitation is responsible for keeping planets in:
A) Rest
B) Orbit
C) Air
D) Water
Answer: B
The gravitational force between Earth and objects makes them:
A) Float
B) Fly
C) Fall
D) Disappear
Answer: C

Here are 100 Very Short Answer Questions from Class 9 – Gravitation
(Each answer is 1–2 lines, exam-oriented format)

✅ GRAVITATION – VERY SHORT ANSWERS

1–20: Basic Concepts

What is gravitation?
Ans: The force of attraction between any two masses in the universe.
Who discovered the law of gravitation?
Ans: Sir Isaac Newton.
State the universal law of gravitation.
Ans: 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 between them.
Write the formula for gravitational force.
Ans: F = Gm₁m₂/r²
What is the value of G?
Ans: 6.67 × 10⁻¹¹ N m²/kg².
Is gravitational force attractive or repulsive?
Ans: Always attractive.
What type of force is gravity?
Ans: Non-contact force.
What is free fall?
Ans: Motion of an object under gravity alone.
What is acceleration due to gravity?
Ans: The acceleration of an object caused by Earth’s gravity.
What is the value of g on Earth?
Ans: 9.8 m/s².
What is the SI unit of g?
Ans: m/s².
Does g change with height?
Ans: Yes, it decreases with height.
Does g change with depth?
Ans: Yes, it decreases with depth.
Where is g maximum on Earth?
Ans: At the poles.
Where is g minimum on Earth?
Ans: At the equator.
What is mass?
Ans: The quantity of matter in a body.
What is the SI unit of mass?
Ans: Kilogram (kg).
What is weight?
Ans: The force with which Earth attracts a body.
Write formula of weight.
Ans: W = mg.
What is the SI unit of weight?
Ans: Newton (N).

21–40: Mass, Weight & Moon

Does mass change from place to place?
Ans: No.
Does weight change from place to place?
Ans: Yes.
Weight on Moon is how much of Earth?
Ans: 1/6th.
Does mass change on Moon?
Ans: No.
What is the value of g on Moon?
Ans: About 1.6 m/s².
What measures mass?
Ans: Beam balance.
What measures weight?
Ans: Spring balance.
What is thrust?
Ans: Force acting perpendicular to a surface.
Define pressure.
Ans: Thrust per unit area.
Write formula of pressure.
Ans: P = F/A.
What is SI unit of pressure?
Ans: Pascal (Pa).
What is 1 Pascal?
Ans: 1 N/m².
Why do camels have broad feet?
Ans: To reduce pressure on sand.
Why do sharp knives cut easily?
Ans: Due to high pressure from small area.
In which direction do liquids exert pressure?
Ans: In all directions.
Does liquid pressure increase with depth?
Ans: Yes.
What is buoyant force?
Ans: Upward force exerted by a liquid.
What is buoyancy?
Ans: The ability of a fluid to exert upward force.
Who gave Archimedes’ principle?
Ans: Archimedes.
State Archimedes’ principle.
Ans: A body immersed in a fluid experiences an upward force equal to the weight of fluid displaced.

41–60: Floating & Density

What is density?
Ans: Mass per unit volume.
Write formula of density.
Ans: Density = Mass/Volume.
What is SI unit of density?
Ans: kg/m³.
What is relative density?
Ans: Ratio of density of substance to density of water.
Does relative density have unit?
Ans: No.
When does an object float?
Ans: When buoyant force equals weight.
When does an object sink?
Ans: When weight is greater than buoyant force.
Why does ice float?
Ans: Because it is less dense than water.
Why do ships float?
Ans: Due to large volume and low average density.
What does hydrometer measure?
Ans: Relative density of liquids.
What does lactometer measure?
Ans: Purity of milk.
What causes tides?
Ans: Gravitational pull of Moon and Sun.
Which affects tides more?
Ans: Moon.
What keeps planets in orbit?
Ans: Gravitational force.
Who measured value of G experimentally?
Ans: Henry Cavendish.
What happens to g at centre of Earth?
Ans: It becomes zero.
What is weightlessness?
Ans: Condition when apparent weight becomes zero.
When is weightlessness experienced?
Ans: During free fall.
What is Kepler known for?
Ans: Laws of planetary motion.
What shape is Earth nearly?
Ans: Spherical.

61–80: Conceptual

Is gravitational force weak or strong?
Ans: Weak compared to other forces.
Is gravitational force long-range?
Ans: Yes.
Does gravity act between small objects?
Ans: Yes, but very weakly.
Does gravity depend on colour?
Ans: No.
What happens to force if distance doubles?
Ans: It becomes one-fourth.
What happens to force if mass doubles?
Ans: It doubles.
Does air exert pressure?
Ans: Yes.
Why are dams thicker at bottom?
Ans: Due to higher pressure at depth.
What is centre of gravity?
Ans: Point where weight acts.
Where is centre of gravity of sphere?
Ans: At its centre.
Can gravitational force be zero between two masses?
Ans: No.
Is gravitational force contact force?
Ans: No.
What is inertia?
Ans: Resistance to change in motion.
Does mass measure inertia?
Ans: Yes.
What is artificial satellite?
Ans: Man-made object orbiting Earth.
Is satellite in free fall?
Ans: Yes.
What holds atmosphere around Earth?
Ans: Gravity.
Why do objects fall to Earth?
Ans: Due to gravitational force.
Does g vary at equator?
Ans: Yes, it is minimum.
Does g vary at poles?
Ans: Yes, it is maximum.

81–100: Mixed Short Facts

What is universal gravitational constant?
Ans: Constant in gravitational formula.
Does G change anywhere?
Ans: No.
Is weight scalar or vector?
Ans: Vector.
Is mass scalar or vector?
Ans: Scalar.
What happens to g with altitude?
Ans: Decreases.
What happens to g with depth?
Ans: Decreases.
Is Moon’s gravity weaker than Earth’s?
Ans: Yes.
What is apparent weight?
Ans: Observed weight in a fluid.
Why does a stone sink?
Ans: Density greater than water.
Why does wood float?
Ans: Density less than water.
What is gravitational field?
Ans: Region around a mass where it attracts other masses.
Does gravity act in space?
Ans: Yes.
What force keeps Moon in orbit?
Ans: Earth’s gravity.
What force keeps Earth in orbit?
Ans: Sun’s gravity.
What is the nature of gravitational force?
Ans: Attractive.
Can gravity push objects?
Ans: No.
Is gravitational force infinite in range?
Ans: Yes.
What is main cause of falling rain?
Ans: Gravity.
Does gravity act on gases?
Ans: Yes.
Why is gravity essential for life?
Ans: It holds atmosphere and keeps objects on Earth.

✅ GRAVITATION – SHORT ANSWERS

1. Define gravitation.

Answer:
Gravitation is the force of attraction that exists between any two objects having mass in the universe. It is a universal force and acts between all bodies, whether small or large. This force keeps planets in orbit and causes objects to fall toward the Earth.

2. State the Universal Law of Gravitation.

3. Write the mathematical formula of gravitational force and explain terms.

Answer:
The formula is: F = Gm₁m₂/r².
Here, F is gravitational force, m₁ and m₂ are the masses of two bodies, r is the distance between their centres, and G is the universal gravitational constant.

4. What is free fall?

Answer:
Free fall is the motion of an object when it falls toward the Earth under the influence of gravity alone, without considering air resistance. In free fall, the object experiences constant acceleration equal to g.

5. What is acceleration due to gravity?

Answer:
Acceleration due to gravity (g) is the acceleration acquired by an object when it falls freely under Earth’s gravitational force. Its average value near the Earth’s surface is 9.8 m/s².

6. Why does the value of g decrease with height?

Answer:
The value of g decreases with height because gravitational force depends inversely on the square of the distance from Earth’s centre. As height increases, distance increases, reducing the gravitational pull and thus decreasing g.

7. Why does the value of g decrease with depth?

Answer:
As we move inside the Earth, the effective mass contributing to gravitational force decreases. Therefore, gravitational pull reduces gradually with depth and becomes zero at the centre of the Earth.

8. Differentiate between mass and weight.

Answer:
Mass is the quantity of matter in a body and remains constant everywhere. Weight is the gravitational force acting on a body and changes with location. Mass is measured in kilograms, while weight is measured in newtons.

9. Why is weight different on Earth and Moon?

Answer:
Weight depends on acceleration due to gravity. Since the Moon’s gravity is about one-sixth of Earth’s gravity, a person’s weight on the Moon becomes one-sixth of their weight on Earth, though mass remains the same.

10. Define thrust and pressure.

Answer:
Thrust is the force acting perpendicular to a surface. Pressure is defined as thrust per unit area. Pressure increases when the area decreases for the same applied force.

11. Why do camels have broad feet?

Answer:
Camels have broad feet to reduce pressure on desert sand. Since pressure equals force divided by area, increasing the area of contact reduces pressure. This prevents camels from sinking into the soft sand while walking.

12. Why do sharp knives cut better?

Answer:
Sharp knives have a smaller area of contact at the cutting edge. According to the pressure formula, smaller area results in greater pressure for the same force. This higher pressure makes cutting easier.

13. Explain Archimedes’ Principle.

Answer:
Archimedes’ Principle states that when a body is immersed in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by it. This principle explains why objects float or sink in liquids.

14. What is buoyant force?

Answer:
Buoyant force is the upward force exerted by a fluid on an object immersed in it. It acts opposite to the direction of gravity and depends on the density of the fluid and the volume of fluid displaced.

15. Why do ships float although made of iron?

Answer:
Ships float because they are hollow and have a large volume, which reduces their average density below that of water. According to Archimedes’ principle, they displace enough water to balance their weight.

16. Define relative density.

Answer:
Relative density is the ratio of the density of a substance to the density of water at 4°C. It has no unit because it is a ratio of two similar quantities.

17. Why does ice float on water?

Answer:
Ice floats because its density is lower than that of liquid water. When water freezes, it expands and becomes less dense, allowing ice to float on the surface.

18. Why are dams thicker at the bottom?

Answer:
Water pressure increases with depth. Therefore, the bottom of a dam experiences greater pressure compared to the top. To withstand this higher pressure, dams are constructed thicker at the bottom.

19. What causes tides in oceans?

Answer:
Tides are caused by the gravitational pull of the Moon and the Sun on Earth’s oceans. The Moon has a greater effect due to its closer distance from Earth.

20. What is weightlessness?

Answer:
Weightlessness is the condition in which a body’s apparent weight becomes zero. It is experienced during free fall, such as by astronauts orbiting Earth in a spacecraft.

21. What is density?

Answer:
Density is defined as mass per unit volume of a substance. It is calculated by dividing mass by volume. The SI unit of density is kilogram per cubic meter (kg/m³).

22. Why does a stone sink in water?

Answer:
A stone sinks because its density is greater than the density of water. The buoyant force acting on it is less than its weight, causing it to sink.

23. What keeps planets in orbit?

Answer:
Gravitational force between the Sun and planets keeps them in orbit. This force provides the necessary centripetal force required for circular motion.

24. What happens to gravitational force if distance doubles?

Answer:
If the distance between two bodies doubles, gravitational force becomes one-fourth because it is inversely proportional to the square of the distance.

25. What happens to gravitational force if mass doubles?

Answer:
If one of the masses doubles, gravitational force also doubles because force is directly proportional to the product of masses.

26–100 (Compact Exam-Ready Format)

Why does air exert pressure?
Air has weight and exerts force on surfaces due to gravity.
Why is g maximum at poles?
Due to smaller radius and no centrifugal effect.
Why is g minimum at equator?
Due to larger radius and Earth’s rotation.
What is centre of gravity?
Point where entire weight of body acts.
What is universal gravitational constant?
Constant value in gravitational formula.
Why is gravitational force called universal?
It acts between all objects in universe.
What is artificial satellite?
Man-made object orbiting Earth.
Why is satellite weightless?
It is in continuous free fall.
Why does gravity hold atmosphere?
It attracts air molecules toward Earth.
What is inertia?
Resistance to change in motion.
Does gravity act in space?
Yes, it acts everywhere.
What is SI unit of G?
N m²/kg².
Why do astronauts float in space?
Due to apparent weightlessness.
Why do objects fall downward?
Due to Earth’s gravitational pull.
What is apparent weight?
Weight measured in fluid.

✅ GRAVITATION – LONG ANSWERS

1. State and explain the Universal Law of Gravitation.

Answer:
The Universal Law of Gravitation states that every object in the universe attracts every other object with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. Mathematically, it is written as F = Gm₁m₂/r², where F is the gravitational force, m₁ and m₂ are the masses of the objects, r is the distance between them, and G is the universal gravitational constant. This force is always attractive and acts along the line joining the centres of the two bodies. The law explains the motion of planets, falling of objects, tides, and the structure of the solar system. It is called universal because it applies to all objects in the universe, irrespective of their size or location.

2. Define acceleration due to gravity and explain its variation with height and depth.

Answer:
Acceleration due to gravity (g) is the acceleration gained by an object when it falls freely under the influence of Earth’s gravitational force. Its average value near the Earth’s surface is 9.8 m/s². The value of g decreases with increase in height because gravitational force decreases as distance from Earth’s centre increases. Similarly, g decreases with depth because as we move toward the centre of Earth, the effective mass pulling the object decreases. At the centre of Earth, g becomes zero. Thus, acceleration due to gravity is not constant everywhere and depends on location.

3. Differentiate between mass and weight in detail.

Answer:
Mass is the amount of matter present in a body and is a measure of inertia. It remains constant everywhere and does not change with location. The SI unit of mass is kilogram (kg), and it is measured using a beam balance. Weight, on the other hand, is the force with which Earth attracts a body. It depends on acceleration due to gravity and varies from place to place. Its SI unit is Newton (N), and it is measured using a spring balance. Mass is a scalar quantity, while weight is a vector quantity because it has both magnitude and direction.

4. Explain free fall and write the equations of motion under gravity.

Answer:
Free fall is the motion of a body when it falls under the influence of gravity alone, without air resistance. In free fall, the object experiences constant acceleration equal to g. The equations of motion under gravity are:
v = u + gt
s = ut + ½gt²
v² = u² + 2gs
Here, u is initial velocity, v is final velocity, s is displacement, t is time, and g is acceleration due to gravity. For upward motion, g is taken as negative because gravity acts downward. These equations help calculate velocity, displacement, and time during vertical motion.

5. What is thrust and pressure? Explain with examples.

Answer:
Thrust is the force acting perpendicular to a surface. Pressure is defined as thrust per unit area. Mathematically, Pressure = Force/Area. Its SI unit is Pascal (Pa). Pressure increases when area decreases for the same applied force. For example, sharp knives cut easily because they have a small area, producing high pressure. Camels have broad feet to reduce pressure on sand and prevent sinking. Thus, pressure depends on both force and area of contact.

6. State and explain Archimedes’ Principle with applications.

Answer:
Archimedes’ Principle states that when a body is immersed wholly or partially in a fluid, it experiences an upward force equal to the weight of the fluid displaced by it. This upward force is called buoyant force. The principle explains floating and sinking of objects. Applications include designing ships and submarines, determining density using hydrometers, checking milk purity using lactometers, and calculating relative density. It plays an important role in fluid mechanics and engineering.

7. Explain why objects float or sink in a liquid.

Answer:
An object floats or sinks depending on the relationship between its weight and buoyant force. If the buoyant force is equal to or greater than the object’s weight, the object floats. If the weight is greater than buoyant force, it sinks. Density also plays a role. If the density of the object is less than the density of the liquid, it floats; if greater, it sinks. For example, wood floats in water, while a stone sinks.

8. Why do ships made of iron float on water?

Answer:
Ships are made of iron, which is denser than water, yet they float because they are hollow and have large volume. This increases the volume of water displaced, creating sufficient buoyant force to balance the ship’s weight. The average density of the ship becomes less than that of water. According to Archimedes’ Principle, when buoyant force equals weight, the ship floats.

9. Explain the phenomenon of tides.

Answer:
Tides are the periodic rise and fall of sea levels caused by the gravitational pull of the Moon and the Sun. The Moon has a greater effect due to its proximity to Earth. When the Moon is aligned with the Earth and Sun, strong tides called spring tides occur. When they form a right angle, weak tides called neap tides occur. Tides are important for navigation and marine life.

10. Explain weightlessness and its causes.

Answer:
Weightlessness is the condition in which the apparent weight of a body becomes zero. It occurs when a body is in free fall, such as astronauts in a spacecraft orbiting Earth. Although gravity still acts on them, they continuously fall toward Earth without touching it, so no normal reaction force acts on them. Hence, they feel weightless.

11–20 (Concise but Full-Length Format)

Explain variation of g at poles and equator.
g is maximum at poles because Earth’s radius is smaller and there is no centrifugal force due to rotation. At equator, radius is larger and centrifugal force reduces effective gravity, making g minimum.
What is relative density? Explain its importance.
Relative density is the ratio of density of substance to water. It helps determine whether object floats or sinks and has no unit.
Describe gravitational constant G.
G is universal constant with value 6.67×10⁻¹¹ N m²/kg². It was measured by Cavendish and remains constant everywhere.
Why is gravitational force called a weak force?
Because its magnitude is very small compared to nuclear and electromagnetic forces, though it acts over infinite distance.
Explain why Moon has no atmosphere.
Moon’s gravity is weak and cannot hold gases, so it lacks atmosphere.
Explain importance of gravitation in daily life.
It keeps us on Earth, causes rainfall, tides, and maintains planetary motion.
Explain centre of gravity.
It is the point where the entire weight of a body acts; for uniform sphere, it lies at centre.
Describe buoyant force in detail.
Buoyant force is upward force by liquid on immersed object, depending on density and displaced volume.
Explain motion of satellite around Earth.
Satellite stays in orbit due to balance between gravitational pull and forward velocity, resulting in circular motion.
Explain why g becomes zero at Earth’s centre.
At centre, gravitational pulls from all directions cancel each other, making net acceleration zero.

Here are 50 Assertion–Reason Questions from Class 9 – Gravitation
(Following CBSE pattern)

Directions:
A. Both Assertion (A) and Reason (R) are true and R is the correct explanation of A.
B. Both A and R are true but R is NOT the correct explanation of A.
C. A is true but R is false.
D. A is false but R is true.

✅ GRAVITATION – ASSERTION–REASON

1

Assertion (A): Every object in the universe attracts every other object.
Reason (R): Gravitational force acts only between planets.
Answer: C

2

A: Gravitational force decreases when distance increases.
R: Gravitational force is inversely proportional to the square of distance.
Answer: A

3

A: Gravitational force increases if masses increase.
R: Gravitational force is directly proportional to product of masses.
Answer: A

4

A: Weight of a body changes from place to place.
R: Weight depends on acceleration due to gravity.
Answer: A

5

A: Mass of a body remains constant everywhere.
R: Mass is independent of gravity.
Answer: A

6

A: Value of g decreases with height.
R: Gravitational force decreases as distance from Earth increases.
Answer: A

7

A: Value of g becomes zero at the centre of Earth.
R: Gravitational pulls from all sides cancel out.
Answer: A

8

A: g is maximum at poles.
R: Earth’s radius is smaller at poles.
Answer: A

9

A: g is minimum at equator.
R: Earth’s rotation produces centrifugal force at equator.
Answer: A

10

A: Moon has weaker gravity than Earth.
R: Moon has smaller mass than Earth.
Answer: A

11

A: Astronauts feel weightless in a satellite.
R: Satellite is in free fall.
Answer: A

12

A: Pressure increases when area decreases.
R: Pressure equals force divided by area.
Answer: A

13

A: Sharp knives cut easily.
R: Smaller area produces greater pressure.
Answer: A

14

A: Camels do not sink in sand easily.
R: Broad feet reduce pressure.
Answer: A

15

A: Liquids exert pressure in all directions.
R: Liquids have weight.
Answer: B

16

A: Pressure in a liquid increases with depth.
R: Weight of liquid column increases with depth.
Answer: A

17

A: Dams are thicker at bottom.
R: Water pressure increases with depth.
Answer: A

18

A: Ice floats on water.
R: Density of ice is less than water.
Answer: A

19

A: A stone sinks in water.
R: Density of stone is greater than water.
Answer: A

20

A: Ships made of iron float on water.
R: Ships are hollow and displace large volume of water.
Answer: A

21

A: Relative density has no unit.
R: It is a ratio of two similar quantities.
Answer: A

22

A: Hydrometer works on Archimedes’ principle.
R: It measures density of liquids.
Answer: B

23

A: Gravitational force is always attractive.
R: It depends on electric charges.
Answer: C

24

A: Gravitational force is a non-contact force.
R: It acts without physical contact.
Answer: A

25

A: Artificial satellites revolve around Earth.
R: Earth’s gravitational force provides centripetal force.
Answer: A

26

A: Value of G is constant everywhere.
R: G depends on location.
Answer: C

27

A: Weight of a body on Moon is one-sixth of Earth.
R: g on Moon is one-sixth of Earth.
Answer: A

28

A: Mass is a scalar quantity.
R: It has magnitude only.
Answer: A

29

A: Weight is a vector quantity.
R: It has both magnitude and direction.
Answer: A

30

A: Air exerts pressure.
R: Air has mass and weight.
Answer: A

31

A: Gravitational force becomes one-fourth if distance doubles.
R: Force is inversely proportional to square of distance.
Answer: A

32

A: Gravitational force becomes double if mass doubles.
R: Force is directly proportional to mass.
Answer: A

33

A: Moon does not have atmosphere.
R: Moon’s gravity is weak.
Answer: A

34

A: Rain falls on Earth due to gravity.
R: Gravity attracts water droplets.
Answer: A

35

A: Centre of gravity of uniform sphere lies at centre.
R: Weight is evenly distributed.
Answer: A

36

A: Buoyant force acts upward.
R: Liquid pressure increases with depth.
Answer: A

37

A: An object floats when buoyant force equals weight.
R: Floating occurs when net force is zero.
Answer: A

38

A: Pressure is directly proportional to force.
R: Pressure equals force divided by area.
Answer: A

39

A: Gravitational force acts between small objects.
R: It acts only between large planets.
Answer: C

40

A: Tides are mainly caused by Moon.
R: Moon is closer to Earth than Sun.
Answer: A

41

A: Value of g depends on Earth’s radius.
R: g = GM/R².
Answer: A

42

A: g decreases inside Earth.
R: Effective mass decreases with depth.
Answer: A

43

A: Gravitational force is weakest among fundamental forces.
R: Its constant G is very small.
Answer: A

44

A: Weight becomes zero at centre of Earth.
R: g becomes zero at centre.
Answer: A

45

A: A body thrown upward slows down.
R: Gravity acts downward.
Answer: A

46

A: Relative density of water is 1.
R: It is compared with itself.
Answer: A

47

A: Pressure at same depth in liquid is equal.
R: Pressure depends only on depth.
Answer: B

48

A: Buoyancy depends on density of liquid.
R: Greater density produces greater upward force.
Answer: A

49

A: Gravitational force has infinite range.
R: It decreases with distance but never becomes zero.
Answer: A

50

A: Gravity is essential for life on Earth.
R: It holds atmosphere and keeps objects on surface.
Answer: A