Class 9 Science Gravitation

Class 9 Science Chapter 10 Gravitation Notes with PDF | Complete NCERT Study Material

1. Introduction

Gravitation is one of the most important ideas in science because it explains why objects fall to the ground, why planets move around the Sun, why the Moon revolves around the Earth, and why we remain attached to the surface of the Earth instead of floating away. Every object in the universe attracts every other object with a force called gravitation. This force is present everywhere, even though we do not always notice it directly in daily life.

The study of gravitation helps us understand motion on Earth and in space. It connects ordinary experiences such as falling objects, the weight of a body, and the flow of water with large-scale natural phenomena such as the movement of planets and satellites. This chapter introduces the universal law of gravitation, free fall, acceleration due to gravity, mass and weight, thrust and pressure, buoyancy, Archimedes’ principle, and relative density. These concepts are not isolated facts. They are deeply linked with one another and explain many events around us.

Gravitation is also a chapter that builds a bridge between mechanics and astronomy. It shows that the same force that makes an apple fall from a tree also keeps the Moon in orbit around the Earth and the Earth in orbit around the Sun. That is why this chapter is one of the most beautiful and logically connected chapters in physics.

2. What Is Gravitation?

Gravitation is the force of attraction between any two objects in the universe. It is a universal force because it acts between all objects, whether they are small or large, near or far, living or non-living. The force exists between two stones, between a stone and the Earth, between the Earth and the Moon, between the Sun and a planet, and even between two people standing close to each other. The force is usually very small unless at least one of the objects has a large mass.

The word gravitation should not be confused with gravity. Gravitation is the attractive force between any two masses in the universe. Gravity is the force with which the Earth attracts objects toward its centre. Thus, gravity is a special case of gravitation, while gravitation is the broader universal force.

In everyday life, we mostly experience gravitation through Earth’s gravity. Objects fall downward because the Earth attracts them. We walk on the ground because the Earth pulls us inward. Rivers flow downhill because of gravity, and the atmosphere remains bound to the Earth because of gravitational attraction.

3. Universal Law of Gravitation

The universal law of gravitation was discovered by Isaac Newton. It states that 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 their centres.

In simple words, larger masses attract each other more strongly, and the attraction becomes weaker as the distance between them increases.

Statement of the Law

The force of gravitation between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

Mathematical Form

F = Gm₁m₂ / r²

Here, F is the gravitational force, m₁ and m₂ are the masses of the two objects, r is the distance between their centres, and G is the universal gravitational constant.

Meaning of the Formula

If the mass of either object increases, the gravitational force increases.
If the distance between the objects increases, the gravitational force decreases rapidly.
The force depends on both masses and the square of the distance.

Universal Gravitational Constant

The constant G is called the universal gravitational constant. Its value is approximately 6.67 × 10^-11 N m² kg^-2.

The value of G is the same everywhere in the universe. It does not change from place to place. That is why the law is called universal.

Importance of the Law

This law explains:

why objects fall to the ground,
why the Moon revolves around the Earth,
why planets revolve around the Sun,
why ocean tides occur,
and why every object in the universe attracts every other object.

4. Why Objects Fall Toward the Earth

Objects fall toward the Earth because the Earth exerts an attractive force on them. This force acts toward the centre of the Earth. When a stone is dropped, it moves downward because gravity pulls it. When a ball is thrown upward, it slows down and eventually comes back down because the Earth’s gravity acts against its upward motion.

The attraction of the Earth is responsible for keeping us on the ground and for giving a downward direction to all freely falling bodies. Without gravity, we would not be able to stand, water would not flow downward, and the atmosphere would not stay around the Earth.

This force also gives direction to the movement of rain, the motion of objects in free fall, and many natural processes involving matter on Earth.

5. Free Fall

When an object falls under the influence of gravity alone, with no other force acting on it except air resistance, it is said to be in free fall. In strict physics, free fall means motion under gravity only. In real life, air resistance is often present, but in many problems it is ignored for simplicity.

In free fall, all bodies accelerate toward the Earth. This acceleration is called acceleration due to gravity. If air resistance is neglected, all objects fall with the same acceleration, regardless of their mass. This is an important and surprising idea because many people think heavy objects fall faster than light ones. In the absence of air resistance, this is not true.

Examples of Free Fall

A stone dropped from a height
A fruit falling from a tree
A body released in air after being held at rest

Galileo studied falling bodies carefully and showed that the distance covered by a freely falling body increases with time in a special way. His work laid the foundation for understanding gravity properly.

6. Acceleration Due to Gravity

The acceleration produced in a freely falling body due to the gravitational pull of the Earth is called acceleration due to gravity. It is denoted by g.

Near the surface of the Earth, the value of g is about 9.8 m/s². This means that in every second of free fall, the velocity of a body increases by 9.8 m/s, if air resistance is ignored.

Why g Is Important

It explains how fast objects fall.
It helps in calculating weight.
It is used in motion equations for freely falling bodies.
It is central to many physics problems.

Direction of g

The direction of acceleration due to gravity is always toward the centre of the Earth. Therefore, even though we often use g as a positive value in calculations, its direction is downward toward the Earth’s centre.

Variation of g

The value of g is not exactly the same everywhere. It changes slightly:

with altitude,
with depth below the Earth’s surface,
and from one planet to another.

g is slightly smaller at higher altitudes because the distance from the Earth’s centre increases. It is also slightly smaller near the equator than at the poles because of the Earth’s shape and rotation.

7. Difference Between G and g

Students often confuse the universal gravitational constant G with acceleration due to gravity g. They are not the same.

Universal Gravitational Constant G

Appears in the law of gravitation.
Its value is the same everywhere in the universe.
It is a constant of nature.
Its SI unit is N m² kg^-2.

Acceleration Due to Gravity g

It is the acceleration of freely falling bodies near a planet’s surface.
Its value depends on the mass and radius of the planet.
It changes from place to place.
Its SI unit is m/s².

G is universal and constant, while g varies depending on location and celestial body. This distinction is very important for accurate understanding.

8. Motion of Objects Under Gravity

When an object moves vertically up or down under the effect of gravity, its motion is affected by gravitational acceleration. A body thrown upward slows down because gravity acts downward opposite to the motion. At the highest point, its velocity becomes zero for a moment. Then it falls downward with increasing speed.

In free fall, the equations of motion can be used with acceleration a replaced by g. For example:

v = u + gt
s = ut + 1/2 gt²
v² = u² + 2gs

These equations are useful for solving problems related to falling bodies, upward motion, and motion under gravity.

9. Mass

Mass is the amount of matter contained in an object. It is a scalar quantity and remains the same everywhere in the universe. Mass does not depend on location. A body has the same mass on Earth, on the Moon, or in outer space.

Mass is measured in kilogram in the SI system. It is also a measure of inertia. The greater the mass of a body, the greater its inertia. This means it is harder to change the motion of a body with large mass.

Characteristics of Mass

It is the amount of matter in a body.
It is a scalar quantity.
It remains constant everywhere.
It is measured in kilogram.
It is a measure of inertia.

Mass should not be confused with weight. Mass is not a force, while weight is a force. This distinction is very important in gravitation.

10. Weight

Weight is the force with which an object is attracted toward the Earth. It is a vector quantity and acts downward, toward the centre of the Earth. Weight depends on gravity.

Weight = mass × acceleration due to gravity

So, W = mg

Since weight depends on g, it changes from place to place. A body weighs less on the Moon than on Earth because the Moon’s gravitational pull is weaker.

Characteristics of Weight

It is a force.
It is a vector quantity.
It depends on the local value of g.
Its SI unit is newton.
It acts toward the centre of the attracting body, usually the Earth.

Weight Versus Mass

Mass remains constant; weight changes with location.
Mass is measured in kilogram; weight is measured in newton.
Mass is the amount of matter; weight is gravitational force.
Mass is scalar; weight is vector.

This difference is often asked in exams and must be understood clearly.

11. Thrust

Thrust is the force acting perpendicular to a surface. It is the force applied in a direction normal to the surface. For example, when a book rests on a table, the force exerted by the book on the table is a thrust.

Thrust can arise due to the weight of objects or due to applied force. It acts on a surface and is important in understanding pressure.

Examples of Thrust

The force of a book on a table
The force of water on the walls of a tank
The force of a person standing on the ground

Thrust is not the same as pressure. Thrust is the force itself, while pressure is the effect produced by this force on a given area.

12. Pressure

Pressure is the force acting on a unit area of a surface. It tells us how concentrated a force is over a surface. If the same force acts on a smaller area, the pressure becomes greater. If the same force acts on a larger area, the pressure becomes smaller.

Pressure = thrust / area

So, Pressure = Force / Area

The SI unit of pressure is pascal. One pascal is equal to one newton per square metre.

Why Pressure Matters

Pressure helps explain many daily life situations. A sharp knife cuts better because the force acts on a very small area, creating large pressure. A camel can walk on sand more easily because its broad feet reduce pressure on the sand. A school bag has straps to spread the force over a larger area on the shoulders.

Examples of Pressure

Needles and pins have sharp tips to increase pressure.
Snow shoes increase area to reduce pressure on snow.
Buildings use wide foundations to distribute pressure.
Wheeled vehicles reduce pressure by spreading load through tyres.

Pressure is important in engineering, nature, medicine, and everyday activities.

13. Fluid Pressure

Liquids and gases are called fluids because they can flow. Fluids exert pressure in all directions. The pressure in a fluid increases with depth because the weight of the fluid above increases with depth.

This is why the lower part of a dam is built thicker than the upper part. Water pressure is greater at greater depth, so the structure must be stronger at the bottom.

Fluid pressure is also the reason why pressure in the lungs, blood vessels, and atmospheric systems is important in biology and environmental science.

14. Buoyancy

When an object is placed in a fluid, the fluid exerts an upward force on the object. This upward force is called buoyant force or upthrust. Because of this force, objects appear lighter in a fluid than in air.

Buoyancy is the tendency of a fluid to exert upward force on an object immersed in it. Whether an object sinks, floats, or remains suspended depends on the balance between its weight and the buoyant force.

Examples of Buoyancy

A ship floats on water.
A cork floats on water.
A stone sinks in water.
A balloon filled with helium rises in air.

Why Objects Float or Sink

If the buoyant force is equal to or greater than the weight of an object, the object may float. If the weight is greater than the buoyant force, the object sinks. The density of the object and the fluid is also very important in determining this behaviour.

15. Archimedes’ Principle

Archimedes’ principle states that when an object is immersed wholly or partially in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by the object.

This principle is the scientific explanation for buoyancy. It helps us understand why some objects float and others sink. It also allows us to calculate the buoyant force acting on immersed bodies.

Meaning of the Principle

If a body displaces a large amount of fluid, the buoyant force becomes larger. That is why big hollow objects like ships can float even though they are made of heavy materials.

Applications of Archimedes’ Principle

Design of ships and boats
Working of submarines
Hydrometers for measuring density
Lactometers for checking purity of milk
Finding the density of solids and liquids

Archimedes’ principle is one of the most practical and useful laws in this chapter.

16. Relative Density

Relative density compares the density of a substance with the density of water. It is defined as the ratio of the density of the substance to the density of water.

Relative density = density of substance / density of water

Since it is a ratio of two similar quantities, relative density has no unit.

Importance of Relative Density

It helps compare substances.
It is used in determining whether a material will float or sink in water.
It is useful in laboratories and industry.

If the relative density of a substance is less than 1, it tends to float on water. If it is greater than 1, it tends to sink.

17. Density and Its Relation to Floating

Density is mass per unit volume. It tells us how tightly packed the matter in a substance is. A substance with high density contains more mass in the same volume than a substance with low density.

Density plays a major role in floating and sinking. Objects with density less than the fluid density tend to float, while objects with greater density tend to sink. However, shape and displaced fluid volume are also important.

This is why a large ship made of steel floats on water. Though steel is denser than water, the ship’s shape causes it to displace a large volume of water, creating enough buoyant force to support its weight.

18. Practical Examples from Daily Life

Gravitation and pressure appear everywhere around us.

Walking: We walk because Earth provides friction and gravity keeps us grounded.
Falling objects: Gravity pulls them downward.
Weight changes: A person weighs less on the Moon than on Earth.
Pressure in sharp tools: Knives, needles, and nails work because of high pressure at a small area.
Floating of boats: Buoyancy and Archimedes’ principle explain it.
Atmospheric effects: Air pressure changes with altitude and affects weather and breathing.

These examples show that gravitation is not just a theory but a force governing countless real-life events.

19. Common Misconceptions

Students often make a few common mistakes while studying gravitation. These should be avoided.

Thinking that heavy objects always fall faster than light objects.
Confusing mass with weight.
Assuming G and g are the same.
Thinking buoyancy acts only on objects that float.
Believing that action and reaction forces cancel each other.
Mixing up thrust and pressure.

Correct understanding of these ideas helps in solving numerical and conceptual problems confidently.

20. Important Formulae

Universal law of gravitation: F = Gm₁m₂ / r²
Acceleration due to gravity: g is approximately 9.8 m/s² near Earth’s surface
Weight: W = mg
Pressure: Pressure = Force / Area
Relative density: Relative density = density of substance / density of water

These formulae are the backbone of numerical work in this chapter.

21. Quick Revision Notes

Gravitation is the attraction between any two masses.
Gravity is Earth’s attraction on objects.
Newton’s law of gravitation explains the force between two masses.
Free fall means motion under gravity alone.
Acceleration due to gravity is denoted by g.
Mass is the amount of matter and does not change with location.
Weight is the gravitational force acting on a body and changes with location.
Thrust is force acting normally on a surface.
Pressure is thrust per unit area.
Buoyancy is the upward force exerted by a fluid.
Archimedes’ principle explains buoyant force.
Relative density is a unitless comparison with water.

22. Practice Questions

Define gravitation and distinguish it from gravity.
State and explain the universal law of gravitation.
What is free fall? Why do all bodies fall with the same acceleration in vacuum?
Define acceleration due to gravity and write its approximate value near the Earth’s surface.
Differentiate between mass and weight.
What is thrust? What is pressure? How are they related?
Explain buoyancy and give daily life examples.
State Archimedes’ principle and mention two applications.
What is relative density? Why has it no unit?
Why does a ship made of steel float on water?

Class 9 Science Gravitation Notes PDF

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23. Final Understanding

Gravitation is the invisible force that shapes much of the universe. It keeps us grounded on Earth, holds the atmosphere around the planet, causes objects to fall, keeps the Moon in orbit, and controls the motion of planets and satellites. It is a simple idea in statement but enormous in its effects.

This chapter also shows how one force leads to many important concepts. Mass, weight, pressure, buoyancy, and relative density are all connected. The same gravitational force that makes an object fall also helps explain why things float, why pressure changes with area, and why our weight changes from one place to another.

If you understand the universal law of gravitation, free fall, acceleration due to gravity, and the difference between mass and weight, the rest of the chapter becomes much easier. Try to relate each idea to an everyday example. That will make the subject memorable and practical.

Study this chapter carefully, practice the formulae, and revise the concepts regularly. Gravitation is one of the most important and elegant parts of Class 9 Science, and a strong understanding here will help in higher classes as well.

Class 9 Science Gravitation Notes with PDF | NCERT Science Notes - Monelitho