GRAVITATION, CBSE CLASS IX, PHYSICS NOTES PART I

 

CBSE CLASS IX, GRAVITATION, SCIENCE (PHYSICS) NOTES-(PART I)

                       GRAVITATION

According to the CBSE Syllabus 2025-26

CBSE Class 9 Science Chapter 10 Gravitation Notes

In Class 9 Science Chapter 10 Gravitation.


Topics in the Chapter

1. INTRODUCTION

2. NEWTON'S UNIVERSAL LAW OF GRAVITATION

a. RELATIONSHIP BETWEEN NEWTON'S 3RD LAW AND NEWTON'S LAW OF GRAVITATION

b. IMPORTANCE OF UNIVERSAL LAW OF GRAVITATION

3. FREE FALL OF AN OBJECT AND ACCELERATION DUE TO GRAVITY

a. VALUE OF G

b. RELATIONSHIP BETWEEN G AND g

4. MASS

5. WEIGHT

6. DIFFERENCE BETWEEN MASS AND WEIGHT

7. FACTORS THAT AFFECT THE VALUE OF g

8. THRUST AND PRESSURE

9. BUOYANCY

10. DENSITY

11. ARCHIMEDE'S PRINCIPLE

a. APPLICATION OF ARCHIMEDES' PRINCIPLE

12. RELATIVE DENSITY

13. SOLVED NUMERALS

Introduction to Gravitation

This chapter discusses gravitation and the Universal Law of Gravitation. The motion of objects under the influence of gravitational force on Earth is also examined closely. Students will also understand how weight varies from place to place and the conditions required for objects to float on water.

Gravitation

Gravitation, or just gravity, is the force of attraction between any two bodies. All the objects in the universe attract each other with a certain amount of force, but in most cases, the force is too weak to be observed due to the very large distance of separation. Besides, gravity’s range is infinite but the effect becomes weaker as objects move away.

Some examples of gravity are:

1. The force that causes the ball to come down is known as gravity.

2. Gravity keeps the planets in orbit around the sun.

3. Gravity is the force that causes a rock to roll downhill.

Type of Forces

There are four fundamental forces in the universe, and they are:

1. Gravitational force

2. Electromagnetic force

3. Strong nuclear force

4. Weak nuclear force


Gravitational Force

The gravitational force is the weakest force out of the four forces. When gravitational force is considered for massive objects, such as the sun or giant planets, the gravitational force is considered to be stronger as the masses of these objects are also large. On an atomic level, this force is considered weak.

Electromagnetic Force

The electromagnetic force is a type of physical interaction that occurs between electrically charged particles. It acts between charged particles and is the combination of magnetic and electrical forces. The electromagnetic force can be attractive or repulsive.

Strong Nuclear Force

The strong force holds together quarks, the fundamental particles that make up the protons and neutrons of the atomic nucleus, and further holds together protons and neutrons to form atomic nuclei.

Weak Nuclear Force

The weak force is the force existing between the elementary particles, which are responsible for certain processes to take place at a low probability.

Gravitational Force of Earth                                                                                            

If we release a small stone without pushing it from a height, it accelerates towards Earth.

The stone is accelerated towards Earth, which means some force is acting on it. The force that pulls the objects towards the centre of the Earth is known as the gravitational force of the Earth.

Here, stone also attracts earth. It means every object in the universe attracts every other object.

Newton’s Universal Law of Gravitation

Sir Isaac Newton 1687 proposed a law about the force of attraction between two objects in the universe, which is known as Newton’s law of gravitation.

Universal Law of Gravitation                                                

Every mass in this universe attracts every other mass with a force which is directly proportional to the product of two masses and inversely proportional to the square of the distance between them.

• Let masses (M) and (m) of two objects be distance (r) apart, then the force of attraction (F) between them,

F ∝ M × m
F ∝ 1/r2
F ∝ Mm/r2
F = (GMm)/r2

where,

G is a constant and is known as the Gravitational constant.
Value of G = 6.67×10-11 Nm2/kg2
G is called the universal gravitational constant.

If the unit of F is in Newton, m is in kg, and r is in metre, then the unit of G can be calculated as:
G = (F×r2)/Mm, the unit will be N m2/ kg2

Relation between Newton’s third law of motion and Newton’s law of gravitation

i. According to Newton’s third law of motion, “Every object exerts an equal and opposite force on another object, but in the opposite direction.”
ii. According to Newton’s law of gravitation, “Every mass in the universe attracts every other mass.”
iii. In the case of a freely falling stone and the earth, the stone is attracted towards the earth means the earth attracts the stone.
iv. But according to Newton’s third law of motion, the stone should also attract the Earth.
 It is true that stone also attracts the earth with the same force F = m × a but due to very less mass of the stone, the acceleration (a) in its velocity is 9.8 m/s2 and acceleration (a) of earth towards stone is 1.65×10-24 m/s2 which is negligible and we cannot feel it.

Importance of universal law of gravitation

(i)  The force that binds us to the earth.
(ii) The motion of the moon around the Earth.
(iii) The motion of Earth around the sun.
(iv) The tides due to the moon and the sun.

Factors affecting the value of g

i. Earth is not a perfect sphere. The radius of Earth increases when we go from the pole to the equator.
ii. Therefore, we can take g as constant at the surface of Earth or closer to it. But, as we move away from Earth, we can use the equation g = GM/d2 to solve problems.
iii. The value of gravitational acceleration ‘g’ at a distance of 12800 km from the centre of the Earth is ¼.                                                                                
iv. The value of gravitational acceleration ‘g’ decreases with increasing height.                                
v. The weight of an object on the moon is one-sixth of the weight on Earth.
• Let the mass of an object be m, its weight on Earth means the force by which earth attracts it towards the centre.
Now, Fe = (GMem)/(Re)2 .... (i)
where,
 G = Gravitational constant
 Me = Mass of the Earth
 m = Mass of object
 Re = Radius of the Earth
• Weight of an object on moon (Fm) = (GMmm)/(Rm)2 .... (ii)
where,                                                                                                                   Mm = Mass of the moon                                                                                               Rm = Radius of the moon
• Dividing equation (i) by equation (ii), we get. 
We know that the mass of Earth is 100 times the mass of the moon.                          So, Me = 100 Mm, and the radius of Earth is 4 times the radius of the Moon.

Gravitational Acceleration and its value at the surface of Earth

The uniform acceleration produced in a freely falling object due to the gravitational force of Earth is called acceleration due to gravity.
It is represented by ‘g’ and it always acts towards the centre of the Earth.

Acceleration due to Gravity

F=mg………………………….(i)

and also,

F = GMm/R2………………….(ii)

From equations (i) and (ii)

g = GM/R2

Values of G = 6.673 × 10-11Nm2Kg-2

M (mass of Earth) = 6 × 1024 kg  and 

R = 6 × 106 m, to get the value of g as ≈ 9.8ms-2

This is the acceleration due to gravity and the acceleration felt by any freely falling body towards the Earth.

The value of g keeps changing due to the variation of Earth’s radius.

Free fall of an object and acceleration (g)

When an object is thrown upward, it reaches a certain height, then it starts falling down towards the earth. It is because the Earth’s gravitational force exerts on it.
This fall under the influence of Earth is called ‘free fall of an object’.
During this free fall, direction does not change, but velocity continuously changes, which is called acceleration due to gravity. It is denoted by ‘g’.
Its unit is the same as acceleration m/s2.

The Moon’s Falling – The Moon’s Revolution around Earth

The moon revolves around the Earth due to the centripetal force, which is the force of gravity of the Earth. If the force of attraction between the Earth and the moon ceases, then the moon will continue to travel in a straight-line path tangential to its orbit around the Earth.

Free Fall and Motion

When an object is under free fall, acceleration due to gravity is constant at g = 9.8ms-2.

Value of g does not depend on mass i.e any object big or small experiences the same acceleration due to gravity under free fall. All three equations of motion are valid for freely falling objects as it is under uniform motion.

The sign of convention = towards earth g is +ve, and away from earth g is -ve.

Centripetal Force

When a body undergoes circular motion, it experiences a force that acts towards the centre of the circle. This centre-seeking force is called a centripetal force. Centripetal force is given by the following equation:

F=mv2 / r

Value of ‘g’ on the surface of earth

The force acting on an object is F = (GMem)/R2 .... (i)
where,
Me = Mass of Earth
m = Mass of an object
R = Radius of Earth
If acceleration due to gravity is ‘g’ due to force F, then, F = m×g  .... (ii)

Relationship and difference between ‘G’ and ‘g’

G = Gravitational constant

g = Acceleration due to gravity                                                                 

g = GM/R2







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