CBSE Class 9 Science, Gravitation: Chapter notes (Part-I)

Get the CBSE Class 9 Science notes on chapter 10 ‘Gravitation’ (Part-I) from this article. These chapter notes are prepared by the subject experts to give you a crisp and brief explanation of all the important topics related to the gravitational force of the Earth. At the end of the notes you can try a few questions to assess your knowledge. These questions will help you to assess your preparation level and get a hold on the subject.

Also Read: CBSE Class 9 Science, Gravitation: Chapter notes (Part-II)

Main topics covered in this part of CBSE Class 9 Science,  Gravitation: Chapter Notes, are:

Also Read: CBSE Class 9 Complete Study Material for 2020-2021

Keynotes for Chapter - Gravitation are:


Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitational pull.

Universal Law of Gravitation

Every object in the universe attracts other object by a force of attraction, called gravitation, which is directly proportional to the product of masses of the objects and inversely proportional to the square of distance between them. This is called Law of Gravitation or Universal Law of Gravitation.

Let masses (M) and (m) of two objects are distance (d) apart. Let F  be the attractional force between two masses.


CBSE Class 9 Science Syllabus 2020-2021

Importance of The Universal Law of Gravitation

Free Fall

When an object falls from any height under the influence of gravitational force only, it is known as free fall.

Acceleration Due to Gravity

When an object falls towards the earth there is a change in its acceleration due to the gravitational force of the earth. So this acceleration is called acceleration due to gravity.

The acceleration due to gravity is denoted by g.

The unit of g  is same as the unit of acceleration, i.e., ms−2

Mathematical Expression for g

From the second law of motion, force is the product of mass and acceleration.    

            F = ma

For free fall, acceleration is replaced by acceleration due to gravity. 

Therefore, force becomes:                      

            F = mg            ….(i)

But from Universal Law of Gravitation,

Factors Affecting the Value of g

To Calculate the Value of g

Value of universal gravitational constant, G = 6.7 × 10–11 N m2/ kg2,

Mass of the earth, M = 6 × 1024 kg, and

Radius of the earth, R = 6.4 × 106 m

Putting all these values in equation (iii), we get:


Thus, the value of acceleration due to gravity of the earth, g = 9.8 m/s2.

Difference between Gravitation Constant (G) and  Gravitational Acceleration (g)

S. No.

Gravitation Constant (G)

Gravitational acceleration (g)


Its value is 6.67×10-11Nm2/kg2.

Its value is 9.8 m/s2.


It is a scalar quantity.

It is a vactor quantity.


Its value remains constant always and everywhere.

Its value varies at various places.


Its unit is Nm2/kg2

Its unit is m/s2.

Motion of Objects Under the Influence of Gravitational Force of the Earth

Let an object is falling towards earth with initial velocity u. Let its velocity, under the effect of gravitational acceleration g, changes to v after covering the height h in time t.

Then the three equations of motion can be represented as:
Velocity (v) after t seconds, v = u + ght
Height covered in t seconds, h = ut + ½gt2
Relation between v and u excluding t, v2 = u2 + 2gh
The value of g is taken as positive in case of the object is moving towards earth and taken as negative in case of the object is thrown in opposite direction of the earth.

Mass & weight
Mass (m)

Weight (W)

Weight of an Object on the Surface of Moon
Mass of an object is same on earth as well as on moon. But weight is different.

Weight of an object is given as,

Hence, weight of the object on the moon = (1/6) × its weight on the earth.

Try the following questions:

Q1. State the universal law of gravitation.

Q2. When we move from the poles to the equator, the value of g decreases. Why?

Q3. If two stones of 150 gm and 500 gm are dropped from a height, which stone will reach the surface of the earth first and why ? 

Q4. Differentiate between weight and mass.

Q5. Why is the weight of an object on the moon 1/6th its weight on the earth??

Also Read: CBSE Class 9 Science Chapter Notes - All Chapters


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