Class 9 Science

# Gravitation

• Universal Law of Gravitation
• SI unit of G

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

In fact, every object in the universe attracts all other objects by this force. In other words, every object in the universe has gravitation or gravitation pull. The sun, the moon, planets and stars, even a rubber has gravitation or gravitation 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.

The distance is considered between the centres of the objects.

Suppose there are two objects having mass M and m respectively.

The distance between their centres is equal to d.

The force of attraction is F.

Therefore, from Law of Gravitation which states that force of attraction by which an object attracts other object is directly proportional to the product of their masses,

Thus, FpropM*m -----(i)

Now, Law of Gravitation also states that force of attraction by which an object attracts other object is inversely proportional to the square of distance between them.

Therefore, F prop 1/d^2 ----(ii)

Now from equation (i) and (ii) we get

F prop (M*m)/d^2
=>F=G*(M*m)/d^2 -----(iii)

Where, G is the proportionality constant and called Universal Gravitation Constant.

From equation (iii)

Fxxd^2 = G*M*m
=>G= (Fd^2)/(M*m) ----(iv)

The expression (iii) and (iv) are called expression for Universal Law of Gravitation.

This Law is applicable everywhere in universe, thus it is known as UNIVERSAL LAW OF GRAVIATION.

## SI Unit of G

The SI unit of G is Nm^2kg^(-2)

The accepted value of G is 6.673xx10^(-11)Nm^2\ kg^(-2)

The value of G was found out by Henry Cavendish, a British philosopher and scientist.

#### Importance of Universal Law of Gravitation

• This Law is applicable to every object in the universe.
• This law explains the cause of revolution of moon around earth and revolution of planets around sun.
• This law explains the cause of neap and tide due to moon and sun.