# Gravitation

Question 1 State the universal law of gravitation?

Question 2 Define gravitation constant?

Question 3 What is the value of G on earth and on the moon?

Question 4 Which force is responsible for the moon revolving round the earth?

Question 5 Describe how the gravitational force between two objects depends on the distance between them?

Question 6 What happen to the gravitational force between object when the distance between them is doubled and halved?

Question 7 What is centripetal force.Explain?

Gravitation
A force is necessary to produce motion in a body,to change the speed or direction of motion of an object.

If we drop a stone from height,the stone falls down towards the earth.Because the earth exert a force of attraction on the stone and pull it down.

Issac newton was sitting in his garden under a tree.When an apple from tree fell on him.He said that an apple fall down from tree towards earth because the earth exerts a force of attraction on the apple in the downward direction.

This force of attraction exerted by earth is called gravity.

The force with which the earth pulls the objects towards it is called gravitational Force of Earth.

The gravitational force of earth is responsible for holding the atmosphere above earth,for rain falling on earth,for flow of water in rivers,keeps us firmly on ground,ball thrown upwards also falls back to earth due to gravity.

Every object in this universe attract every other object with a certain force.The force with which two objects attract each other is called gravitational force.

If the masses of object are large,gravitational force between them is very large.

The force that cause acceleration and keeps the body moving along the circular path is acting towards the centre.This is called centripetal force.

For Ex:Take a piece of thread,tie a small stone at one end,hold the other end of thread and whirl it round,note the motion,release it,note the motion.

Observation:Before the thread is released,stone moves in a circular path with certain speed and changes direction at every point.The change in direction at every point.The change in direction involves change in velocity.

The motion of moon around the earth is due to centripetal force.

Universal Law of Gravitation or Newton’s Law of Gravitation

This law states that every body in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to square of distance between them.

m1  m2
F=————-
r2

This formula is applicable anywhere in this universe.

If we double distance between two bodies then force become one-fourth.

If we half distance between two bodies then force become four times.

This law is called universal law because it is applicable to all bodies having mass.
The value of Universal gravitation constant(G) has been found to be 6.67 x 10 11 N  m 2 kg 2
The small value of G tells us that force of gravitation between any two ordinary objects will be very very weak.
The earth has a very large mass and this force is quite large between earth and other objects.