# Gravitation

## Introduction

• In our dairy life we have noticed things falling freely downwards towards earth when thrown upwards or dropped from some height.
• Fact that all bodies irrespective of their masses are accelerated towards the earth with a constant acceleration was first recognized by Galileo (1564-1642).
• The motion of celestial bodies such as moon, earth, planets etc. and attraction of moon towards earth and earth towards sun is an interesting subject of study since long time.
• Now few question arises that are
(a) what is the force that produces such acceleration with which earth attract all bodies towards the center
(b) What is the law governing this force.
(c) Is this law is same for both earthly and celestial bodies.
• Answer to this question was given by Newton as he declared that
Laws of nature are same for earthly and celestial Bodies.
The force between any object falling freely towards earth and that between earth and moon are governed by the same laws.
• Kepler (1571-1631) Studied the planetary motion in detail and formulated his three laws of planetary motion, which were available Universal law of gravitation.

• You must have observed that whenever you throw any object upwards it reaches a certain height and then falls downward towards the Earth. So these objects are acting under the gravitational pull of the Earth or gravitational forces which are forces of attraction.
• Gravitational force or gravity of earth is responsible for pulling you and keeping you on earth.
• Now each and every object in this universe that has mass exerts a gravitational force on every other mass and the size of that pull depends on how large or small are the masses of two objects under consideration.
• So for smaller masses like two human beings the gravitational force of attraction is very small and is negligible because two peoples are not very massive
• Now when you consider massive objects like planets, Sun, Earth, Moon or other celestial bodies, the gravitational pull becomes very strong.
• So here you must note that gravitational force depends on how massive objects under consideration are.
• Gravity is very important on earth . It is the gravitational pull of earth that keeps our planet orbiting round Sun.
• The motion of moon is also affected by both Sun and Earth.

## Why don't Moon Fall down

• You must wonder If gravitational force is a force of attraction then why does moon not fall into earth?
• To understand this consider a person whirling a stone tied to a thread along a circular path as shown below in the figure.
• If he releases the stone then it flies along the tangent , at that point on the circular path.
• Before the release of thread it is centripetal force responsible for the motion of stone in the circular path where the stone moves with a certain speed and changes direction at every point.
• During this motion the change in direction involves change in velocity which produces acceleration. This force which is called centripetal force , causes this acceleration and keeps the body moving along the circular path is acting towards the center.
• Now when the thread is released the stone does not experience this force and flies off along a straight line that is tangent to the circular path.
• The motion of the moon around the earth is due to the centripetal force. The centripetal force is provided by the force of attraction of the earth. If there were no such force, the moon would pursue a uniform straight line motion.