Class 9 Physics Gravitation Introduction

Introduction

• If earth can attract an apple why not moon or other celestial bodies?
• If moon is attracted by earth why it does not fall  towards it?
• Why earth is not moving towards apple?

These are some questions which came to Newton when he saw an apple falling from a tree. • He conjectured that the nature of the force is same whether it is moon or earth, and earth attracts all bodies.

• Now as force is there then why there is no acceleration? Well there is acceleration, called CENTRIPETAL acceleration and the force which causes this is termed as CENTRIPETAL FORCE, it acts towards center.

• An object moving in a circle experiences centripetal force, it changes the direction of object at every moment.

These two points can explain the movement of planets around sun.

1. The motion of moon around the earth is due to centripetal force. The centripetal force is provided by the force of attraction of earth.

1. The earth is also attracted towards the apple by the same amount of force, in accordance with the Newton’s third law, but we cannot see it moving because acceleration is proportional to mass and mass of earth is very large in comparison to apple.

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