The motion of the planets, the moon and the Sun was the interesting subject among the students of Trinity college at Cambridge in England.

Isaac Newton was also one among these students. In 1665, the college was closed for an indefinite period due to plague. Newton, who was then 23 years old, went home to Lincolnshire. He continued to think about the motion of planets and the moon. One day Newton sat under an apple tree and had tea with his friends. He saw an apple falling to ground. This incident made hint to think about falling bodies. He concluded that the same force of gravitation which attracts the apple to the Earth might also be responsible for attracting the moon and keeping it in its orbit. The centripetal acceleration of the moon in its orbit and the downward acceleration of a body falling on the Earth might have the same origin. Newton calculated the centripetal acceleration by assuming moon’s orbit (Fig) to be circular.

Acceleration due to gravity on the Earth’s surface, **g = 9.8 m s ^{-2}**

Centripetal acceleration on the moon, **a _{c} = v^{2}/r**