The first man who came up with the idea of gravity was Isaac Newton. It was an evening in 1665 when he was trying to solve the mystery of why planets revolve around the Sun. Suddenly an apple fell from the tree under which he was sitting. The idea of gravity flashed in his mind. He discovered not only the cause of the falling apple but also the cause that makes the planets revolve around the Sun and the moon around the Earth. This unit deals with the concepts related to gravitation.