A force is required to change the state of rest or of motion of a body. All objects when dropped from a height fall towards the earth. Why do objects fall towards the earth?
Bodies thrown vertically upward come back to the earth. Even if an object is dropped from some height, it falls towards the earth. Similarly tree leaves and fruits fall toward the earth when they are separated from the branches. Why does it happen so? This must be due to some force acting on the bodies like leaves or fruits. What type of force is acting on them? It was Issac Newton who answered this question.
There is an interesting story about Newton. It is said that while Newton was sitting under an apple tree, an apple fell on him. The fall of the apple set Newton thinking, why did the apple fall down? If some force is acting on the apple then it must be in accelerated motion.
Newton knew that bodies fall towards the earth due to force of gravity. He further thought, if the earth can attract an apple or a stone, can it also attract the moon? He was also curious to know whether the same force was responsible for keeping the planets go around the sun in their orbits.
Newton concluded that in order to move in a circular orbit the moon must be attracted by the earth continuously. Arguing in the same lines he said that there exists a force between the sun and the planets. The force is known as the gravitational force. He stated that gravitational force exists everywhere in the universe. All objects in the universe attract each other. It is always attractive whatever may be the size of bodies.