Anything that is dropped falls to the ground; it does not go towards the sky. Similarly fruits from trees fall on the ground. Why does it happen like this? Questions like this used to bother the ancient man also, but he could not find their answers. But today, it is an accepted fact that the Earth attracts everything towards its centre. That is why the fruits from trees or the ball thrown up, are all attracted towards the Earth. This invisible force of attraction between the Earth and any other body is called the force of gravity. The centre of gravity of the Earth lies in its centre. Imagine what would happen if a hole is drilled in the Earth from one side to the other, passing through its centre and a ball is dropped in this hole. The ball, in fact, will stop at the centre of Earth; it will not go to the other side. The weight of a body will be more, if it is nearer the centre of gravity of the Earth. Similarly the weight will be less if the body is away from it. This is why a body weighs more at the poles than at the equator, since the poles are nearer the centre than the places on the equator. Not only the Earth, but all other planets also have this force of gravity. As a matter of fact, every body in this universe attracts every other body with this force of gravitation and it is this force which keeps all the planets and stars suspended in the sky. It is this gravitational pull that keeps the moon revolving round the Earth and the Earth revolving round the sun. Naturally the moon also attracts the Earth and tides in the sea are mainly due to the gravitational pull of the moon.
Up to the end of the fifteenth century, it was assumed that if two bodies were simultaneously dropped from the same height in vacuum, the heavier body will hit the ground first. But this assumption was baseless. The famous scientist Galileo proved it in 1590 for the first time that irrespective of their masses, all the objects dropped simultaneously from the same point in a vacuum will reach the ground at the same time. From the Leaning Tower of Pisa, he dropped one ball of 100 pounds and another of just half pound at the same time and he demonstrated it in the presence of thousands of people that both the balls hit to the ground simultaneously.
Subsequently Newton propounded the law of gravitation. According to this law, the force of attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. It follows from this that the force of attraction will be doubled if the mass of one of the two bodies is doubled. On the other hand if the distance between them is doubled, the force will be reduced to one-fourth of the initial value.
Scientists have not been able to fully explain the existence of the gravitational force of the Earth or of other heavenly bodies. The velocity of a freely falling body towards the Earth increases by 9.8 metres or 32 feet every second. This is called acceleration due to gravity.
