Kepler's law for gravity

 What do you observe when you drop a stone from a height? The stone which was initially at rest starts moving towards the ground and attains a maximum speed just before it touches the ground. Is the stone moving with a constant speed? No, it is not moving with uniform speed. Its speed keeps changing at every instant, which means that the stone is accelerating. According to Newton's second law of motion, a force is required to produce an acceleration in a body. No force was exerted on the stone. Where did the force come from? The solution for this problem was given by Sir Issac Newton, when he saw an apple falling from a tree. His argument was that the Earth attracts the apple and the apple attracts the Earth. The force exerted by the Earth on the apple is very large and hence the apple lands on Earth. Whereas the apple is not able to pull the Earth, because the force exerted by it on the Earth is very small. Thus we can conclude that the acceleration produced in the stone is due to the large force of attraction exerted by the Earth, which is very massive.

 Fig 4.1 Acceleration produced in the stone is due to the large force of attraction exerted by the Earth

It is clear from the above example that it is this force of attraction that holds our complex universe together, keeps the moon revolving around the Earth and holds all the planets in its orbit around the Sun and it is this force which helps us to walk properly on the surface of the Earth. This type of force of attraction which exists between any two objects in the universe is known as force of gravitation or gravitation.
The force of attraction or force of gravitation between Earth (or any planet) and any other material objects in the universe is known as force of gravity or gravity.

Kepler's laws

Johannes Kepler derived three laws, which govern the planetary motion. These are,
1. The orbit of a planet is an ellipse with the Sun at the one of the foci. 2. The line joining the planet and the Sun sweeps equal areas in equal intervals of time.
3. The cube of the mean distance of a planet from the Sun is proportional to the square of its orbital period. Sir Isaac Newton used the third law of Kepler to show how gravity decreases with distance.