Newton's Law of Gravitation

There is a very popular story behind the law of Gravitation.
One day, Sir Issac Newton was sitting below an apple tree. At that moment, an apple felled on the ground. This phenomenon was not unusual. It happens everyday and nearly everybody has seen this phenomenon. Nobody cares to think about it. It would be better to say that nobody finds anything thinkable about this.
But this is the difference between a normal person and a genius. Newton started thing why this apple felled on ground and did not go up?
A silly question! Is'nt it? But this was the question, which led to the discovery of a law, which changed the course of Physics. He said that Earth pulls everything with a force. He named that force as "Force of Gravitation". Later, he generalised it to all bodies. He said that:

All the bodies attract each other with a force which
is directly proportional to their masses and inversely
proportional to square of distance between them.

In mathematical terms:

Where:

                m1 = Mass of first body


                m2 = Mass of second body


                r = Distance between the bodies.

Substituting a universal gravitation constant (G), we get:

Where,