How did Newton know that there is force between two mass?

Question

Newton's Law of Gravitation states:

$$F=\frac{GM_1M_2}{r^2}$$

How did he recognize that there is a force between two masses?

Answer 1

Sir Isaac Newton spent years studying math, physics and astronomy and eventually he theorized the laws of motion. It is really hard to describe/understand HOW he did it since he was the first in his field. Basically he did like any other scientist do: He made an observation and thereafter he created the model to predict future behaviour of such events. He observed the planets and stars as well as objects on earth behave under influence of a force and then he created a model that fit the observed behaviours.