**Law of Universal Gravitation** The forces of interaction between bodies has been the subject of research for many years, great geniuses of mathematical physics have devoted many efforts to understand and explain how mutual attraction between two bodies occurs.

Sir Isaac Newton and Albert Einstein managed to describe and explain the phenomenon from different points of view, the first from the point of view of classical mechanics and the second from the point of view of relativistic mechanics.

## Explanation

The law of gravitation expressed in this way is valid only when the dimensions of the bodies are small compared to the distances between them, that is, when the bodies can be considered as material points.

When determining the force of mutual attraction between two bodies that cannot be considered as points, we must proceed in the following way. A body is divided into particles so small that they can be taken as points, in the second body a particle is chosen and the resultant of the attractive forces by all the particles of the first body is determined. Then the same is done with all the other particles of the second body and the sum is taken; that sum represents the force with which the first acts on the second. The force acting on the first body is determined by the third law .

Calculations made for spheres of homogeneous material have shown that the resulting gravitational force is applied to the center of each sphere and complies with Newton’s stated law.

The first measurement of a force of gravity was made by Cavendish in 1798 with the help of a torsion balance. This same principle was also later used to measure gravitational forces.