The following law, due to Newton, that describes the force of attraction that exists between two particles. Say particles P and S have gravitational masses m and M, respectively. Let $\mathbf{r}=\stackrel{}{SP}$→ be the position vector of P relative to S and let r = |r|. Then the force F experienced by P is given by

where G is the gravitational constant. Since r/r is a unit vector, the magnitude of the gravitational force experienced by P is inversely proportional to the square of the distance between S and P. The force experienced by S is equal and opposite to that experienced by P.