The following law that describes the force of attraction that exists between two particles, assumed to be isolated from all other bodies. Consider two particles P and S, with gravitational masses m and M, respectively. Let r be the position vector of P relative to S (that is, the vector represented by and let r=|r|. Then the force F experienced by P is given bywhere 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.