© Blayne Heckel.
According to Newton's law of universal gravitation, a point mass, m1, exerts a gravitational force, , on another point mass, m2. The gravitational force is attractive, proportional to the product of the masses, and inversely proportional to the square of the distance, r, between the masses. Mass, m2, exerts an equal and opposite force on m1. The force is directed along the line connecting the centers of the two particles, denoted by . (Unit: 3)