© Blayne Heckel.

According to Newton's law of universal gravitation, a point mass, m_{1}, exerts a gravitational force, , on another point mass, m_{2}. 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, m_{2}, exerts an equal and opposite force on m_{1}. The force is directed along the line connecting the centers of the two particles, denoted by . (Unit: 3)