Newton"s third law of motion states that for every action force there is a reaction force that has the same magnitude but points in the opposite direction. Newton"s laws of motion can be used to predict the paths of object in the night shy such as the moon, the planets, and the stars. Newton developed a simple expression for the gravitational force between two objects of mass m and mass m that are separated by a distance r. m. This expression does not depend on the composition of the object. The unit of force is called a newton, in honour of sir isaac newton. If mass m is in kilograms kg and acceleration a is in m/s2.