  In order to describe the velocity of two bodies after they collide, Newton
developed a phenomenological equation known as "Newton\' s Experimental Law"
(NEL). In this way, he was able to practically bypass the complication
involving the details of the force that occurs during the collision of the two
bodies. Today, we use NEL together with momentum conservation to predict each
body\'s velocity after collision. This, indeed, avoids the complication of
knowing the forces involved in the collision, making NEL very useful. Whereas
in Newton\'s days the quantity of kinetic energy was not known, today it is a
basic quantity that is in use. In this paper we will use the loss (or gain) of
kinetic energy in a collision to show how NEL can be derived.
