The collisions of real particles do not have to be either completely elastic or completely inelastic. There is a coefficient K, known as coefficient of restitution, that can vary from 0 to 1. It measures the degree of elasticity.

In addition, collisions may not be frontal. Therefore, there is an impact parameter P that varies between 1 (for a frontal collision) and 0 (for the case in which one body sweeps past the other).

The change of these two parameters allows the explanation of practically any type of collision between particles. In the following visual we will study the effects of a real collision between two billiard balls. We will assume in every case that one ball is initially at rest while the other is in motion towards it in order to make comprehension easier.