Collisions Between Rods: A Visual Analysis

One of the key subjects in introductory physics is the problem of collisions. It provides a nice example where conservation laws of energy and momentum are essential. Two extreme cases are usually solved: elastic and perfectly inelastic collisions. In the very simple one-dimensional case, velocities before and after collision are readily related through the masses of the colliding bodies. Similar solutions can be found for partially inelastic collisions, provided that the degree of energy loss is known. Otherwise, the energy balance equation cannot be written down. Usually, one can reasonably assess whether the collision is perfectly inelastic (for instance, a bullet impinging onto a piece of wood). However, it is a matter of faith to consider a priori a collision as elastic or as being in any intermediate situation. We hope this statement will become clearer to the reader by the end of this paper.