The observation by Thomas Kane a few years ago, that long-used relationships for predicting post-collision motion of a system of rigid bodies can imply a significant increase in kinetic energy during collision, has revived interest in this type of problem. This paper is intended to clarify understanding of the sources of this difficulty, and to suggest an alternative to some of the previously used assumptions for making such predictions. An organization of the pertinent equations of kinetics is presented, which provides a more direct means of examining the aforementioned question and of obtaining rebound predictions.