I hadn't played snooker since I was in the Sixth Form. I still can't get the angles right. But this is such a brilliant example of the laws of mechanics that there should really be one in every lab. The idea is that momentum is preserved in each collision. The balls have equal mass. An elastic collision is one in which momentum is conserved AND kinetic energy is conserved. In this case kinetic energy cannot be conserved because you hear the click of one ball into another - some kinetic energy is transformed into sound. I shot the white ball with initial velocity u into a red ball that was initially at rest. After the collision both balls moved on. Say the white ball has v1 and the red ball v2. Here are the equations for conservation of momentum and kinetic energy:
Combining the equations you get
This means that kinetic energy can only be conserved if v1=0 and the white ball stops dead. So the fact that the white ball goes on a bit is evidence of an inelastic collision. I hadn't realised that before.
