When balls collide. As an example, consider a billiard ball travelling with speed V 1 colliding head-on with a second ball that is initially at rest. Assume that the balls have equal masses and that the collision is elastic. The first thing to do is to write down the total momentum of the system before and after the collision. So the 2nd ball rolls off with the same velocity that the incoming ball had.
Elastic collisions conserve kinetic energy.
Intro to muscles.