Suppose you hit a stationary pool ball dead center (head-on) with another pool ball so that the collision is collinear and the balls move without spinning immediately after impact. What will be the resulting motion of both balls (Figure 10)?

The ball that was initially moving will become stationary upon impact, while the other ball will start moving in the same direction as the incoming ball. If you measure the speed of both balls just before and just after collision, you will find that the speed of the incoming ball is almost the same as that of the outgoing ball. Using the equation of kinetic energy of a body as the product

it can be shown that the final kinetic energy of the system is almost the same as the initial kinetic energy of the system.

Key terms

• Kinetic energy: energy possessed by a body due to its motion.