Ivchenko recently provided a simple mathematical description and a nice animation of a ball colliding head-on with two other balls at rest but connected by a spring.1 He was interested primarily in calculating the amount of energy lost due to vibration of the second pair of balls. In order to do that, he assumed that the collision between the incident ball and the first ball of the spring-connected pair was perfectly elastic. He concluded that the collision was actually inelastic since energy was subsequently lost due to vibration of the two connected balls. In effect, he assumed that the coefficient of restitution was unity to calculate the ball speeds immediately after the initial collision, then found that the coefficient of restitution was less than unity due to the subsequent energy loss.

There is nothing wrong with Ivchenko’s calculations but it raises an interesting question. That is, what is the...
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