DISCUSSION
In this paper, dynamic hysteresis curves have been presented for a number of common ball types bouncing off a heavy brass rod. The results indicate that all balls studied ~apart from the plasticene ball! rebound in a slightly compressed state, but the major energy loss occurs during the bounce rather than after the bounce. The study was limited to impacts at low ball speeds off a flat surface. The technique could easily be extended to study impacts at higher speeds or to study other balls. Such a study would be particularly useful in regard to the testing and approval of balls used in ball sports.11 The current rules regarding tennis balls are quite specific regarding static compression tests, although the specified equipment to be used is relatively ancient and somewhat operator dependent. There are no rules at all regarding the static compression of a golf ball or a baseball. In regard to dynamic tests, a tennis ball must have a COR of 0.745 62.3% when dropped from a height of 100 in. onto a concrete slab. There are no rules regarding the COR of a tennis ball in a high-speed collision. Surprisingly, there are no of- ficial rules at all concerning the COR of a baseball. The dynamic rule for a golf ball is that it must not travel faster than 250 ft ~76.2 m! per second when hit by apparatus speci- fied in the rules. Particularly in the case of tennis balls, where a wide range of pressurized and unpressurized balls are manufactured to meet current specifications, it is observed that different balls can behave quite differently under actual playing conditions. The techniques described in this paper would provide a useful method of distinguishing and understanding these differences.