VIII. 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 official
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 specified
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.