The
model hysteresis curve bears a resemblance to the tennis ball
data, for an appropriate choice of g, but it does not give a
good fit and is not relevant to any of the other balls. There is
no evidence of any velocity-dependent force acting on any of
the other balls, since F50 at t50 for all of the balls. Consequently,
the energy loss in all cases appears to be due to a
time-dependent relaxation of the internal stresses in the ball.
Such an effect is referred to simply as an ‘‘elastic aftereffect’’
in the rheology literature