The break in the curve at maximum compression is
due to relaxation of internal stress in the ball during the one
minute pause between the compression and expansion
cycles. The static and dynamic hysteresis curves cannot be
compared directly since ~a! the dynamic curve is plotted as a
function of the y displacement of the center of mass, and the
static curve is plotted as a function of the ball compression,
x; and ~b! both sides of the ball were compressed equally in
the static test,