where vx is the velocity parallel to the surface, I5amr2 is
the moment of inertia of the ball, c5ru˙ is the peripheral
velocity of the ball’s surface, u˙ is the rotational velocity or
spin of the ball, and r is the radius of the ball. In this notation,
a ball with c.0 would have backspin while a ball with
c,0 would have topspin. As a result of these conserved
quantities, there also is a reversal of the surface velocity of
the ball at the point of contact