II. KINEMATICS OF A PARTIALLY INELASTIC
SUPERBALL
The simple model of an ultraelastic rough ball or superball
begins with two assumptions about the ball during its collision
with a flat surface:1,2 ~a! the kinetic energy is conserved
during the collision, and ~b! there is no slip at the point of
contact. Because the collision is assumed to occur at a point
on the ball’s surface, all of the forces act through this point
and the torque is zero. As a result, the angular momentum
about the point of contact is conserved during the collision.
From these two conservation laws, the spin and velocity
parallel to the surface after the collision are expressed in
terms of the initial spin and velocity of the ball: