For a partially inelastic, partially smooth ball, both the
assumption of energy conservation and no slip at the point of
contact do not necessarily apply. In realistic collisions, there
exists the possibility of sliding at the point of collision as
well as a loss of tangential strain energy that is stored during
the collision.4–7 In many studies of the dynamics of systems
of inelastic spheres, such as in granular flows8 and granular
gases,9 the models of the particle collisions attempt to take
these rotational energy losses into account. The simplest
model involves the addition of a coefficient of tangential
restitution into the change in the surface velocity, Eq. ~3!,
such that there is no longer a perfect reversal of the velocity
of the ball’s surface at the point of contact. This simplistic
model does not capture the more complicated dynamics of
slip and elastic restitution over the area of contact between
the ball and the surface,5,10 but it does provide a simple extension
of the original superball description to capture additional aspects of the ball’s motion. Although the addition of a
tangential coefficient of restitution to Garwin and Strobel’s
original equations was recently presented by Cross,3,11 it is
revisited here with particular attention to its implications for
the dynamics of the superball.