where ____. For the completely elastic and rough superball, ___, the multiple collision matrix reduces to
which means that after two bounces, the ball returns to its initial state. Figure 2(a) illustrates this sequence for a ball with zero initial horizontal velocity and c<0, dropped from an arbitrary height. This result also is a consequence of the symmetry under time-reversal 14,15 and an explicit expression for M is not necessary in order to derive result. For 1 <B<1, because B'"-+0 as n-oo, the collision matrix becomes lim M (5) (a+ 1) When B 0, it is straightforward to show that v c, which means that the ball undergoes simple rolling at the point of contact after the first bounce. Thus for -1<B<1, Eq. (15) indicates that the ball undergoes a transition to simple rolling as n- oo. Figure 2(b) shows this transition for the initial conditions used in Fig 2(a). This transition captures an as- pect of superball motion that many find very useful; it is much easier to pick up a dropped ball if you wait until the erratic bouncing decays to simple rolling. Finally, consider the eigenvalues and eigenvectors of M The first eigenvalue for this matrix is A 1 and the core. responding eigenvector is v(I) (16) This result can be understood by noting that the velocity at the point of contact for this eigenvector is (vr+ c) (-c c) 0. The point of contact is stationary with respect to the surface, and therefore there is no exchange of energy be- tween the translation motion parallel to the surface and the