This paper is concerned with the non-colinear impact of two elastic
spheres of similar materials. It is convenient to restrict attention to the
case of a sphere impacting a half-space, since the more general problem
presents no new features. It will be assumed that the coefficient of sliding
friction ~1 between the surfaces is a constant and that friction is the only
source of energy dissipation (and hence that the coefficient of restitution
in a normal impact is unity).