  We experimentally examine the dynamics of two-particle collisions occuring on
a surface. We find that in two-particle collisions a standard coefficient of
restitution model may not capture crucial dynamics of this system. Instead, for
a typical collision, the particles involved slide relative to the substrate for
a substantial time following the collision; during this time they experience
very high frictional forces. The frictional forces lead to energy losses that
are larger than the losses due to particle inelasticity. In addition, momentum
can be transfered to the substrate, so that the momentum of the two particles
is not necessarily conserved. Finally, we measure the angular momenta of
particles immediately following the collision, and find that angular momentum
can be lost to the substrate following the collision as well.
