  We present a model for the motion of hard spherical particles on a two
dimensional surface. The model includes both the interaction between the
particles via collisions, as well as the interaction of the particles with the
substrate. We analyze in details the effects of sliding and rolling friction,
that are usually overlooked. It is found that the properties of this
particulate system are influenced significantly by the substrate-particle
interactions. In particular, sliding of the particles relative to the substrate
after a collision leads to considerable energy loss for common experimental
conditions. The presented results provide a basis, that can be used to
realistically model the dynamical properties of the system, and provide further
insight into density fluctuations and related phenomena of clustering,
structure formations, and inelastic collapse.
