  By means of Molecular Dynamics simulations, we investigate the elementary
process of avalanches and size segregation by surface flow in 2 dimensions: a
single ball confined to moving along an inclined line consisting of balls. The
global characteristics of the motion depend strongly on the size of the moving
ball relative to the size of the balls on the line, as well as the distribution
of the balls on the line. We find that in the steady state the friction force
acting on the ball is independent of material properties like the Coulomb
friction coefficient and the coefficient of restitution. Contrary to previous
notions about the details of the motion, we find that it is very regular and
consists of many small bounces on each ball on the line. As a result of this
regularity, introducing a random spacing between the balls on the line has
mainly the same influence as a regular spacing of adequate length. The
insensitivity of the steady state velocity to material properties and to the
detailed arrangement of the balls on the line allows for an analytical
estimation of the mean velocity which fits the simulation results very well. We
find that results from the 2D case can probably not be transferred to the 3D
case of a ball moving on a rough inclined plane as easily as has been suggested
previously.
