  The motion of an elastic body over a rough surface is simulated in one
dimension using Newtonian dynamics. We drive the body by prescribing a constant
velocity. We extract the friction force from the work done by the applied force
in order to maintain the motion of the body. A positive amount of work is
needed because energy is converted into the internal elastic degrees of freedom
of the body. The velocity dependence of the friction force is studied as
function of the roughness of the surface and the elastic properties of the
mobile body. We pay special attention to the low velocity limit. It is shown
that the friction force goes to a finite value as the velocity goes to zero
when the substrate is sufficiently rough to induce elastic instabilities. For
smooth substrates, where elasitc instabilities are absent, the friction force
goes to zero when the velocity goes to zero.
