  Using molecular dynamics we study the dependence of the friction force on the
sliding speed when an elastic slab (block) is sliding on a rigid substrate with
a ${\rm sin} (q_0 x)$ surface height profile. The friction force is nearly
velocity independent due to phonon emission at the closing and opening crack
tips, where rapid atomic snap-in and -out events occur during sliding. The
rapid events result from lattice trapping and are closely related to the
velocity gap and hysteresis effects observed in model studies of crack
propagation in solids. This indicates that the friction force is dominated by
processes occurring at the edges of the contact area, which is confirmed by
calculations showing that the friction force is independent of the normal
force. The friction force increases drastically when the sliding velocity
approaches the solid transverse sound velocity, as expected from the theory of
cracks.
