  Wearless dry friction of an elastic block of weight N, driven by an external
force F over a rigid substrate, is investigated. The slider and substrate
surfaces are both microscopically rough, interacting via a repulsive potential
that depends on the local overlap. The model reproduces Amontons's laws which
state that the friction force is proportional to the normal loading force N and
independent of the nominal surface area. In this model, the dynamic friction
force decays for large velocities and approaches a finite static friction for
small velocities if the surface profiles are self-affine on small length
scales.
