When rubber slides on a hard, rough substrate, the surface asperities of th
e substrate exert oscillating forces on the rubber surface leading to energ
y "dissipation" via the internal friction of the rubber. I present a discus
sion of how the resulting friction force depends on the nature of the subst
rate surface roughness and on the sliding velocity. I consider in detail th
e case when the substrate surface has a self affine fractal structure. I al
so present a theory for the area of real contact, both for stationary and s
liding bodies, with elastic or elastoplastic properties. The theoretical re
sults are in good agreement with experimental observation. (C) 2001 America
n Institute of Physics.