  We propose a statistical model for static and sliding friction between rough
surfaces. Approximating the contact between rough surfaces by the contact of an
ensemble of one-dimensional viscoelastic elements with a rough rigid surface,
we study the collective behavior of the elements. We find that collective
response of the contacts can lead to macroscopic behavior very different from
the microscopic behavior. Specifically, various observed features of friction
emerge as collective phenomena, without postulating them directly at the
microscale. We discuss how parameters in our model can be related to material
and surface properties of the contacting surfaces. We compare our results to
commonly used rate and state phenomenological models, and propose a new
interpretation of the state variable.
