  We investigate the velocity dependence of kinetic friction with a model which
makes minimal assumptions on the actual mechanism of friction so that it can be
applied at many scales provided the system involves multi-contact friction.
Using a recently developed master equation approach we investigate the
influence of two concurrent processes. First, at a nonzero temperature thermal
fluctuations allow an activated breaking of contacts which are still below the
threshold. As a result, the friction force monotonically increases with
velocity. Second, the aging of contacts leads to a decrease of the friction
force with velocity. Aging effects include two aspects: the delay in contact
formation and aging of a contact itself, i.e., the change of its
characteristics with the duration of stationary contact. All these processes
are considered simultaneously with the master equation approach, giving a
complete dependence of the kinetic friction force on the driving velocity and
system temperature, provided the interface parameters are known.
