  We investigate a mechanism of the appearance of kinetic friction in granular
materials. We consider a small number of intervening inelastic particles
between two rough surfaces as one of the simplest dynamical models to study
granular friction. The resistance force applied to the upper surface is
numerically calculated. We find that the resistance force F(t) is scaled as
F'(vt) for a small pulling velocity v. The time average F_0=<F(t)> in the limit
v->0 is not zero owing to the mutual collisions between the intervening
particles. The nonzero F_0 implies the appearance of kinetic friction in this
simple dynamical system.
