Kinetic Friction by a Small Number of Intervening Inelastic Particles between Rough Surfaces

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.