Dynamics of a grain on a sandpile model.

The dynamics of a macroscopic grain rolling on an inclined plane composed of fixed identical grains is investigated both experimentally and theoretically. As real sand, the system exhibits an hysteretic transition between static and dynamical states: for angles smaller than straight phi(d), the roller always stops, for angles larger than straight phi(s), it spontaneously starts rolling down. But for angles between straight phi(d) and straight phi(s), it can be either at rest or in motion with a constant velocity.