  The dynamics and the transition to the centrifugal regime are studied
analytically and numerically for particles in rotating drum. The importance of
the particle-wall friction coefficient is demonstrated by studying first the
motion of one non-rotating particle where three different regimes are found in
the transition to the centrifugal motion. When a few rotating particles are
considered, they behave similarly to one non-rotating particle in the low
friction limit. A critical particle number is necessary to reach the
centrifugal regime for which an analytic expression is derived in the limit of
negligible inter-particle friction.
