  The interplay between Coulomb friction and random excitations is studied
experimentally by means of a rotating probe in contact with a stationary
granular gas. The granular material is independently fluidized by a vertical
shaker, acting as a 'heat bath' for the Brownian-like motion of the probe. Two
ball bearings supporting the probe exert nonlinear Coulomb friction upon it.
The experimental velocity distribution of the probe, autocorrelation function,
and power spectra are compared with the predictions of a linear Boltzmann
equation with friction, which is known to simplify in two opposite limits: at
high collision frequency, it is mapped to a Fokker-Planck equation with
nonlinear friction, whereas at low collision frequency, it is described by a
sequence of independent random kicks followed by friction-induced relaxations.
Comparison between theory and experiment in these two limits shows good
agreement. Deviations are observed at very small velocities, where the real
bearings are not well modeled by Coulomb friction.
