  We consider the effects of a velocity-independent friction force on
cantilever damping. It is shown that this dissipation mechanism causes
nonlinear effects in the cantilever vibrations. The size of the nonlinearity
increases with decreasing cantilever velocity. Our analysis makes it possible
to understand experiments [Stipe et al, PRL 87, 096801 (2001)] where an
amplitude dependence of the cantilever eigenfrequency and anomalous dissipation
was observed only at small amplitudes.
