  A decade ago, Budakian and Putterman (Phys. Rev. Lett., {\bf 85}, 1000
(2000)) ascribed friction to the formation of bonds arising from contact
charging when a gold tip of a surface force apparatus was dragged on
polymethylmethacrylate surface. We propose a stick-slip model that captures the
observed correlation between stick-slip events and charge transfer, and the
lack of dependence of the scale factor connecting the force jumps and charge
transfer on normal load. Here, stick-slip dynamics arises as a competition
between the visco-elastic and plastic deformation time scales and that due to
the pull speed with contact charging playing a minor role. Our model provides
an alternate basis for explaining most experimental results without ascribing
friction to contact charging.
