  We present a theoretical study of the dynamics of a tip scanning a graphite
surface as a function of the applied load. From the analysis of the lateral
forces, we extract the friction force and the corrugation of the effective
tip-surface interaction potential. We find both the friction force and
potential amplitude to have a power law dependence on applied load with
exponent $\sim 1.6$. We interpret these results as characteristic of sharp
undeformable tips in contrast to the case of macroscopic and elastic
microscopic contacts.
