Force microscopy images of boric acid crystals were obtained experimentally and simulated with the use of a two dimensional mechanical model. An analysis of the stick and slip movement of the microscope tip shows that the energy dissipation mechanism is strongly influenced by the non-linear dynamics of the sliding system. The contributions of stick and viscous forces on the energy dissipation (or friction forces) are studied as a function of the relative scanning velocity. At low relative velocities, the stick forces are shown to be responsible for the energy dissipation. This energy is velocity dependent, due to the coupling between the two degrees of freedom of the sliding system. As the scanning velocity increases the stick forces are damped, then the viscous force is predominant in the energy dissipation process.