  The suppression of friction between sliding objects, modulated or enhanced by
mechanical vibrations, is well established. However, the precise conditions of
occurrence of these phenomena is not well understood. Here we address these
questions focusing on a simple spring--block model, which is relevant to
investigate friction both at the atomistic as well as the macroscopic scale.
This allows to investigate the influence on friction of the properties of the
external drive, of the geometry of the surfaces over which the block moves, and
of the confining force. Via numerical simulations and a theoretical study of
the equations of motion we identify the conditions under which friction is
suppressed and/or recovered, and evidence the critical role played by surface
modulations and by the properties of the confining force.
