Size-scale effects on the friction coefficient

An overview of the classical friction laws holding at the macro-scale and the new developments at the nano-level are proposed. Furthermore, two opposite phenomena are addressed: the former concerning the apparent weakness of the San Andreas fault, the latter regarding the strong frictional behavior which appears at the nano-scale. An interpretation of these size effects on the friction coefficient is attempted making use of the renormalization group procedure which allows to explain the frictional phenomena over all the scales.