Work and Energy in the Presence of Friction: The Need for a Mesoscopic Analysis

This paper deals with some of the difficulties involved in correctly linking the mechanical and thermodynamic descriptions of situations where there are bodies in motion and changes in temperature and/or heat exchanges. It proposes and discusses a definition of internal energy and shows that the total work of the non-conservative forces, such as frictional forces, always equals zero. The energy balances of two bodies in motion in the presence of sliding friction are analysed in this way; the calculation of the work of frictional forces is discussed, taking into account the mesoscopic level.