Quando la termodinamica incontra la meccanica: bilanci energetici di oggetti in movimento

In the study of subjects at the borderline between Thermodynamics and Mechanics, typical aspects of one of the two sciences are sometimes overlooked and incomplete, contradictory or partially erroneous descriptions are given. We find this type of difficulty in situations where there are objects in motion and, at the same time, variations in temperature and/or heat exchanges. It is therefore necessary to clarify the concepts of work, heat, internal energy and the connection between the macroscopic and microscopic levels. In this article I offer my point of view on the definition of the internal energy, I present a few results and definitions concerning the work and energy of the systems of material points, I discuss the variations in energy in cases of friction. I shall try to demonstrate, among other things, that the total work of the forces of friction always equals zero.