On the mechanical power transfer and unit-cell averaged energy-current density in general lattices

An elementary derivation is giveni) for the power transfer in a general system of coupled masses within the framework of classical mechanics andii) for the instantaneous energy-current density which is defined as an average over each unit celln. As a first application of these general expressions, it is shown for lattice waves in a general periodic harmonic lattice that the time average of the mentioned energy-current density is equal to the mean energy density times the group velocity.