The work done by frictional forces has usually been calculated incorrectly. The key to a correct treatment lies in making a careful distinction between a purely mechanical integral of Newton’s second law on the one hand and the first law of thermodynamics on the other. These two equations are the same for point particles but differ for deformable systems, which include systems subject to sliding friction. A model‐independent calculation is supplemented by applications to current models for friction. Heat transfer is treated in detail for the case of lubricated friction. Rotational friction is analyzed. An invariant form of the energy equation is presented.