Using Knight's Physics : A Strategic Approach (3rd edition): I am on the section concerning dissipative forces. A box is pulled with a rope on a surface with friction at a constant speed. The book says Wtension=Change in thermal energy. By Newton's First Law, the book says: Change in thermal energy=TΔs=kinetic friction times Δs. Then it says: "You might wonder why we didn’t simply calculate the work done by friction. The rather subtle reason is that work is defined only for forces acting on a particle. [...] There is work being done on individual atoms at the boundary as they are pulled this way and that, but we would need a detailed knowledge of atomic-level friction forces to calculate this work. The friction force fk is an average force on the object as a whole; it is not a force on any particular particle, so we cannot use it to calculate work. Further, increasing thermal energy is not an energy transfer from the book to the surface or from the surface to the book. Both book and surface are gaining thermal energy at the expense of the macroscopic kinetic energy." The book seems to be contradicting itself. Everywhere on the web, I see problems where we find the work done by friction with formula W=FΔs. Why is Wtension okay but not Wfriction? Why are there so many questions with Wfriction if it is wrong?