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Friction

What about not-so-conservative forces? In the real world a lot of energy gets dissipated through what is loosely known as friction. Nowhere will you find an entirely satisfactory definition of precisely what friction is, so I won't feel guilty about using the cop-out and saying that it is the cause of all work that does not ``get stored up as potential energy.'' That is, when I do work against frictional forces, it will not reappear as kinetic energy when I ``let go.''

Where does it go? We have already started getting used to the notion that energy is conserved, so it is disturbing to find some work just being lost. Well, relax. The energy dissipated by work against friction is still around in the form of heat, which is something like disordered potential and kinetic energy.^11.14 We will talk more about heat a few chapters later.