Nonconservative forces cannot be represented in terms of potential energy. But the effects of these forces can be described in terms of kinds of energy other than kinetic and potential energy. When a car with locked brakes skids to a stop, the tires and the road surface both become hotter. The energy associated with this change in the state of the materials is called internal energy. Raising the temperature of a body increases its internal energy; lowering the body's temperature decreases its internal energy.
To see the significance of internal energy, consider a block sliding on a rough surface. Friction does negative work on the block as it slides, and the change in internal energy of the block and surface (both of which get hotter) is positive. Experiments show that the increase in internal energy is exactly equal to the absolute value of the work done by friction:
ΔUint = −Wother
where ΔUint is the change in internal energy. If