A. kinetic energy.
Friction converts kinetic energy into heat, and so it represents a net loss of mechanical energy. When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into heat. As demonstrated by the use of friction created by rubbing pieces of wood together to start a fire, kinetic energy is converted to heat whenever motion with friction occurs. The friction produced by brake pads in a car must generate a quantity of heat equal to the kinetic energy of the car and as a result, the brakes get quite hot.