Galileo's cannonball in the opening drawing converts potential energy to kinetic energy as it falls faster and faster.

Heat is a form of energy, too, but in a degraded form. On the molecular level, heat is the uncoordinated vibration or motion of individual molecules. It is easy to convert work or other kinds of energy into heat. If you rub one hand hard against another on a cold day, the warmth generated by friction is just a conversion of the work that you do into heat.

When Galileo's cannonball strikes the pavement, the potential energy that had first changed into kinetic energy of motion as it fell, is changed again into heat as it collides with the ground. The reverse transformation of heat into motion, as in an automobile engine, is accomplished only with difficulty; not all of the heat can be converted.

We will come back to these ideas when we talk about order and disorder in the next chapter. For the moment, the important idea is that potential energy, kinetic energy, heat, and work all are different kinds of energy, and with certain limitations they can be interconverted from one form to another.