It turns out that gravitational potential and spring potential energies we discussed above are examples of one of the two fundamental types of energy–the type that depends on the positions of masses. An energy that depends only on the relative positions of the particles (objects), and not on their past history (the path they took) or on their speeds, is called a potential energy. “Positional energy” is a more descriptive word, but for historical reasons, these are labeled potential energy.
The second fundamental type of energy depends only on the speeds of particles (objects). This kind of energy is called energy of motion or kinetic energy.
All of the various kinds of energy fit into one of the two fundamental types. For example, chemical bonds involve the electric potential energies that depend on the locations of the electrons of the atoms as well as their kinetic energies as they whirl around the nucleus. Thermal energy is a combination of the kinetic and potential energies of individual atoms due to their random motions. Bond energy is a potential energy due to the force individual atoms exert on each other. The elastic energy of a spring is a potential energy because it depends only on the positions of the elements of the spring, not on their speeds. When an object rotates, it has energy due to its rotational motion—rotational kinetic energy.
All Energy Types Are Created Equal... Or Are They?
One energy is as good as any other—almost. On a microscopic scale, energy is energy and any kind can be turned into any other kind. But on a macroscopic scale, where lots of particles (atoms and molecules) are involved, it turns out that all kinds of energy can be turned into thermal energy, but there are restrictions on turning thermal energy back into other kinds. We can convert some, but not all thermal energy to other forms. (We delve into the mysteries of entropy and the second law of thermodynamics in Chapter 4.)