Kinetic energy is the energy of a moving object. The basic energy model says that kinetic energy can be transformed into potential energy without loss, but where is the potential energy? If energy is real, not just an accounting fiction, what is it that has potential energy?
Potential energy is stored in fields. We’ve not yet introduced fields in this textbook, although we’ll have a lot to say about electric and magnetic fields in later chapters. Even so, you’ve no doubt heard of magnetic fields and gravitational fields. Our modern understanding of the fundamental forces of nature, the long-range forces such as gravitational and electric forces, is that they are mediated by fields. How do two masses exert forces on each other through empty space? Or two electric charges? Through their fields!
When two masses move apart, the gravitational field changes to a new configuration that can store more energy. Thus the phrase “kinetic energy is transformed into gravitational potential energy” really means that the energy of a moving object is transformed into the energy of the gravitational field. At a later time, the field’s energy can be transformed back into kinetic energy. The same holds true for the energy of charges and electric fields, a topic we’ll take up in Part VII.
What about elastic potential energy? Remember that all solids, including springs, are held together by molecular bonds. Although quantum physics is needed for a complete understanding of bonds, they are essentially electric forces between neighboring atoms. When a solid is placed under tension, a vast number of molecular bonds stretch just a little and more energy is stored in their electric fields. What we call elastic potential energy at the macroscopic level is really energy stored in the electric fields of a vast number of molecular bonds.
The theory of field energy is an advanced topic in physics. Nonetheless, this brief discussion helps complete our picture of what energy is and how it’s associated with physical objects.