IntroductionTo understand politics, it is said, you need to follow the money. To understand physics, it is said, you need to follow the energy. If we understand where energy is coming from, how it flows, and where it ends up in any physical situation, then we understand a lot about the physics of the situation.
"What is energy?" you may ask. Actually energy is harder to explain than you may think. In the history of physics, the concept of energy did not suddenly arrive as a mature concept out of Newtonian theory, like Athena emerging from Zeus's head. Rather, it began as a hazy idea which grew in richness and clarity during the 1800s. In this chapter we can do no better, so we will introduce the concept slowly.
In standard English, energy is defined as the capacity for performing something useful. We can obtain energy from various places and then we can use it usefully or squander it, and once it is gone, it is gone.
In physics also, energy is the capacity for doing useful things. On the other hand, energy is a thing which cannot be created from nothing or destroyed, only transformed from one form to another. Following energy through its forms is what much of physics is all about.
As you can see, the physics understanding is distinct from the popular understanding, and we will want to pay attention to the differences between the two.