Most of us have an intuitive concept of energy that goes something like this:

Energy is the stuff we need to accomplish physical actions such as walking, lifting a glass, heating some water, or powering a television set.

Although this definition is correct, its a bit indirect because it really only conveys to us what energy is used for, not what energy is, or even how it behaves (for example, what happens to it after you use it?). A curious person might still ask questions like: Is energy a thing? Or is it a property or a condition of a thing? How do we really define it? How was it discovered? What are its properties? These are some of the questions we will try to answer in this and following sections, as completely, briefly, and simply as possible.

With perhaps the exception of energy in the form of light, energy is not a thing per se. Rather, energy refers to a condition or state of a thing.

As we will discuss in more depth later, a book sitting on a table, for example, possesses energy ("potential energy") because of its condition of being able to fall if nudged off the table. A ball flying through the air has energy ("kinetic energy") because of its relative velocity with respect to the ground, and it also possesses potential energy because of its height above the ground.

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