We’ve talked a lot about forces, but there is another physics concept that is perhaps equally important: *energy*. From our point of view, the important thing is that energy concepts sometimes provide an alternative way to solve problems that can prove tricky or even impossible to solve using the approach of forces. Just like momentum, this is because there is a powerful law of conservation of energy (which we’ll talk about shortly). In fact, conservation of energy and momentum can be applied together to solve problems involving collisions, as we shall see in Chapter 5 and, in more detail, in Chapter 11.

Unlike momentum, energy is a bit trickier to define. Before we can do so, we need to introduce a closely related concept—that ...

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