The calculus has survived the test of time because it has proved to be both practical and useful. The derivative quantizes how one quantity varies when a related quantity changes. The sections in Chapter 3 cover different applications of the derivative, some geometric, some physical, some analytical. There are many other fields and applications - biology, finance, etc., that could have appeared in Chapter 3. But with a solid foundation in the meaning of the derivative, and exposure to a handful of applications, it should be possible to interpret any number of other applications in more diverse fields of study.