INTRODUCTION
Most of us love the dear old Earth – in fact we’re quite attracted to it. That attraction arises from the Earth’s large mass and thus the Earth’s pull of gravity. When we lift a book away from the center of dear old Earth, we do work on that book. We do work because we must counteract the book’s attraction to the Earth in the process. We do work by applying a force over a distance. What does that do for us? Well, it tires us. The book, however, gets energy out of the deal. Any