Serway & Jewett — Energy of a System: Problem 34.

Why is the following situation impossible? A librarian lifts a book from the ground to a high shelf, doing \(20.0\text{ J}\) of work in the lifting process. As he turns his back, the book falls off the shelf back to the ground. The gravitational force from the Earth on the book does \(20.0\text{ J}\) of work on the book while it falls. Because the work done was \(20.0\text{ J} + 20.0\text{ J} = 40.0\text{ J}\), the book hits the ground with \(40.0\text{ J}\) of kinetic energy.