ABSTRACT
When you push a book that is lying on a polished table, part of the work that you do on the book is converted into kinetic energy and the book begins to move with a certain velocity (Figure 10.1). After you release it, however, the book does not continue moving with the same velocity; it slows down and eventually stops. What happens to the initial kinetic energy of the book? Since there is no change in the potential energy, as the book remains on the table, the mechanical energy is not conserved. As we pointed out in Chapter 5, friction has taken away some of the energy. Although it might be difcult to verify without sophisticated equipment, the book and the table are warmer than before. From our experience, however, we know that friction causes objects to become warmer. When the weather is cold, for example, we rub our hands together to warm them up; the wheels of a motorcycle become hot during braking due to rubbing with the brake pads; and the tires get warmer due to friction (Figure 10.2).
