Work and energy
As explained on the previous page, work is the relationship that connects force and energy. When energy is dissipated from a system, which we referred to as “waste” in chapter 6, the energy dissipated is equivalent to the work done by the dissipating force. Recall that the “waste” we worked with in chapter 6 always had a negative value, representing energy leaving the system. When a force is exerted in the opposite direction as an object’s motion, the work done by the force is negative.
The common example we consider in this book is the force of friction. As an object slides across a surface, friction does work on the object. The energy dissipated is the work done by friction. The work done by friction is negative because friction always acts on an object in the opposite direction than the object is moving.