Work Done by Sliding Friction

Friction does work when two objects in contact slide past one another. This process is studied using a simple numerical model consisting of two flexible asperites, one for each object. The force of friction is modeled as a conservative electric force that can be either attractive or repulsive. The concept of pseudowork plays an important role throughout the analysis. The difference between work and pseudowork is equal to the change in internal energy. Because the asperites are not rigid, the work done in a typical interaction is greater than the psuedowork, leading to an increase in internal energy. For a real physical system, this is the source of thermal energy. The processes discussed here fall into two categories: forced sliding and free sliding. With forced sliding, applied forces keep the objects moving with constant velocities. With free sliding, the only force on one of the objects (in the direction of motion) is friction. For free sliding, friction not only increases the internal energy but also decreases or increases the object's translational kinetic energy. The change in translational kinetic energy is determined by the pseudowork done by friction.