Suppose you drag a heavy sofa across the floor at a steady speed. You are doing work, but the sofa is not gaining kinetic energy. And when you stop pulling, the sofa almost instantly stops moving. Where is the energy going that you’re adding to the system? And what happens to the sofa’s kinetic energy when you stop pulling?
You know that rubbing things together raises their temperature, in extreme cases making them hot enough to start a fire. As the sofa slides across the floor, friction causes the bottom of the sofa and the floor to get hotter. An increasing temperature is associated with increasing thermal energy, so in this situation the work done by pulling is increasing the system’s thermal energy instead of its kinetic energy. Our goal in this section is to understand what thermal energy is and how it is related to dissipative forces.