|When friction is made by two atoms rubbing — it makes heat. But how and why? — GN, Marine City, MI
When two surfaces slide across one another, some of the mechanical energy in those surfaces is converted to thermal energy (or heat). That's because the surfaces are microscopically rough and their atoms collide as the surfaces slide pass one another. Each time a collision occurs, the atoms that collide begin to vibrate more vigorously than before. In this process, the surfaces lose some of their overall mechanical energy but the atoms gain some randomly distributed local vibrational energy—more thermal energy. Those surface atoms become hotter. As the sliding continues, large regions of the surfaces become hotter and the surfaces lose much of their energy. If you don't push them to keep them sliding across one another, they'll come to a stop as all their mechanical energy is converted into thermal energy.