My 10-year old son understands that body temperature is related to the speeds/kinetic energies of the molecules inside you, but does friction play a role as well? — MR
You’re both right about temperature being associated with kinetic energy in molecules: the more kinetic energy each molecule has, the hotter the substance (e.g. a person) is. But not all kinetic energy “counts” in establishing temperature. Only the disordered kinetic energy, the tiny chucks of kinetic energy that belong to individual particles in a material contributes to that material’s temperature. Ordered kinetic energy, such as the energy in a whole person who’s running, is not involved in temperature. Whether an ice cube is sitting still on a table or flying through the air makes no difference to its temperature. It’s still quite cold.
Friction’s role with respect to temperature is in raising that temperature. Friction is a great disorderer. If a person running down the track falls and skids along the ground, friction will turn that person’s ordered kinetic energy into disordered kinetic energy and the person will get slightly hotter. No energy was created or destroyed in the fall and skid, but lots of formerly orderly kinetic energy became disordered kinetic energy—what I often call “thermal kinetic energy.”
The overall story is naturally a bit more complicated, but the basic idea here is correct. Once energy is in the form of thermal kinetic energy, it’s stuck… like a glass vase that has been dropped and shattered into countless pieces, thermal kinetic energy can’t be entirely reconstituted into orderly kinetic energy. Once energy has been distributed to all the individual molecules and atoms, getting them all to return their chunks of thermal kinetic energy is hopeless. Friction, even at the molecular level, isn’t important at this point because the energy has already been fragmented and the most that any type of friction can do is pass that fragmented energy about between particles. So friction creates thermal kinetic energy (out of ordered energies of various types)… in effect, it makes things hot. It doesn’t keep them hot; they do that all by themselves.