What is Friction?

When we think about friction, there is one more place for energy to go - heat. When you push a book across a table, does the energy of the book change? Not if in the end, it is sitting still at the same height. Where did the energy from the work you did pushing the book across the table go? Into fighting friction, which does work but does not change the total energy of the book. It does no useful work. The energy used fighting friction goes into heat generated where the objects touch, the surface of the book and the surface of the table. Friction always uses up energy, because it always works against motion.