Friction is a force between two objects that slide over each other. Coulomb's friction law states that the friction force is proportional to the normal force between the bodies, and always acts to reduce the sliding velocity and thus transforms kinetic energy into heat. If it weren't for friction, a car would only need to be pushed once, and could then ride around without the engine on for as long as it wants. In fact, without friction you would also drop your pants and slide off your chair!

While friction seems to be simple enough at a first look, it is one of the most challenging of all problems in science, coupling electromagnetic interaction on the atomic scales with macroscopic everyday physics of everything we see around us.