The Force That Always Acts Against Motion
Friction occurs anywhere that two different objects rub against each other. It can be desirable, such as on a bath mat that keeps you from slipping in the shower. It can be undesirable, such as between moving parts in an engine. The field of tribology is devoted to studying it. Friction affects the mechanical advantage you get from using different simple machines to do the same job. The Bill Nye YouTube video discusses friction. The examples below show many instances of friction in everyday life, and show fluid friction and rolling friction, too.