Answer to Question #6616 in Mechanics | Relativity for Anonymous
When do action and reaction forces within a system not cancel each other?
Action and reaction forces do not cancel each other; if they did, no motion would ever be possible, since for every action there is an equal and opposite reaction. This means that for example, when my hand pushes on a door, the door is also pushing on my hand, with equal force and in an opposite direction. Nonetheless, the door still moves.