|If forces are always equal but opposite, how can a hammer drive a nail into a wall? Don't the forces on the nail cancel?
Although forces always appear in equal but oppositely directed pairs, the two forces in each pair act on different objects. The nail and hammer experience one of these force pairs—the hammer pushes on the nail just as hard as the nail pushes on the hammer. Because the nail's force on the hammer is the only force that the hammer experiences, the hammer accelerates away from the nail and the wall. The nail and wall experience the other force pair—the wall pushes on the nail just as hard as the nail pushes on the wall. The nail thus experiences two horizontal forces: the hammer pushes it toward the wall and the wall pushes it away from the wall. As long as all the forces are gentle, the two forces on the nail cancel and it doesn't accelerate at all. But if you hit the nail hard with the hammer, the wall can't exert enough support force on the nail to prevent it from enter the wall. The two forces on the nail no longer cancel and it accelerates into the wall.