From Newton's Third Law of Motion, I know that there is a reaction force--equal in magnitude and opposite direction--for every action force. But what I find a bit puzzling is what this author of an article says about friction, "Thus the force of friction has a remarkable property of adjusting its magnitude so as to become exactly equal to the applied force tending to produce motion. However, after a certain limit the force of friction cannot increase further. If the applied force exceeds this particular limit then the block starts moving as the two forces, i.e., the force of friction and the applied force are not balanced. " I thought forces were always equal in magnitude and opposite in direction. But if that were true, nothing would move. Observing motion in reality, it would make sense that, depending on the surface, friction has some threshold, which the allow the eventual motion.