Today in class, my physics teacher demonstrated Newton's 3rd Law by nailing a hammer in a piece of wood. She drew a free body diagram and there were three force pairs: force pair between hammer and nail, force pair between nail and wood from gravity/normal force, but however, she also put a 3rd force pair that she said was the force of the nail on the wood. I am confused by this 3rd force pair as she stated the force exerted by the hammer was 10 N, but the force of the nail on the wood was only 5 N. How is this logical? Isn't this 3rd force pair just from the nail pushing against the wood? I thought this force just equals the 10 N? What is really confusing me is, if forces come in pairs, shouldn't the nail exert a force of 10 N from the 10 N exerted by the hammer? So the nail is exerting 10 N on the wood block and the wood block is exerting 10 N back on the nail. How can hammering a nail into wood be possible?