wave propagating along a string -- Newton's 3rd Law I'm a bit confused about the action-reaction forces (ie. Newton's Third Law) when a wave propagates along a string. I think I'm confused about a point not mentioned elsewhere in the forum. Let's imagine the string is made up of many tiny particles, and that it carries a wave propagating in the right to left direction. A person holding the right end of the string rapidly displaces that end upwardly, then rapidly displaces that end back to its original position. Then if one point somewhere along the string (let us call it "x") waves upward, it places a force on the point just to its left (x+dx). By Newton's third law, wouldn't this point (x+dx) push on point "x" with an equal and opposite force? It seems to me that such an equal and opposite force woud tend to return the string just behind the wave, to its original position. But I thought that it was the downward movement of the hand that returns the string to its original position? Thanks!