Hi, please could someone help clarify the reason why a mechanical wave is inverted at a boundary as I'm really stuck! Some sources I have read seem to suggest it can be explained by Newton's 3rd law whilst others suggest its to do with conservation of momentum. Newton's 3rd law - consider the crest of a wave pulse of a rope approaching a fixed end. The last particle of the rope will receive an upward displacement which exerts an upward force on the first particle of the fixed boundary. This is an equal and opposite reaction and the wave is inverted and reflected. Is that correct? Conservation of energy - the rope supposedly has a forward momentum. When the rope hits the boundary in order to conserve momentum an inverted wave is reflected back. I don't understand this as how can a rope wave have mass (and therefore momentum) as the motion is only the propagation of energy (the particles themselves move perpendicular). Can energy not be conserved by the fixed end (+ earth) receiving momentum as in the case of a tennis ball hitting the wall. I don't see the need for the wave to be inverted to conserve energy? I know its a bit long but I would greatly appreciate some help.