if you have a rope or slinky and send a single pulse through the medium (for example a crest), the crest travels all the way to the end of the rope basically at the same height. I know that for disturbances to be waves, the motion must be rhythmic and one component of a wave is a restoring force, and in the rope, gravity I think would be the assumed restoring force. How come as the crest travels, gravity does not slowly push this crest down toward the equilibrium position and then to the trough position? Why is the force of gravity or any other restoring force in other waves not able to change the height and orientation of a single pulse? Thanks!