This is not really a homework problem, I do not understand the concepts. The attachment below is as informative as the slides provided are. The course seems to be based on H.J. Pain's book "waves and vibrations". I know that the information provided is limited, but there is little I can provide more on this question, hopefully someone can explain what is going on. I am embarrassed to ask my lecturer as he assumed it to be straightforward. 1) Firstly, is the beam bellow showing what is happening with the same slices of the beam at some later time ? 2) Why is psi (up to that point the notation was for the usual wave equation psi = Aexp(kx-wt) ) and psi + delta psi pointing inward ? Is psi + delta psi supposed to mean a reflected wave ? 3) With the "compression" case (on the left) are the forces pointing outward because some kind of restoring force is being shown ? 4) Why is the pressure wave (also it was called the acoustic wave) different from longitudinal compression wave ? 5) What is A(p + psip) ? The force ? Why is it pointing inwards now ? Thanks !