1. The problem statement, all variables and given/known data Its not really a problem - more like a question my teacher asked us. 2. Relevant equations My problem is that in my opinion the wave concept is quite vague. Are we talking about a single narrow pulse propagating through the medium or collective vibrations of all the atoms in the solid? If all atoms are performing simple harmonic motion then the momentum averaged over one period is 0. But who says all waves in solids are like this, i.e. when you release a spring a single longitudional wave travels down the spring and it obviously carries momentum. What do you think my teacher referred to when he said elastic waves? 3. The attempt at a solution Edit: I looked it up in my book and it said waves (which kind I don't know) carry no physical momentum but act as if they had momentum p=hbar*k. What is that supposed to mean!?