I've got a review question on classical waves (SHM) where I'm given the frequency as f = 20 (sec^-1) and velocity v = 80m/s. The question is: How far apart are two points whose displacements are 30 degrees apart in phase? My own reasoning doesn't seem to agree with the solution I was provided. Here is my thinking: Phase = wt +/- kx For the first point, let x = x1 and t = t. For the second point, let x = x2 (and t = t) So then the phase difference would be (wt-kx1) - (wt-kx2) = k(x2-x1). Then, given v and f, lambda = v/f = 4 m, so k = 2pi/lambda = pi/2. So phase difference, which is given to be 30 degrees, or pi/6 rad, becomes: pi/6 = (pi/2)(x2-x1) -where solving for x2-x1 gives an answer of 1/3. The solution I was given says that the phase difference = 2pi(x2-x1) = pi/6, which gives x2-x1 = 1/12. I don't understand how the 2pi comes into it! Could someone tell me where my thinking went wrong?