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Homework Help: Fourier Series/Wave Problem

  1. Feb 14, 2017 #1
    1. The problem statement, all variables and given/known data
    A violin string is plucked to the shape of a triangle with initial displacement:

    y(x,0) = { 0.04x if 0 < x < L/4
    (0.04/3)(L-x) if L/4 < x < L

    Find the displacement of the string at later times. Plot your result up to the n = 10 term, for t = L/10v, L/5v, and L/2v, where v = p T /µ is the speed of the wave. Are all harmonics excited?

    2. Relevant equations
    The normal Fourier Series equations (the ones I use are here)

    3. The attempt at a solution
    The solution is supposed to be of the form y(x,t) = Σansin(nπx/L)cos(nπvt/L)

    My only issue is conceptually. I understand how to go through the math to get an. Why is it that the vt is within the cosine and not the sine? How am I even supposed to know to add the vt in? I feel like this is something unbelievably basic that I'm missing.
  2. jcsd
  3. Feb 14, 2017 #2


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    Why would you think it should be a sine? The string is released from rest. How would you solve the problem?
  4. Feb 17, 2017 #3


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    Notice first that the solutions terms are a product of sin(function of x only) and cos(function of t only). What do you think these two "separated" pieces represent, conceptually?
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