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Fourier, motion of a rope

  1. Apr 27, 2013 #1
    Hello,
    1. The problem statement, all variables and given/known data
    A rope of mass M and length L is tend with tension T between two rings free to oscillate along a rod parallel to the y axis. Initially the rings are maintained at y=0 while we give to the rope a y(x,0)=dsin²(pix/L).
    Give the complete expression of motion of the rope in term of its fourier components.

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    2. Relevant equations
    y(x)=Ʃ Ancos(npix/L)
    An=2/L∫y(x)cos(npix/L) dx from 0 to L


    3. The attempt at a solution
    The spatial period si P=L and it has to be symetric around points x=0 and x=L, so all the Bn=0.
    So I tried to calculate An.
    An=2/L∫dsin²(pix/L)*cos(npix/L) dx from 0 to L
    I used the identities:
    sina*cosb=1/2[sin(a+b)+sin(a-b)] to make the integral easier then I had
    An=d/L∫sin(pix/L)sin(a+b)+sin(pix/L)sin(a-b) dx
    The problem is that when I calculated everything I found that An=0 ...?
    I'm supposed to find that
    y(x)=[d/2-d/2*cos(2*pi*x/L)]*cos(wnt)
    I arrived to
    An=(-d/2)[sin(2π+nπ)/(2π+nπ)+sin(2π-nπ)/(2π-nπ)]
    Then I used sin(a + b) = sin a. cos b + sin b. cosa and as n= 1,2,3... we found An=0
    I also did it using sin²(x)=1/2cos(2x).. same result

    Thanks
     
    Last edited: Apr 27, 2013
  2. jcsd
  3. Nov 8, 2016 #2
    if you try to transform a periodic function between its period points you will always have 0 dude
     
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