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Plucked string, potential energy

  1. Apr 14, 2013 #1
    Hello, I don't understand the second question, i don't know what I have to do:
    1. The problem statement, all variables and given/known data
    A string of length L, which is clamped at both ends and has a tension
    T, is pulled aside a distance h at its center and released.
    (a) What is the energy of the subsequent oscillations?
    (b) Find the approximative expression of this energy for small oscillations


    2. Relevant equations



    3. The attempt at a solution
    In a/ I found that U=2Th²/L
    For the second question I took a segment dx, so we have that
    dU=T(ds-dx)
    ds=dx(1+1/2(dy/dx)²)
    I replaced it so that
    U= T/2∫(dy/dx)²dx from 0 to L
    I'm stuck here, how can I find the exact potential energy for small oscillations, i'm not even sure that i'm using the good method since in the next questions, they ask to show that the energy in b is the sum of potential energies of each mode, so I can't use y(x,t) to find dy/dx in this question, right ?
    Thanks
     
  2. jcsd
  3. Apr 14, 2013 #2

    haruspex

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    Looks to me that you have answered (b), not (a). I would have taken your answer to (a) as correct if it were not for the fact that it goes on to ask (b). This makes me think that (a) is asking for an exact expression, with no approximations.
     
  4. Apr 14, 2013 #3
    I don't understand, in the question (a) it's asking for the exact expression, and b for an approximation of U for small oscillations
     
  5. Apr 14, 2013 #4

    haruspex

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    We agree on that, but your answer to (a) is not exact, in several ways. It is only an approximation valid for small h.
     
  6. Apr 14, 2013 #5
    I'm really confused.
    So the answer to (a) must be U=Tn²pi²A²/4L we use y(x,0)
    And for (b) it must be U=2Th²/L
    Then, for c/, they ask to show that the potential energy is the sum of potential energy of each mode by finding the same expression as b/
    So I have to find U=2Th²/L, right ?
    The problem now I don't know how to find b/ by using "small oscillation argument" without answering to the question c/ at the same time.
     
  7. Apr 14, 2013 #6

    haruspex

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    The initial half length is L/2. If the centre point is displaced h orthogonally to that, what is the exact extension?
    That's my interpretation.
    I'm not sure what c is asking for. It sounds like they want you to do the Fourier analysis to find the energy at each harmonic, but I wouldn't know how to do that other than by assuming the total of those is what you calculated in b.
     
  8. Apr 14, 2013 #7
    I did c/ and I found U=2Th²/L
    I don't understand what you're asking for a/ ..?
     
  9. Apr 14, 2013 #8

    haruspex

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    The initial length is L. If the centre point is displaced h orthogonally to that, what is the exact length of the wire now? How much longer is that than L?
     
  10. Apr 14, 2013 #9
    It says that we neglect the extension of the wire.
    Now, I don't know how to deal with the a/ question , is it a n function ?
     
    Last edited: Apr 14, 2013
  11. Apr 15, 2013 #10

    haruspex

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    Not in the OP, it doesn't :grumpy:. Please post the question exactly as given.
     
  12. Apr 15, 2013 #11
    Sorry, I didn't notice that i forgot it. "We neglect the extension of the wire and the change of the tension."
     
  13. Apr 15, 2013 #12

    haruspex

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    That changes things. Now I agree with your answer for (a) but I'm mystified as to what (b) is asking. Note that you've referred to potential energy a few times, but in the question as you posed it it just says energy.
     
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