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Transverse Standing Wave, Maximum Potential Energy Per Unit Length (Answers Disagree)

  1. Nov 10, 2012 #1
    1. The problem statement, all variables and given/known data

    A wire is fixed at both ends vibrating fundamentally. For what value of x (x position on the wire, with 0 being one edge and L being the other) is the potential energy per unit length has the maximum value?

    Known: Length of wire (L), Tension in wire (T), Mass of wire (m), Amplitude (A).


    2. Relevant equations

    y(x,t) = A sin(kx) cos(wt)

    dU/dx ~= T (dy/dx)^2 /2
    (dy/dx is a partial derivative)

    3. The attempt at a solution

    dy/dx = Ak cos(kx) cos(wt)

    dU/dx = T (Ak)^2 cos(wt)^2 cos(kx)^2

    d/dx(dU/dx) = -T (Ak)^2 cos(wt)^2 2k cos(kx) sin(kx)
    d/dx(dU/dx) = -T (Ak)^2 cos(wt)^2 k sin(2kx)

    Critical points exist at d/dx(dU/dx) = 0, so since everything else is a constant, sin(2kx) = 0, and thus 2kx = 0 (x = 0), 2kx = pi (x = L/2), and 2kx = 2pi (x = L) are all solutions.

    dU/dx = 0 at x = L/2, but when cos(wt)^2 > 0 then dU/dx is greater at x = 0, L than at x = L/2

    The textbook says that the maximum potential energy per unit length occurs at the middle of the wire. My math says that it's at the two endpoints? Why?

    Also, I googled the problem and they are taking dy/dx at 0 to get the result. I don't understand this. Can anyone explain?
     
    Last edited: Nov 10, 2012
  2. jcsd
  3. Nov 10, 2012 #2

    Simon Bridge

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    Re: Transverse Standing Wave, Maximum Potential Energy Per Unit Length (Answers Disag

    What does the potential energy per unit length depend on? Where would these things be maximum?
    Who is "they"?
     
  4. Nov 10, 2012 #3
    Re: Transverse Standing Wave, Maximum Potential Energy Per Unit Length (Answers Disag

    The potential energy per unit length depends on the partial derivative dy/dx (at least as an approximation) and the tension (which is constant).

    The partial dy/dx seems to be maximum at x = 0.

    I looked here on page 78. This "solution" is what I don't understand. (I had worked out this problem without checking the solution to it first).

    http://www.uccs.edu/~rtirado/Ch16%20ISM.pdf [Broken]
     
    Last edited by a moderator: May 6, 2017
  5. Nov 10, 2012 #4

    Simon Bridge

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    Re: Transverse Standing Wave, Maximum Potential Energy Per Unit Length (Answers Disag

    well ... I see you are stuck on thinking in terms of equations.
    Have a look at a string that is vibrating like that and use physics.

    Each length-element of the string is in constant motion, up and down.
    So each mass element on the string is constantly exchanging kinetic and potential energy. When y is a maximum, the string between x and x+dx has maximum potential energy for that bit, when y=0, then the kinetic energy is a maximum. From this, without reference to any equations, what value of x would you expect to find the bit of the string that gets the most potential energy?

    Note:
    The reference in the link does not have a page 78 ... I found page 1678, but if any of the examples there used dy/dx I did not see it. There is a problem 78 too - but also not apropos. Some of the problems used dU/dx etc though but I stopped hunting.

    The books pattern of getting you to "picture the problem" first is a good one for this problem too.
     
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