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Homework Help: Wave equation help

  1. Mar 3, 2010 #1
    how do i show that v(x,t)=u(x,t)-ue(x) satisfies the wave equation? =(

    i get that ue(x)=gx2/2c2 + ax + b where a and x are just constants but how does this satisfy the wave equation?
  2. jcsd
  3. Mar 15, 2010 #2


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    Can you clarify your question? I don't understand what ue(x) actually is in terms of the function u(x,t)? Is u(x,t) an arbitrary function, or one that itself satisfies the wave equation? Perhaps I'm missing something obvious. :)

    Generally to show that a function satisfies a DE, you'll need to show that its derivatives actually have the relationship in question. So I'd start by differentiating your definition of v twice with respect to each variable.
  4. Mar 16, 2010 #3
    thanks for taking a look. still stumped. Ue(x) is the sagged equilibrium position ( when Q(x,t)=-g and the boundary conditions are u(0)=0 and u(L)=0 or fixed boundary/ends of the string). i hope that clarifies it a bit?
  5. Mar 16, 2010 #4


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    Ah, I see. :) So you've got a string with a uniform loading (which you've called Q) along the x-direction, sagging in the shape of a parabola because of that (note that in reality, if this loading were due to the weight of the string, the equilibrium shape would be a catenary, not a parabola).

    And you're trying to prove that if u(x,t) is a solution to the wave equation on an equivalent (same boundary conditions) unloaded string, then:

    [tex]u(x,t) - u_e(x)[/tex]

    Will be a solution to the wave equation on the loaded string.

    Did I understand the question correctly?

    If so, then look up the superposition theorem for linear differential equations (such as the wave equation). This states that the sum of two solutions to a DE will also be a solution to the DE - So in this case, if both ue(x) and u(x,t) are solutions then it immediately follows that v(x,t) is a solution.

    If you also need to show that it's the solution that you're looking for, then you'll need to check it satisfies the appropriate boundary/initial conditions.
  6. Mar 16, 2010 #5


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    Are we supposed to be psychic? Why don't you give us the equation you are trying to satisfy? How does Q enter in to it?
  7. Mar 16, 2010 #6
    wow you totally got the question. thanks alot. i see it clearly now :)

    LCKurtz: Q(x,t) is just force acting on the string, so gravity in most cases. i got the answer now, sorry for not being more precise.
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