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Derivation of the ordinary time derivative of the energy associated with a wave

  1. Oct 21, 2008 #1
    Okay, so I'm at a loss for words to describe my irritation and curiousity on how this is solved.
    Given the one-dimensional wave equation
    (i.e. u_tt=c^2*u_xx 0<x<L, t>0) with no source and constant velocity, we define the energy associated with the wave to be
    E=integral from 0 to L of (1/2)*(u_t)^2 with respect to x plus the integral from 0 to L of (c^2/2)*(u_x)^2 with respect to x. I get that E is the sum of the kinetic and potential energies. I am, however, having a hard time grasping how the ordinary time derivative of the energy function was derived, where dE/dt=c^2*u_t(L,t)*u_x(L,t)-c^2*u_t(0,t)*u_x(0,t).
    The problem I am having is that I don't know what an ordinary derivative of a function of two variables is. What I'm trying to say is that I don't know what d/dt [(u_x)^2] or d/dt[(u_t)^2] are.

    If you could throw me a bone here if I'm going in the correct direction in the derivation or tell me before I hit the wall of wrongness, that would be golden!
  2. jcsd
  3. Oct 22, 2008 #2


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    Science Advisor

    Where did you see the phrase "ordinary time derivative"?
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