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Homework Help: Weird ODE containing cos.

  1. Nov 11, 2008 #1
    1. The problem statement, all variables and given/known data

    I can not determine the solution to the diff. eq.

    2. Relevant equations

    [tex]\ddot{x}+k \cos{x}=0[/tex].

    The constant k is postive.

    3. The attempt at a solution

    I tried solving it with the methods I know and it all ended up in a big mess. I am just not used to the second term, \cos. Doe's anyone know what to do? :confused:
  2. jcsd
  3. Nov 11, 2008 #2


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    It's GOT to be something of the form

    [tex]x=arccos(y)[/tex] for some y a function of t, based on an argument of 'this is really fricking hard otherwise'. Try the substitution and see what you get
  4. Nov 11, 2008 #3


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    Since the independent variable, which I will call "t", does not appear explicitely in the equation, this is a candidate for "quadrature". Let [itex]y= dx/dt[/itex]. Then [itex]d^2x/dt^2= (dy/dx)(dx/dt)[/itex] by the chain rule. But since [itex]y= dx/dt[/itex], that is [itex]d^2x/dt^2= y dy/dx[/itex] and the equation converts to the first order equation, for y as a function of x, [itex]y dy/dx= -cos(x)[/itex] which is [itex]y dy= -cos(x)dx[/itex]. Integrating, [itex](1/2)y^2= sin(x)+ C[/itex]. Now we have [itex]y^2= -2 sin(x)+ 2C[/itex] or [itex]y= dx/dt= \sqrt{2C- 2sin(x)}[/itex] so [itex]dx= \sqrt{2C- 2sin(x)}dx[/itex].

    I am not sure that is going to be easy to integrate, but that gives the solution.
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