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Homework Help: Central Force motion

  1. Mar 5, 2009 #1
    1. The problem statement, all variables and given/known data
    Consider the motion of a particle in the central force F(r) = -kr.
    Solve for the particle's location as a function of time, r(t) and theta(t).

    2. Relevant equations

    3. The attempt at a solution
    [tex]E = 1/2 m r'^2 + \frac{L^2}{2mr^2} + U(r)[/tex]
    I know U(r) = 1/2 k r^2
    [tex]\frac{dr}{dt} = \sqrt{\frac{2}{m}(E-U(r)) - \frac{L^2}{m^2 r^2}}[/tex]
    (Where L is ang. momentum)

    Plugging in U(r) I get a really nasty integral

    [tex]dt = \frac{r^2 dr}{\sqrt{2E/m r^2 - k/m r^4 - L^2/m^2}}[/tex]

    According to my professor I can use a trig sub to solve this, but I am not getting anywhere.
    Is there some sort of relationship that I am missing? I know it's supposed to be an ellipse but I seem to get any sort of substitutions to work.
    Last edited: Mar 5, 2009
  2. jcsd
  3. Mar 5, 2009 #2
    This integral is such a mess.
    I think the easiest way is to solve this problem in cartesion coordinates and convert it to polar coordinates.
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