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Confused with square root ODE

  1. Mar 10, 2010 #1
    hi there,
    I'm trying to plot r against [tex]\phi[/tex] by solving the following ODEs using runge-kutta. The problem i'm having is with the square root. How do I know when it will be positive and when it will be negative? If this is a simple question I apologise I'm not that great with the maths :).

    E and L and M are constants.
    V^2(r) = \left(1 - \frac{2M}{r} \right)\frac{L^2}{r^2}
    \frac{d \phi}{d \lambda} &= \frac{L}{r^2}
    \frac{dr}{d\lambda} &= \pm\sqrt{E^2 - V^2(r)}

    Last edited: Mar 10, 2010
  2. jcsd
  3. Mar 11, 2010 #2
    [itex]\pm[/itex] sign shouldn't be there in the first place. You have to decide whether it is positive or negative sign. Just look back at your model again.
  4. Mar 11, 2010 #3
    The model is of a photon's path around a black hole. The [tex]\pm[/tex] is due to the fact that the photons distance from the blackhole can increase and decrease in the same orbit.

    I've been advised to differentiate dr/d[tex]\lambda[/tex] to get around the [tex]\pm[/tex] but I am unsure as to how this will solve my problem.

    I have the [tex]d^2r/d \lambda ^2[/tex] as [tex]\frac{d^2r}{d\lambda^2} &= \frac{L^2(r - 3M)}{r^4}[/tex]
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