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Speed at perihelion and aphelion?

  1. Jan 16, 2009 #1
    SO the equations for the speed at the aphelion and perihelion are, respectively:

    v=SQRT[GM( (2/r_p) -(1/a) )]

    v=SQRT[GM( (2/r_a) - (1/a) )]

    where M is mass of sun, r_a & r_p are distances from sun at aphelion and perihelion , and a is length of semi major axis.

    How do you derive them? I am having trouble seeing where they come from, and a quick Google turns up NOTHING on them unfortunately.

    ANy help?
  2. jcsd
  3. Jan 16, 2009 #2

    D H

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    Those equations result simply from inserting the perifocal and apofocal distances into the vis-viva equation,

    [tex]v^2 = GM\left(\frac 2 r - \frac 1 a\right)[/tex]

    This equation follows directly from conservation of energy. The total energy (kinetic plus potential) of a point mass m separated by a distance r from some other point mass M and moving with a velocity v relative to that other point is

    [tex]E = \frac 1 2 m v^2 - \frac {G M m}{r}[/tex]

    The total energy of a point mass in an elliptical orbit is also given (see any intermediate-level classical mechanics text) by

    [tex]E = - \frac {G M m}{2a}[/tex]

    Equating the two expressions leads directly to the vis-viva equation.
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