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Homework Help: Orbital Energetics Help

  1. Feb 7, 2008 #1
    Given:
    Planet is orbiting a star
    G = 6.67259 x 10^-11 Nm^2/kg^2
    Mstar = 1.99 x 10^30 kg
    mplanet = 5.98 x 10^24 kg
    distance from perihelion to star = 1.69 x 10^11 m
    distance from aphelion to star = 1.745 x 10^11 m
    potential energy at perihelion = -4.698 x 10^33 J


    Need:
    Kinetic energy at perihelion

    To solve for perihelion kinetic energy I used GMm/2r and checked it with -.5 x potential energy, but apparently this was incorrect as my answer (2.3493 x 10^33 J) was incorrect.

    Am I missing something? Or could the "correct" answer be incorrect (this has been known to occur).
     
  2. jcsd
  3. Feb 7, 2008 #2

    D H

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    Staff Emeritus
    Science Advisor

    The total energy is [tex]E=-\frac {GMm}{2a}[/tex], not [tex]-\frac {GMm}{2r}[/tex]. The latter equation (the one you used) is only valid for circular orbits.
     
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