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Homework Help: Velocity of a planet around the sun

  1. Apr 8, 2015 #1
    1. Asking because the answer I got seems too simple...a planet of mass m moves in an elliptical orbit about the sun. The minimum and maximum distances of the planet from the sun are called the perihelion and aphelion respectively. If the speed of the planet at p is vp, what is its speed at a? Assume the distances rp and ra are known.

    2. Relevant equations

    dA/dt = 1/2r2ω, ω=v/r

    3. The attempt at a solution

    dA/dt is constant, so 1/2rp2ωp = 1/2ra2ωa. Substitute v = ωr, that gives 1/2rpvp = 1/2rava, va = rpvp/ra. Is this correct?
  2. jcsd
  3. Apr 8, 2015 #2

    Simon Bridge

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    Try verifying the result from another rule - like conservation of momentum and energy...
  4. Apr 8, 2015 #3
    So using conservation of momentum I got the same thing...dA/dt = L/2m = mr2ω/2m. Given that ω = v/r, rpvp/2 = rava/2, va = rpvp/ra.
  5. Apr 8, 2015 #4


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    Yes, it is the same thing as Kepler's law. Good job.
  6. Apr 8, 2015 #5
    Great, thank you!
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