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Homework Help: Comet elliptical orbit

  1. Nov 25, 2007 #1
    1. The problem statement, all variables and given/known data

    There is a system of 3 bodies, consisting of the sun, the earth and a comet. When the comet is at perihelion it is at a distance half that of the earth's orbital radius. At this point it has a speed twice that of the earth's. Ignore the gravitational forces between the earth and the comet.

    What is the orbital speed of the comet when it crosses the earth's orbit (should be given in terms of Ve)? What is the angle at which the orbit's cross?

    Will the comet escape from the solar system, never to return?

    Where Ve is the orbital speed of the earth.
    2. Relevant equations

    3. The attempt at a solution

    By considering conservation of angular momentum, i get:

    [L] = [L of comet at earths orbital radius] = [L of comet at perihelion]
    [L] = [Re x mv] = [Re/2 xm2Ve]

    where Re is the radius of the earth, and the square brackets show it is magnitude we are considering. From this i get the perpendicular speed of the comet when crossing earths orbit with respect to the position vector r as Ve. But i want to know the tangential speed of the comet at this point.
    Any ideas how to do this?

    I was told to use conservation of angular momentum and conservation of energy in working out this problem.
    Last edited: Nov 26, 2007
  2. jcsd
  3. Nov 26, 2007 #2
    For conservation of energy, i get

    using k = GMm

    (m/2Ve^2)/2 - k/(r/2) = (mVc^2)/2 - k/r

    where vc is the tangential speed of the comet and is what i want to find. I am given the data for G, M and r (the earths orbital radius). But the question wants Vc in terms of Ve and i was also told that i could get rid of the GMm/r terms somehow. I think there was a mention of centrifugal forces enabling me to get rid of the terms.

    Please any help is greatly appreciated, thanks.
  4. Nov 26, 2007 #3

    Your steps seem good. However, I'm not too sure about how to solve this either :/ . Hopefully someone else will come along and give you some more help.


  5. Nov 27, 2007 #4


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    Homework Helper

    Consider the earth to move in a circular orbit. This will give you ve squared in terms of G,Ms (sun) and Re (earth).
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