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Homework Help: Radius of orbit

  1. Jun 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A satellite orbits the sun with a period of 2.1 day. What is the radius of its orbit?

    2. Relevant equations


    3. The attempt at a solution

    If the time is given in days I have to convert to seconds right? Also how would I find velocity would I use V=square root (Gms/r)? For some reason gravity has got me confused
  2. jcsd
  3. Jun 5, 2009 #2


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    That would be useful, as the units of G involve seconds.

    Recognize that

    v = 2πr/T

    and m*v2/r = GMm/r2 or as you have written

    v2 = GM/r

    Combine with

    v2 = (2πr/T)2 = GM/r
  4. Jun 5, 2009 #3
    so that actually gets rid of the actual speed of the object as far as usefulness in the equation.
    Last edited: Jun 5, 2009
  5. Jun 5, 2009 #4


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    Alternatively you could also use [itex]m \omega^2 r[/itex] as the centripetal force.
  6. Jun 5, 2009 #5
    That is an awesome way to check yourself since he wants the work done by using the gravity equations but just to double check myself I can use that! Thats a big help also thank you!
  7. Jun 5, 2009 #6


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    Well ... yes. They only ask you for the radius. Were you invested in wanting to find the speed?
  8. Jun 5, 2009 #7
    No, but I just didn't even think about using the equations that deal with velocity. So I was getting lost. Is there another way to go about this problem?
  9. Jun 5, 2009 #8
    Last edited by a moderator: Apr 24, 2017
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