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Homework Help: Orbital motion

  1. Mar 6, 2013 #1
    1. At what altitude will a satellite complete a circular orbit of the Earth in 4.4 hours?

    2. Relevant equations

    T^2 = ((4π^2)(r^3))/GM

    3. The attempt at a solution

    Used the equation and plugged all the information, and solved, but for a weird reason it comes wrong, maybe I should convert the hours to minutes?
  2. jcsd
  3. Mar 6, 2013 #2


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    What units did you use for the quantities in your formula?

    G = 6.674*10^-11 m^3/kg*s^2

    so time may have to be in seconds rather than minutes.

    I don't understand why folks think that the units will take care of themselves when making calculations.
  4. Mar 6, 2013 #3
    cuberoot((((15840seconds)^2)(6.67384E-11 N m^2 Kg^-2)(5.974E24Kg))/(4pi^2)) = r

    is that correct?
    Last edited: Mar 6, 2013
  5. Mar 6, 2013 #4
    That formula looks good.
    Last edited: Mar 6, 2013
  6. Mar 6, 2013 #5


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    That gives an answer in metres. What distance is that calculating, exactly? The question asks for altitude.
  7. Mar 6, 2013 #6
    I do it, but, the answer that it gives me is totally different from the answer that my practice problem gives
  8. Mar 6, 2013 #7
    the answer that this equation gives me is something like,13632 and the real answer should be 7260 or something approximated
  9. Mar 6, 2013 #8
    That formula gives you the distance r between the center of the Earth and the satellite. To get the altitude of the satellite you have to subtract the Earth radius from r which gives you the correct answer.
  10. Mar 6, 2013 #9
    thank you!!!
  11. Mar 6, 2013 #10
    You're welcome.
  12. Mar 6, 2013 #11


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    No, I meant what distance does that number represent. I was trying to point out that you were not calculating altitude. Anyway, looks like you have it now.
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