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Homework Help: Planetary Motion, calculation of orbital period

  1. May 21, 2010 #1
    Hi everyone,

    I'm really confused with a particular question:

    1. The problem statement, all variables and given/known data

    A space shuttle orbits the Earth at 6720 km from its centre. The gravitational field strength is 8.9N/kg. Calculate the shuttle's orbital period in minutes...

    2. Relevant equations

    g=4pi^2 x R /T^2

    3. The attempt at a solution
    The thing is, I know how to solve it using g=4pi^2 x R /T^2, but I want to know why I can't just use g=r^3/T^2 (the constant for all objects orbiting a particular mass)...if I have r and g, why can't i just use this equation???


  2. jcsd
  3. May 21, 2010 #2


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    Science Advisor
    Homework Helper

    Because r^3/T^2 is a constant. But that constant isn't the same as g. It doesn't even have the right dimensions.
  4. May 21, 2010 #3
    ahh ok...thanks :)
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