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Homework Help: Determining Orbital Period of a Planet

  1. Oct 16, 2012 #1
    Using the equation P2 = (4 * pi2 * a3)/(G(M+m)) to find the orbital period of Earth where:

    P = orbital period
    a = semi-major axis = 1.50e11 m
    G = 6.67e-11 m3 kg-1 s-2
    M = mass of sun (in this case) = 1.989e30 kg
    m = mass of Earth = 5.972e24 kg

    I have been trying to find the orbital period of the Earth but the answers I've been getting make no sense.

    P2 = (4 * pi2 * (1.50e11)3) / (G(1.989e30 + 5.972e24))

    P = 31690989.27

    What are the units on P? I tried following the units as they cancelled each other and I ended up with s kg-2. Any advice would be helpful.


    - Marcin
  2. jcsd
  3. Oct 16, 2012 #2


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    Homework Helper
    Gold Member

    P has units of seconds.
  4. Oct 16, 2012 #3


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    Science Advisor
    Gold Member

    Hello Marcin

    Try calculating how many seconds there are in a year. And compare with your answer.
  5. Oct 16, 2012 #4
    Ah yes. Well that was embarrassing. Thank you Bandersnatch!
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