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Homework Help: Need help with keplers 3rd law of periods problem

  1. Apr 2, 2008 #1
    [SOLVED] need help with keplers 3rd law of periods problem

    1. The problem statement, all variables and given/known data
    A 20 kg satellite has a circular orbit with a period of 2.6 h and a radius of 9.4E6 m around a planet of unknown mass. If the magnitude of the gravitational acceleration on the surface of the planet is 8.6 m/s2, what is the radius of the planet?

    2. Relevant equations

    a gravity=(GM)/r^2

    3. The attempt at a solution

    I used the law of periods to find the mass and then used the second equation to find the radius of the planet but I got a really small number
  2. jcsd
  3. Apr 2, 2008 #2


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    Homework Helper

    Can you show your calculations in more detail? Note that r in your first equation is not the same as r in the second (in case you forgot that).
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