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Using a pendulum to find Planetary mass and radius

  1. Feb 14, 2010 #1
    Long Time reader, first time caller

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

    An explorer wants to find planetary mass M and radius R using a pendulum of length L. The average elevation of the planet is R. At this elevation the pendulum has period T1. He then climbs up 2km where the pendulum has period t2. Express M and R interms of other variables need to calculate. Assume the planet is spherical

    2. Relevant equations

    Fg= GMm/R^2


    3. The attempt at a solution

    g = 4pi^2 L/T^2= GMm/R^2

    I know I need another equation, I was hoping someone could point one out or tell me what Im missing. Obviously this could be solved one of ther variable but at the moment I have 3 var. and 2 equations. I thought about CoM but or CoE but then that brings in a lot more variables.

    any thoughts
  2. jcsd
  3. Feb 14, 2010 #2


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    Homework Helper

    4pi^2 L/T^2= GM/R^2 (note: NOT GMm/R^2) is for ground level. Write another equation that says the pendulum has period t2 2km up.
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