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Homework Help: Mass of a planet using a pendulum

  1. Feb 18, 2010 #1
    1. The problem statement, all variables and given/known data
    An explorer wants to find the mass and radius of a planet he has landed on. He uses a pendulum he has with him and observes it takes a period of T1 to complete. He then climbs 2km up and observes period t2. Find planetary Radius r and Mass m

    2. Relevant equations
    g = GM/r^2

    T = 2pi(sqrt(L/g)



    3. The attempt at a solution

    g= 4pi^2L/T1^2 = Gm/r^2 #1

    -----
    4pi^2L/T2^2= Gm/(r^2 + 2000)#2

    I took #1 and solved for r,

    r=sqrt((GMT1^2)/4pi^2L)


    input that in two and came up with

    GmT1^2 + L8000pi^2= GmT2^2

    after canceling

    M=8000pi^2L/G(t2^2-T1^2)

    when i evaluate this I get a reasonable mass, 5.~ *10^15 but my radius is way to small



    any thoughts?
     
  2. jcsd
  3. Feb 18, 2010 #2

    mgb_phys

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    Homework Helper

    (r^2 + 2000) is not the same as (r+2000)^2
     
  4. Feb 18, 2010 #3
    ya sorry type o there,
     
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