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Homework Help: Angular Momentum of a planet

  1. Nov 23, 2008 #1
    1. The problem statement, all variables and given/known data
    Consider a planet that rotates one revolution in one day and orbits a star in one year. The planet has mass = 5.6 1024 kg, radius = 6.7 106 m, and is 1.7 1011 km from the star.
    (a) Determine the angular momentum for the rotating planet about its rotation axis (assume a uniform sphere).
    kg · m2/s

    (b) Determine the angular momentum for the planet in its orbit around its star (treat the planet as a particle orbiting the star).
    kg · m2/s






    2. Relevant equations



    3. The attempt at a solution

    I started this problem, but I am not sure how to do part b.
    a)
    I=2/5MR^2
    I=2/5(5.6*10^24)(6.7*10^6)^2
    I=1.01*10^38
    w=1rev/day*2pi/1rev*1day/24hrs*1hr/60min*1min/60sec=7.27*10^-5
    L=Iw
    L=(1.01*10^38)(7.27*10^-5)
    L=7.34*10^33
     
  2. jcsd
  3. Nov 24, 2008 #2

    LowlyPion

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    Homework Helper

    The moment of inertia that you calculated in a) is for the rotation about the axis of the planet.

    In b) they are asking you to calculate the angular momentum about it's star. They also suggest to treat it as a point mass.

    I = m*r²

    L = m*r²*ω = 2π*m*r²/t
     
  4. Nov 24, 2008 #3
    Thanks!
     
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