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Determining New Angular Velocity

  1. Apr 19, 2015 #1
    1. The problem statement, all variables and given/known data
    A star with mass 2.4x1030 kg and diameter 1.7x109 m rotates once every 34 earth days. It eventually dies and becomes a white dwarf, shedding about half of its mass and contracting to a diameter of 1.9x108 m. If we approximate the star as a perfect sphere, what is its resulting angular velocity?

    Initial star r = 8.5x108 m
    Initial star m = 2.4x1030 kg
    Initial star ω = 1 rev/34 days = 1.85x107 rad/s
    White dwarf r = 9.5x107 m

    2. Relevant equations
    I=(2/5)MR2

    3. The attempt at a solution
    I'm getting really stuck on this one. I can determine I for the initial star (I=(2/5)(2.4x1030 kg)(8.5x108)2 m=6.94x1047 kgxm2, but from there I'm not sure where to go. I'm not even sure if that's relevant. I'm not looking for a handout here, but any guidance would be appreciated!
     
  2. jcsd
  3. Apr 19, 2015 #2
    Why do you need to know I?
     
  4. Apr 19, 2015 #3
    There are no external torques in this problem, therefore angular momentum is conserved.

    h = Iω
     
  5. Apr 19, 2015 #4
    Yes, so can you write an equation that shows this?
     
  6. Apr 19, 2015 #5

    haruspex

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    Yes, but be careful. What do you think the angular momentum of the shed mass will be at the end?
     
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