1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes, but be careful. What do you think the angular momentum of the shed mass will be at the end?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted