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Homework Help: Conserved angular momentum

  1. Mar 6, 2013 #1
    1. The problem statement, all variables and given/known data

    A molecular cloud 0.2 light years across rotates once every 1.4 ×107 years. If it were to collapse to the size of our Sun, how long would it take to rotate once if it conserved angular momentum given by mvr, where m is mass, v is the circular rotation speed and r is the radius. Is this reasonable in this case?

    2. Relevant equations


    3. The attempt at a solution
    the suns rotation is approx 26.8 days
    the radius i worked out to be 0.1 x 9.4605284 × 10^15 metres = 9.4605284 x 10^14 metres
    rotation= 1.4 x 10 ^7 years
    do i also use the mass of the sun?
    in which case m=1.99 x 10^30 kg?

    then sub into
    l=mvr ?
  2. jcsd
  3. Mar 6, 2013 #2


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    You don't care about the mass or the rotation rate of the Sun. On collapse, the mass will be the same as the original cloud.
    What is the angular momentum of the cloud before collapse? (I don't know whether you're supposed to assume a ball or a disc.)
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