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Homework Help: Period of a star

  1. Mar 28, 2010 #1
    1. The problem statement, all variables and given/known data

    The mass of a star is 1.370×1031 kg and it performs one rotation in 38.10 day. Find its new period (in days) if the diameter suddenly shrinks to 0.330 times its present size. Assume a uniform mass distribution before and after.

    2. Relevant equations
    Ii * Wi = If * Wf
    2/5MR^2 * 1rot/38.1 days = (2/5MR^2*0.330^2) * Wf

    3. The attempt at a solution

    I am not sure what cancels out

    I thought it would be something like

    1rot/38.1days = .330^2 * Wf

    solve for Wf

    But that didn't work. .241s was not correct
  2. jcsd
  3. Mar 29, 2010 #2


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    Take care of the units. rot/day is not second.

  4. Mar 29, 2010 #3
    I still don't understand because if I change the bottom number to seconds I still have rot/sec and that is not seconds. Plus it asks for the period in days so doesn't that factor in?
  5. Mar 29, 2010 #4


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    wi and wf have the same units.

  6. Mar 29, 2010 #5
    it asks for the new period in days! I am not sure what to do with this problem
  7. Mar 29, 2010 #6
    I am getting like 2198.249 as an answer.
  8. Mar 29, 2010 #7


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    You replaced wi by 1rot/39.1days. Then you should do the same with wf: wf=1rot/xdays.

    Rewrite your equation

    1rot/38.1days = .330^2 * Wf


    1rot/38.1days = 0.330^2* (1rot/x days)--->1/38.1 =0.330^2/x.

    Solve for x.

  9. Mar 29, 2010 #8
    thank you, i think i just didn't understand that you needed the x on the other side of the equation and that is what was giving me such a large answer
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