1. Not finding help here? Sign up for a free 30min 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!

Period of a Star Please help

  1. Oct 21, 2007 #1
    Period of a Star...Please help!!!

    1. The problem statement, all variables and given/known data
    The mass of a star is 1.250×1031 kg and it performs one rotation in 36.30 day. Find its new period (in days) if the diameter suddenly shrinks to 0.590 times its present size. Assume a uniform mass distribution before and after.

    I don't know where to begin!!!
     
  2. jcsd
  3. Oct 21, 2007 #2

    hage567

    User Avatar
    Homework Helper

    Think about conservation of angular momentum. What do you know about that?
     
  4. Oct 21, 2007 #3
    Torque(net) * Change in T = Change in L
    Ii * Wi = If * Wf
     
  5. Oct 21, 2007 #4

    hage567

    User Avatar
    Homework Helper

    OK, so what do you think you might do next? What is I for a solid sphere?
     
  6. Oct 21, 2007 #5
    2/5MR^2 but how do we get R?
    1 rotation/36.3days * pi = W
     
  7. Oct 21, 2007 #6

    hage567

    User Avatar
    Homework Helper

    You don't need to know the radius. In the question, you are given the final radius in terms of the first, so it will cancel out.

    Be careful. This isn't quite right. If you are going to convert this, remember there are 2*pi radians in one revolution.
     
  8. Oct 21, 2007 #7
    Right thats what I thought, but I don't know how to convert rotations to revolutions....
     
  9. Oct 21, 2007 #8

    hage567

    User Avatar
    Homework Helper

    1 rotation = 1 revolution. They mean the same thing. There are 2*pi radians per rotation (or revolution).
     
  10. Oct 21, 2007 #9

    hage567

    User Avatar
    Homework Helper

    You could leave Wi in terms of rotation/day for this particular problem since all of your other units will cancel out. Of course, if you're unsure how to do it, it's good practice to give it a try.
     
  11. Oct 21, 2007 #10
    Ii * Wi = If * Wf
    2/5MR^2 * 1rot/36.3 days = If * Wf
    I am not sure how we figure out If?
     
  12. Oct 21, 2007 #11

    hage567

    User Avatar
    Homework Helper

    If will be the same equation where the only thing that changes is R since the star becomes smaller in diameter. You are given information in the question that allows you to determine how much smaller it becomes

     
  13. Oct 21, 2007 #12
    Ii * Wi = If * Wf
    2/5MR^2 * 1rot/36.3 days = (2/5MR^2*0.590) * Wf
     
  14. Oct 21, 2007 #13

    hage567

    User Avatar
    Homework Helper

    This needs to be squared as well, since it is part of the radius term.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?