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Homework Help: Unstretched Spring and Angular Velocity Problem

  1. Apr 12, 2008 #1
    [SOLVED] Unstretched Spring and Angular Velocity Problem

    1. The problem statement, all variables and given/known data

    A disk of mass 73 kg is held by a spring of unstretched length 88 cm and stiffness constant 430 N/cm, and rests on a frictionless table. It is then orbited in a circle at angular velocity .675 rad/s.

    By how much is the spring stretched? Answer in units of cm.

    2. Relevant equations

    After a bit of derivation, I came up with x = r/((k/mw^2)-1).

    3. The attempt at a solution

    r = 88
    k = 430
    m= 73
    w = .675

    It does not seem to work out. Am I being to complicated about it? Any help/hand-holding is greatly appreciated.
     
  2. jcsd
  3. Apr 13, 2008 #2
    Force acting on the disc in circular motion== mv^2/r

    Now use hook's law to calculate extension..
     
  4. Apr 13, 2008 #3

    Doc Al

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    Staff: Mentor

    Looks good to me.

    Make sure you are using standard units. Express lengths (and k) in terms of meters, not cm.
     
  5. Apr 13, 2008 #4
    Thanks for the catch on conversion. I changed length to .88 and K to 4.3, getting an answer in m. I then converted it to cm but the online system still says I am incorrect.

    I can't use mv^2/r because I don't have v. That is why I did that insane derivation.

    Or am I wrong?

    I think I'm a few baby steps away from having this.
     
  6. Apr 13, 2008 #5

    Doc Al

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    Staff: Mentor

    Redo that last one. :wink:
     
  7. Apr 13, 2008 #6
    Uh oh. Did I go the wrong way with it? 43000N/m?
     
  8. Apr 13, 2008 #7

    Doc Al

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    Staff: Mentor

    That's better.
     
  9. Apr 13, 2008 #8
    Oh, thank you so much *sighs in relief*

    Converting correctly makes the problem work. Who knew? :D
     
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