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Homework Help: Rotating ring supported at a point

  1. Feb 3, 2009 #1
    1. The problem statement, all variables and given/known data

    1. A ring 1.5 m in diameter is pivoted at one point on its circumference so that it is free to rotate about a horizontal axis. Initially, the line joining the support and center is horizontal.

    a. If released from rest, what is its maximum angular velocity.

    b. What must its initial angular velocity b if it is to jst make a complete revolution ?
    2. Relevant equations


    3. The attempt at a solution

    I don't know how to treat the ring as. Should I treat it as a hoop ?

    How should I calculate maximum velocity ? I don't know what formula to use.
  2. jcsd
  3. Feb 4, 2009 #2
    Re: Rotation

    Any hint on what equation I should start on ??
  4. Feb 4, 2009 #3


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    Homework Helper

    Re: Rotation

    Rotation suggests that you should be concerned about the moment of inertia and rotational kinetic energy sounds like to me.

    In this case your moment is displaced, but I'm sure you can figure it out with the parallel axis theorem.
  5. Feb 4, 2009 #4
    Re: Rotation

    Yeah, I can definitely find I.

    KE=(1/2)I*w^2 so I am still missing KE to solve for W right ?
  6. Feb 4, 2009 #5
    Re: Rotation

    So I got part a.

    In part b. I use kinetic energy. I need to solve for v initial and I can calculate gravitational potential energy. However, the velocity at the top is not 0 so how can solve this ?
  7. Feb 4, 2009 #6


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    Homework Helper

    Re: Rotation

    The question is asking you:
    How much more initial ω needs to be added to make it past the top when it gets there.

    (Hint: How much more potential energy will it have to have stored when it gets to the top?)
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