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Thin rods (H) rotating about an axis

  1. Feb 10, 2009 #1
    1. The problem statement, all variables and given/known data
    A rigid body is made of three identical thin rods, each with length L=0.600m, fastened together in the form of a letter H. The body is free to rotate about a horizontal axis that runs along the length of one of the legs of the H. The body is allowed to fall from rest from a position in which the plane of the H is horizontal. What is the angular speed of the body when the plane of the H is vertical?

    2. Relevant equations
    Not really sure

    3. The attempt at a solution
    I am not really sure where to start since all that is given is L. I think that each rod must have its moment of inertia calculated separately about the axis using the perpendicular and parallel axis theorems, but we have not reviewed these subjects extensively and I am not sure of how I would go about doing this.
  2. jcsd
  3. Feb 10, 2009 #2


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    Homework Helper

    Yes you are sensing the right solution.

    As I read it the plane of the H rotates about one leg. Since one leg is already the pivot it can be disregarded. (r=0 after all.)

    So that means then you have two more moments to account for.

    Do you know the moment of inertia for a rod pivoted about 1 end? It's easily looked up or determined.

    Then you just need to figure the moment of the 3rd rod. But since it is just falling and not rotating won't it simply act like a point mass at the end of a rod?

    After you have the moment of inertia you know how to figure the potential energy to rotational kinetic energy don't you?
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