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Homework Help: Rotational Kinetic Energy

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data
    A uniform rod AB is hinged smoothly at A. The length L of the rod is 50cm. Determine
    a) The angular velocity,

    of the rod when it makes an angle (Theta) =30o with the verticle.

    ( Moment of inertia of rod about axis through one end and perpendicular to the rod = 1/3 mL2 ; g =9.8m s-2

    2. Relevant equations
    Ki + Ui = Kf + Uf

    3. The attempt at a solution

    0 + mghi = 1/2 Iw2 + mghf

    Where hi = 1/2 L

    and hf = 1/2 L ( 1-cos Theta )

    Then plugging in the information given,
    Angular velocity = 7.1 rad s-1

    When the rod is vertical, let the horizontal line which passes through the center of the rod be the reference line where h=0 and U=0.

    Can anybody tell me why the final potential energy is not just 1/2 L cos Theta.

    Or is it 1/2 L ( 1-cos Theta ) ?

  2. jcsd
  3. Mar 1, 2009 #2


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    Science Advisor
    Homework Helper

    Hi Victorian91! :smile:

    (have a theta: θ :wink:)

    potential energy can be measured from anywhere

    it's your choice …

    if you chose to measure it from θ = 0, then you have make sure that it's 0 at θ = 0 :wink:
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