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Homework Help: Angular Momentum of a conical pendulum

  1. Jul 14, 2007 #1
    1. The problem statement, all variables and given/known data

    A small metallic bob is suspended from the ceiling by a thread of negligible mass. The ball is then set in motion in a horizontal circle so that the thread describes a cone.
    Given: Length of string = 2.8m
    Angle between string and vertical: 21 degrees
    Mass of bob: 2.9kg

    2. Relevant equations

    I = mr^2
    w = square root of [g/(lcos(theta)]
    Momentum = I x w

    3. The attempt at a solution

    I first calculated the angular velocity by plugging numerical values into the equation and got 1.936 rad/s. Then, I found out the moment of inertia of the bob by multiplying 2.9 x 2.8^2 [this is the step I'm not sure of], but the answer wasn't right. Is there something I missed out?
    Thank you for your help!
  2. jcsd
  3. Jul 14, 2007 #2
    The raidus is wrong. It should be r * theta
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