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Homework Help: Rigid body rotation question

  1. Nov 19, 2008 #1
    a plank is conected horizontally at both ends to two points. One of the points dissapears and the other turns into a pivot. show that the planck of length l,mass m pivoted at one end has an acceleration (3/2)*g initially

    i used parallel axis theorm do derive the moment of inertia of a rod with pivot at one end
    I = 1/3 (M*L*L)

    the torque acting at the CM of rod
    TAU=-Mg(postion of cm)=-Mg(x(cm))=-(0.5*l)Mg

    newtons second law in rotation

    ALPHA = TAU/I = (-0.5*l)Mg/(1/3*M*L*L) = (-) 3/2*g*l

    this is not what the question wanted as i have an extra l, what have i missed? thanks
    Last edited: Nov 19, 2008
  2. jcsd
  3. Nov 24, 2008 #2
  4. Nov 24, 2008 #3
    Your answer is correct, but they are asking for the acceleration which is equal to ALPHA*l
    Last edited: Nov 24, 2008
  5. Nov 24, 2008 #4
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