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Homework Help: Problem on rotational dynamics

  1. May 19, 2013 #1


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    1. The problem statement, all variables and given/known data
    Given a rod of length 'l' placed upright on a smooth floor.A slight disturbance causes it to fall.Now required is the velocity with which the COM falls down when the rod makes angle θ with horizontal.

    3. The attempt at a solution
    now let the middle portion of the rod be A,and the lower portion B.Using conservation of angular momentum,we get
    By work energy theorem,
    mgl/2*(1-sinθ)=m(Va)^2/2+ml^2/12*ω^2 (ω about point B)
    What is the mistake I'm making here???
  2. jcsd
  3. May 19, 2013 #2
    Angular momentum is not conserved. The table has a torque on the COM of the rod, meaning there is an external force which has a torque.
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