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Homework Help: Tricky Rotational Problem

  1. Nov 26, 2007 #1
    1. "A small ball of mass 1.06 is attached to each end of a thin steel rod of length 1.2 m and mass 6.4kg. The rod is constrained to rotate in the horizontal plane about a vertical axis through its midpoint. At some instant, it is observed to rotate with an angular velocity of 30 rev / s. Due to friction, it comes to rest 32 seconds later. Assuming a constant frictional torque, compute

    the retarding torque exerted by friction.


    First I converted the initial angular velocity to rad/s which is 188.4 rad/s

    I computed the angular acceleration to be -5.89 rad/s2

    we know the final angular velocity is 0

    I THOUGHT i could do

    torque,net = I*angular acceleration

    and assume that friction is the only torque thats making it slow down ??

    I'm so confused..
    I think I is (1/3)M,rod*L^2

    any help or suggestions would be greatly appreciated.
  2. jcsd
  3. Nov 26, 2007 #2
    I is not that, since a ball is attached to the end, I,total would be I of the rod plus M(ball)(L/2)^2
    hope it helps
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