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Homework Help: Angular Velocity?

  1. Mar 12, 2007 #1
    Angular Velocity????

    A thin uniform rod is rotating at an angular velocity of 9.37 rad/s about an axis that is perpendicular to the rod at its center. As the figure indicates, the rod is hinged at two places, one-quarter of the length from each end. Without the aid of external torques, the rod suddenly assumes a "u" shape, with the arms of the "u" parallel to the rotation axis. What is the angular velocity of the rotating "u"?

    Before: The system is a single rod of mass M and length L rotating about an axis through its center.
    After: The system consists of three parts; a rod of mass M/2 and length L/2 rotating about an axis through its center and two masses M/4 rotating at a distance L/4 from the axis. (Treat these masses as particles.)

    I know I should use the conservation of momentum, moment of inerta for the rod, Net torque= Inertia x angular acceleration but I have no idea where to start!!!

    Help me exam tomorrow!!!
  2. jcsd
  3. Mar 13, 2007 #2


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    Indeed, conservation of momentum is the way to go.
    The momentum before is [itex]I_1\omega_1[/itex] and after it's [itex]I_2\omega_2[/itex].
    So ask yourself what's given, what can you calculate and what is the unknown?
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