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Finding time in a pulley system

  1. Dec 1, 2015 #1
    1. The problem statement, all variables and given/known data
    A horizontal bar with a mass of 3.2 kg and a length L of 64 cm is rigidly mounted to a vertical spindle of negligible mass such that the two objects spin together. The spindle has a diameter of 2.0 cm, and it is attached to the bar a distance of L /4 from its centre of mass. A string is wrapped around the spindle, and is pulled with a steady force of 15.0 N. The string is wrapped four times around the spindle.
    If the system rotates at an angular speed of 5.5 rad/s when the string unwinds fully and drops from from the spindle, after the string has fully unwound, how long does it take for the system to come to a stop?

    2. Relevant equations


    3. The attempt at a solution
    I've tried using Δω=α Δt but I don't know what α is? I'm pretty lost on what to do.
     
  2. jcsd
  3. Dec 1, 2015 #2

    haruspex

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    The question implies it will come to a stop. Why would that be, do you think?
     
  4. Dec 1, 2015 #3
    I'm assuming there is a frictional torque that would cause it to stop, but I don't know how to use that to determine the time.
     
  5. Dec 1, 2015 #4

    haruspex

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    There is enough information to deduce the frictional torque.
     
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