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Homework Help: Angular Momentum/Velocity HW Help

  1. Aug 1, 2012 #1
    1. The problem statement, all variables and given/known data

    A device can be used to measure the speed of a bullet. The device consists of two rotating disks, separated by a distance of d = 0.850 m, and rotating with an angular speed of 95.0 rad/s. The bullet first passes through the left disk and then through the right disk. It is found that the angular displacement between the two bullet holes is 0.240 rad. From these data, determine the speed of the bullet.

    2. Relevant equations

    Angular displacement = arc length/radius
    Angular velocity = angular displacement/time
    3. The attempt at a solution
    I attempted to set up an equation, but I simply couldn't.
    Last edited: Aug 1, 2012
  2. jcsd
  3. Aug 1, 2012 #2
    What is the angular displacement of the first disk during the bullet's flight between the disk? Of the second one?
  4. Aug 1, 2012 #3


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    Gold Member

    The definition of Velocity is distance/time

    The distance is known so you just need to work out the time it takes to travel the distance between the discs.

    The definition of angular velocity is

    Angular velocity = angle/time

    So rearrange that to give an equation for the time. Plug that into the first equation.
  5. Aug 1, 2012 #4
    Please clarify? I've been stumped for a long time.
    Following your advice, time = angle/angular velocity
    But how do I plug it in?
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