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Homework Help: Torque need to stop rotation

  1. Jun 6, 2009 #1
    1. The problem statement, all variables and given/known data

    A 1.02 kg ball and a 2.06 kg ball are connected by a 0.95 m long rigid, massless rod. The rod is rotating clockwise about its center of mass at 38 rpm. What torque will bring the balls to a halt in 8.53 s? (Give an absolute value of torque.)

    2. Relevant equations

    torque = F * r sin theta
    (rpm/60)*2pi= angular velocity (int)
    anglv fin = anglv int + anglaccel * delta t
    accel = anglaccel * r

    3. The attempt at a solution

    First I found the angular velocity initial of the system using the (38rpm/60)*2pi
    then I found angular acceleration need to reach angular velocity final = 0 in 8.53s by
    0 = (38rpm/60)*2pi + angular accel * 8.53s
    I solved for accel by multiplying the answer found for angular accel by radius which is 0.95/2m.
    Then i found the torques (F=ma so mass of ball 1 and 2 * accel * radius) and added them together since, I assumed that both forces were in the same direction. I thought that these torques added together would give me my answer. But I'm getting it wrong. Can any one help? did I overlook some element to this problem.
  2. jcsd
  3. Jun 6, 2009 #2


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    Homework Helper

    Well when you find the angular deceleration,[itex]\alpha[/itex]. I believe you can just use [itex]\tau = I \alpha[/itex], where I is the moment of inertia.
  4. Jun 6, 2009 #3
    How would i solve for the moment of inertia? Is it I=1/12*M*L^2 for a rod with axis at center? what would the M be?
  5. Jun 6, 2009 #4


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    I guess since they said the mass is negligible you can just ignore it and just use I=mr2 for the two masses
  6. Jun 6, 2009 #5
    That did it. Thanks
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