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Homework Help: Need help with hw problem (torque question)

  1. Apr 27, 2008 #1
    The question is:

    A solid disk with m=60.0kg and a diameter of 48.0cm is to be turned from rest through 12.0 revolutions in 6.00s. Calculate the torque required to accomplish this.

    What ive got is this:

    Tau=I*al(angular acceleration)

    I=.5*m*r^2 .5*60.0kg*.240m^2=1.728kg*m^2

    Al = om/t (convert to om and then to al)12revs/6sec=2.00revs/sec = 12.56rads/s / 6

    sec = 2.09 rads/s^2=al

    1.728kg*m^2 * 2.09rads/s^2= 3.61N*m ...is this right?

    This is an online physics course and I am getting a little lost at this point. The course notes are vague at best and I am just not sure if i have the irght answers or not.
    ANy help would be appreciated.
  2. jcsd
  3. Apr 27, 2008 #2


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    Science Advisor

    If a disk has a constant angular acceleration, a rad/s2, then in t seconds, it will have reached angular speed at rad/s and will have moved (1/2)at2 rad or (1/2)[\pi]at2 revolutions. Another way of looking at that is that if it has constant acceleration, a, it will have average speed (1/2)at over time t. Moving through 12.0 revolutions in 6 seconds means it had an angular acceleration of 12/72= 1/6 rev/s2= [itex]\pi[/itex]/6= 0.523 rad/sec2, not 2.09.

  4. Apr 27, 2008 #3
    still confused

    I am sorry.
    One of the problems is that this proffesor, while a good and helpful person, has used his own characters for the variables..ie "om" instead of w for omega, and "al" for angular acceleration. While i know he thinks hes helping, it is causing me great problems when i refer to the text or get help from others, I am getting "lost in translation" because I dont understand the language. I am very frustrated because i know this is simple.

    Is "at" angular acceleration?

    I dont see how to get the acceleration from the given info.
  5. Apr 27, 2008 #4
  6. Apr 27, 2008 #5
    It is unfortunate that your professor does that. I would suggest buying a used textbook and create a key or something so you can translate. Anyway, you are looking for angular acceleration. Heres the equation in the proper language:

    a(avg)=w2-w1/t2-t1 rad/s^2

    average angular acceleration = change in angular speed divided by change in time
    Units: radians per second squared

    Once you find this, you can easily derive the torque. Good luck!
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