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Disk (horizontal axis) with string added masses angular velocity after 2 seconds?

  1. Oct 27, 2008 #1
    1. The problem statement, all variables and given/known data

    A disk with a mass of 4kg and a .20m radius is free to rotate about a horizontal axis. A 4.0kg mass and a 8.0kg mass are attached by a massless string, which is hung over the disk. If the string does not slip, what is the angular velocity of the disk after 2 seconds?

    2. Relevant equations

    Angular acceleration = change in angular velocity/change in time
    I= 1/2mr^2
    Angular velocity= tangential velocity/radius

    3. The attempt at a solution

    I took a picture of my work. It looks like I did everything right, but I'm not sure. I thought I would use inertia in my problem solving, but I didn't need it?? All I did was, I found the acceleration of the heavier block using the formula in the picture (which includes the mass of the disk). I took that acceleration, which should equal the tangential acceleration, and found the tangential velocity after 2 seconds. With the tangential velocity, I found the angular velocity using the radius of the disk shown in the picture. Found it to be 28 rads/sec which is about 44 revolutions per second. I think this would be pretty fast when dealing with only 4 and 8kg weights... Is this correct? If you can't make something out in the picture, let me know. THANKS!!

  2. jcsd
  3. Oct 28, 2008 #2


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    Science Advisor
    Homework Helper

    Hi Mugen112! :smile:

    I don't follow what you've done … I particularly don't follow your T1 - T2 equation … you would need torque = rate of change of angular momentum.

    But why bother to find T1 and T2 when the question doesn't ask for them?

    Just use KE + PE = constant. :smile:
  4. Oct 28, 2008 #3
    Re: Disk (horizontal axis) with string added masses... angular velocity after 2 secon

    I figured it out! Thanks for the help!
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