1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook