Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Two disks (moment of inertia)

  1. Nov 20, 2005 #1
    What is the final angular speed?

    I came up with 9.4 rad/s, which is correct.

    What fraction of the initial rotational kinetic energy is converted to heat in the process?

    I came up with .0588, which is also correct.

    A motor must restore the angular speed of the combination to wo in one revolution. What torque must the motor supply ?

    I used the equation [itex]\tau=I\alpha[/itex] here. I added each moment of inertia (for each disk individually):


    The above I got using the standard formula for the moment of inertia for a disk (1/2)mr2.

    Now I found the acceleration:


    Then I multiplied the acceleration by the moment of inertia to come up with:


    ...however the above is incorrect. Could someone please tell me why? I have gone through my steps many times, meaning that the only way this could be wrong is if I took the wrong steps; my arithmetic is fine. I'd appreciate any input on this.

    Thank you very much.
  2. jcsd
  3. Nov 20, 2005 #2
    your procedure is correct ... your final solution for tau does have some round-off error in it from rounding the final omega from the 1st part of the problem ... 9.4 rad/s.

    If you don't round, you should get tau = .0913092394 Nm

    If this is an on-line problem that is machine graded, that could be the reason.
  4. Nov 20, 2005 #3
    Thank you very much. It's nice to know my problem was in rounding digits rather than my procedure (it is web-based hw by the way).

    Thanks again.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook