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: Average Angular Acceleration of a CD

  1. Nov 27, 2011 #1
    1. The problem statement, all variables and given/known data

    A CD has a playing time of 59.8 minutes. When the music starts, the CD is rotating at an angular speed of 460 revolutions per minute (rpm). At the end of the music, the CD is rotating at 202 rpm. Find the magnitude of the average angular acceleration of the CD. Express your answer in rad/s^2.

    2. Relevant equations

    angular acceleration = change in angular velocity / change in time

    3. The attempt at a solution

    initial angular velocity in rad/s:
    460rpm / 60 * 2 * Pi = 48.17rad/s

    final angular velocity in rad/s:
    202rpm / 60 * 2 * Pi = 21.15rad/s

    time in s:
    59.8min * 60 = 3588s

    angular acceleration:
    (21.15 - 48.17) / 3588 = -.00753rad/s^2

    I'm really not sure why this is incorrect. I feel like I'm making a really obvious/face-palm type mistake.
  2. jcsd
  3. Nov 27, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I agree with your answer.

    It sounds like you are submitting this for online grading. For some reason these are known to have bugs in terms of accepting answers that are slightly off or in a different format from what the accepted answer is. I am always puzzled as to why they can't get it to act right.
  4. Nov 28, 2011 #3
    You're exactly right. I'm using the WileyPlus system and it expected scientific notation without explicitly stating so.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook