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Angular acceleration problem Help!

  1. Sep 6, 2008 #1
    A space station is constructed in the shape of a wheel 23 m in diameter, with essentially all of its 7.9×10^5 kg mass at the rim. Once the station is completed, it is set rotating at a rate that requires an object at the rim to have radial acceleration g, thereby simulating Earth's surface gravity. This is accomplished using two small rockets, each with 150 N thrust, that are mounted on the rim of the station.

    How long will it take to reach the desired spin rate?

    T = rFsinTHETA
    T = I {alpha}
    I = MR^2

    Finding anuglar acceleration:
    2RF = MR^2 {alpha}
    2F = MR {alpha}
    2F/MR = {alpha}

    Finding the desired spin rate:
    a = v^2/r
    = w^2r^2 / r
    = w^2r
    w = sqrt(a/r)
    = sqrt(g/r)

    Finding the time:
    t = w/ {alpha}
    = sqrt(g/r) * MR / 2F
    = sqrt(9.81 / 23) * (7.9*10^5) * (23) / (2 * 150)
    = 3.96 * 10^4

    Mastering Physics says I am wrong. Where is the mistake?
  2. jcsd
  3. Sep 7, 2008 #2

    I don't believe that this is such a difficult problem that noone can help me with a proper solution... This is just basic Physics B. So please help me.

  4. Sep 7, 2008 #3

    Doc Al

    User Avatar

    Staff: Mentor

    23 m is the diameter, not the radius.
  5. Sep 7, 2008 #4
    Wow, that's it? Wow... damn it. I read the problem many times over and never saw this stupid mistake. Thanks a lot. :)
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