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Rotation at speed c

  1. Aug 15, 2012 #1
    I'm assuming the maximum number of revolutions per second for a disk is defined as speed c divided by the circumference of the disk, eg a disk with a circumference of half a meter is allowed to rotate twice as fast per second as a disk with a circumference of one meter.

    C = circumfrence of the disk
    c = speed of light
    mrps = maximum revolutions per second (not meters per second)

    So is the value of mrps nice and simple : mrps = c / C

    Or are there more complicated relativistic affects, for example does the circumference shrink due to length contraction.
  2. jcsd
  3. Aug 15, 2012 #2


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    The relationship that the velocity in the lab frame is 2*pi*r* revolutions / second, where r is the radius in the lab frame, doesn't change in the lab frame. The circumference of the disk in its own "frame" (which is not really a frame!) is different (larger) than 2*pi*r however. See any of the threads about the Ehrenfest paradox.
  4. Aug 15, 2012 #3
    I thought something strange was going to happen but wasn't sure, thanks for the info.

    I found this link, http://en.wikipedia.org/wiki/Ehrenfest_paradox

    But I can't follow most of the math, why are they complicating things by using radius * pi? If I know the circumference I don't need pi. A circumference of 1m means it can rotate 299792458 times a second.
  5. Aug 15, 2012 #4
    Oh wait I think I see why they don't use its circumference.
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