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Homework Help: Rotational Motion of spokes

  1. Oct 16, 2008 #1
    1. The problem statement, all variables and given/known data

    A wheel with 16 spokes rotating in the clockwise direction is photographed on film. The film is passed through a projector at the rate of 24 frames/s, which is the proper rate for the projector. On the screen , however, the wheel appears to rotate counterclockwise at 4.0 rev/min. Find the smallest possible angular speed at which the wheel is rotating.

    2. Relevant equations
    1 revolution= 2pi radians
    angular speed = angular acceleration x time

    3. The attempt at a solution
    Since the wheel is actually turning clockwise, its acceleration will be negative, but when its shown on the projector, the acceleration is positive. I assume that the "proper rate" means that the acceleration as shown on the projector is of the same magnitude as that in real life. I set up angular speed1= acceleration1 x time and angular speed2= acceleration2 x time (where 1 is on the projector and 2 is actual). Since time is the same, I set the two equations equal to each other and got speed1/acceleration1=speed2/acceleration2 or speed2 = (acceleration2 x speed1)/acceleration1. Because the projectors working at the "proper rate" acceleration1= negative acceleration2 and so speed2=-speed1.

    I don't think I'm following this right because I'm only using one of the numbers in the problem, unless the other two are just extraneous information.

    Thanks in advance.
  2. jcsd
  3. Oct 16, 2008 #2


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    Homework Helper

    4 revs in a minute is 15 seconds to make the "virtual" revolution.

    Doesn't that mean the precession will appear slower by 1 revolution over that many frames?
    If it was turning at exactly that speed wouldn't it appear to be not moving at all?
  4. Oct 16, 2008 #3
    I'm sorry but I don't think I'm following what you said in the second part of your reply. Could you explain?
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