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All of this was done with a standing fan set horizontally on a table.

During an experiment, I had to tape a piece of rubber band to one of the standing fan's blade and measured the period of the fan. Taped to the free end of the rubber band is a small metal mass, in the shape of a doughnut. The combined mass of the rubber band and the mass is 1.95 g. At this point, the fan was spinning slow enough that I could approximately count its rotation, just like if I was counting music. I approximated 31 rotations over 5 seconds, which is about 6.2 Hz. Another periodic measurement, using camera, is approximately 187fps over 3.29 seconds, though I'm not sure how to get frequency/period from this.

But when I took the rubber band off, the period/frequency of the fan increased so drastically (that is to say back to its normal period) that I could no longer hear its distinct rotation and count anything. My question is: how did taping a small rubber band with a mass of 2 g affected the period of so drastically?

I have some more data (but not sure which is useful/junk) if any more is needed to calculate anything.

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  • $\begingroup$ Every thrown a balance weight from the wheel of your car? Or even if you haven't, do you understand the consequences of such an event? How might that relate to the issue in front of you? $\endgroup$ Apr 19, 2015 at 20:58
  • $\begingroup$ Maybe I haven't done it, but I don't understand the question. I also have never drove. $\endgroup$
    – Long Vuong
    Apr 19, 2015 at 21:15
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    $\begingroup$ You stated "when I took the rubber band off, the period/frequency of the rubber band increased so drastically". Did you mean to say "the frequency of the fan increased so drastically"? $\endgroup$
    – Floris
    Apr 19, 2015 at 21:24
  • $\begingroup$ Yes, yes I did. That was a mistake. Damn. $\endgroup$
    – Long Vuong
    Apr 19, 2015 at 21:32

2 Answers 2

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By adding the rubber band you did two things:

  • increased the air drag
  • unbalanced the fan

The bearings of a fan don't like to be unbalanced - the friction goes up significantly because as the fan picks up speed there will be a large lateral force (centripetal force keeping the rubber band plus object in their circular orbit). When you balance the fan it can run at its normal speed.

You will sometime see this during the spin cycle of a washing machine: if clothes are not evenly distributed the engine cannot get the clothes up to speed.

You may find that if you film the fan with the camera of your phone, it will actually appear to slow the motion right down because of the way the image sensor "scans". You get some freaky effects (bedding blades, blades that appear to "break", ...) but if you know the frame rate of your camera you can often estimate the fan speed quite accurately.

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The load increased but the input power driving the fan remained same. Moreover in very accurate measurements, the air drag can also not be neglected, all this will hold for a very another reason that the blades rotated by the motor are of very much comparable mass to the taped rubber & stuff.

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