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# Strobe Light on a Fan

1. A strobe lights shines on a fan. By changing the speed of the strobe light, the fan appeared to run slower and sometimes even looked like it was stopping or running backwards. Assume the fan has three identical blades so that once the fan has rotated 1/3 around it looks exactly the same. Before turning on the strobe light we notice that the fan rotates at a rate of 120 rotations per minute (RPM).

b. If we define the period of the fan as the time it takes for the fan to rotate and appear to return to its "starting position", what is the frequency of the fan (Hz)?
c. At what minimum rate (in Hz) should we run the strobe light so that the fan appears to rotate at a speed of 120 RPM?
e. What is the rotation rate of the fan in deg/second?
f. If the fan appears to run backward at a rate of 30 deg/flash calculate one possible strobe light flash rate.

#### Solution Preview

(b)
The frequency is the number of rotations per second = (number of rotations per minute)/(60 seconds) =
120 round/min / ( 60 sec/min) = 2 (round/sec) = 2 Hz,
and the period is T = 1/(2 Hz) = (1/2) sec

(c)
This is a somewhat psychological question on how we get the impression on how the fan rotates.
The likely answer seems to ...

#### Solution Summary

This solution provides a description of a strobe light on a fan.

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