Originally posted by sonhouse
So I know if I blink my eyes as fast as I can, I am making 100 millisecond clips of time. So I blink my eyes like that at my overhead fan, and I notice I can stop the blades ala strobe effect. However, the blades are not crisp images like those strobes which flash for a few microseconds, my eyes are open for 100 millisecond clips. So in that 100 ms, the bla ...[text shortened]... is blurry, looks twice as wide. So with that info, how fast is the blade turning?so it is moving
In 100 milliseconds the blade tip moves 150 mm
so, converting to metres and seconds:
Velocity of tip = 0.15 / 0.1 m/s = 1.5 m/s
blade diameter = 1.3 metres
blade circumference = 1.3 * pi metres
so tip will travel circumference in
1.3 * pi / 1.5 seconds
and the rotational speed is 1.5 /( 1.3 * pi) s^-1
= 0.367 rotations per second, a slowly rotating fan.
The extra constraint is that the blades must appear stopped.
You are could be closing your eyes for 2.6 seconds, which is what would be needed for the first blade to come round again (( 1.3 * pi - 0.15)/1.5 = 2.6), but that sounds a bit long...
So perhaps it is a 4 bladed fan and you are closing your eyes for about 0.66 seconds between snapshots?