Ulysses wrote:
So then if a saw blade is rotating at a speed which is a multiple of 120
(such as 4800 rpm) then it could appear to be stopped, right?

Yes. Say you had marked one tooth on the saw blade with red paint,
then the saw blade would have rotated 4800/120 = 40 times around
between each "flash" of the fluorescent light, and you would see that
red tooth appear to stand still. For this example, if the saw was
going a little bit faster or slower, you could see it freeze with 39 or
41 revolutions for 39 * 120 = 4680 RPM or 41 * 120 = 4920 RPM.
Rotational speed between these RPM numbers would not result in any
freezing action, so for this example you can see that the effect is
rather sensitive to slight variations in the RPMs of the saw blade.

If you didn't mark one saw tooth, then this sitation becomes even more
sensitive to slight variations in RPMs. Say there are 40 identical
teeth on the saw blade, so each tooth is 1/40th of a revolution (=
0.025). At 4800 RPM, you will freeze the blade because it is exactly
40 revolutions, but you can also freeze the blade with exactly 40.025
revolutions, which would be 40.025 * 120 = 4803 RPM, because on the
next flash of the fluorescent light the adjacent saw blade tooth will
be in the same position as the first saw blade tooth was on the first
fluorescent light flash. So you can freeze the blade at 4800 RPM or
4803 RPM, but between those speeds there will be no freezing.

Ken