View Full Version : backward tires/fans
10-08-2001, 09:05 AM
Long ago, I read one of Cecil's columns regarding the optical illusion that makes rotating wheels or fan blades appear to spin in the opposite direction. Some guy wrote in claiming to have seen this, and Cecil basically said he was insane and that the only way you can see this is if it's recorded and played back. The explanation was this weak crap about frame rates of film or television.
Well, I've seen spinning objects appear to rotate in the opposite direction. Does that make me insane, or is there a better explanation than frame rates? I would tell you where to look up the column, but it's from the late '80s.
10-08-2001, 09:31 AM
I remember the column you're referring to, but I cannot find it in the archives. I've searched for minutes to no avail. The way I remember it the column ended with an acknowledgment by Cecil had actually seen the effect himself.
It is simple really. All you need is a light source flashing with a frequency slightly higher than a multiple of the number of spokes * the rotational frequency of the wheel. Say that a car with five-spoke 1m circumference tires moves at just below 20m/s (72km/h) the spokes flash by any point at just below 100Hz. Assume that you are in Europe (or any other part that uses 50Hz AC) most street lights (fluorescent tubes or high pressure sodium) will flicker at 100Hz. That means that for every flash the spokes will appear slightly earlier in their cycle than on the last flash. Thus the apparent retrograde motion.
Off course the effect cannot be seen in daylight, or with incandescent lightbulbs (as they emitt an almost constant light), or if there are several lamps connected to different phases iluminating the object.
10-08-2001, 09:33 AM
One more thing. As this started out as a comment to Cecils column, you should have posted it in the forum aptly named "Comments on Cecil's Columns".
I would venture to guess that one of our friendly moderators will move it accordingly.
10-08-2001, 09:53 AM
It took some doing to find it online, but I finally did. How come the wheels of a moving car appear to rotate backward sometimes? (http://www.straightdope.com/classics/a3_199.html) It can also be found on p. 199 of Return of the Straight DopeOriginally posted by tc
I would venture to guess that one of our friendly moderators will move it accordingly. I can't wait for one of the friendly moderators, so I'll damn well move it myself.
10-08-2001, 10:25 AM
What I was trying to get across is that I don't buy that explanation because I am convinced I have seen this effect in incandescent and sunlight. Now, I'm not discounting the possibility that I'm insane, but I did want to see if anyone had a different explanation.
10-13-2001, 05:57 AM
You might want to read the material at that link to Cecil's column, because it contains a retraction, under the heading Facts Au Jus.
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