Originally published at: https://boingboing.net/2024/06/01/optical-illusion-causes-blades-cutting-vegatables-at-high-speed-to-look-like-they-are-slowly-spinning.html
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How is there not a large safety screen covering all but the bit where the vegetables shoot out?
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Blades will appear stationary if revolutions per second = n* frame rate / number of blades. Assuming n is a positive integer and blades are identical and evenly spaced.
Illusion! Or perhaps disillusionment?
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revolutions per second!? how does this apply to guillotines!?
( ohh… the other kind of revolution. )
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As a bored young passenger in the back of our family van, I noticed this phenomenon, while looking at the hubcap spokes of passing cars.
At certain speeds, the wheels look as if they are moving slowly or even backwards.
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Well you can be sure it Hertz
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With your eyeballs and not a camera?
The phenomenon can be seen at night with the flicker of street light.
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Still not a sufficient frequency.
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My high school physics teacher demonstrated this with a strobe light and a fan. I had some good teachers in high school. Thanks Mr. Brown!
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Mmmmm… floor spice.
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Actually, there’s a door visible, which is open…and doesn’t seem to have a safety switch to lock out the motor. So either the machine is inherently unsafe or someone jimmied it for a cool effect.
ETA: just rewatched it and there appears to be no switch 
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Many moons ago at Rocketdyne, or lab’s vibrations group would spice up the annual open house events with a truly entertaining display. It involved a large balsawood model airplane (5’ wingspan) on one of their shaker tables, turned down lights, and a variable-speed strobe light. Playing with the shaker’s level of vibration and the strobe’s speed, they created an uncanny illusion of the model’s wings smoothly flapping! Many oohs and aahs from everyone. So cool!
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In a similar vein, the Oscylinderscope exhibit at the Exploratorium in San Francisco shows off the effect on guitar strings, without the strobe light.
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I guess you’re just supposed to “know” which the dangerous bits are. Nevermind the fact that a slip/trip or any inadvertant contact could send a person into the “whirligig of doom”.
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I’m wondering if it’s possible you created this memory from memories of seeing TV/movie footage of hubcap spokes. The phenomenon can’t be seen by eyes in the daytime, but memories are incredibly suggestible.
(I have a distinct memory of sitting in the backseat of the car and putting my arms up to frame a sunset which – if it had been real – would have meant the Earth was currently being sucked into the sun. Looking back, I assume I mixed in memories of zoomed-in sunsets from movies, of the kind cowboys ride off into.)
OR! As @vonbobo says, maybe it was the effect of flickering streetlights at night.
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It’s possible it was a night time phenomenon, it was a very long time ago, so the particulars have faded. But, I feel as if I did see this with my own eyes, just don’t call me as a witness in a jury trial. 
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