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SYFY WIRE Bad Astronomy

The Magic of Physics: A Water Spiral

By Phil Plait

Via Boing Boing (which was sent to me from BABloggee Jeremy Huggins) I saw this extremely cool video of water apparently frozen in time, spiraling away from a hose as if by wizardry:

How is this demon magic possible? Easy: It’s not magic. It’s SCIENCE.

The hose is attached to a speaker. Speakers work by vibrating at certain frequencies, pumping the air at those frequencies, which is picked up by our ears and interpreted by our brains as sound.

Video cameras work by taking pictures at a very rapid rate, usually 24 or 30 frames per second. Our eyes send signals to the brain at a rate of about 14 frames per second, so they interpret the much faster video frame rate as continuous motion (old movies used to display frames at a slower rate which was easily detectable, causing the motion to flicker, which is why we still call movies “flicks”. At least I do).

In the video above, the camera is taking 24 frames per second. The speaker is vibrating at the same rate, 24 pulsations per second. The speaker drives the hose to vibrate at that same rate, making a little circle 24 times per second. Every time the camera takes a picture the hose is back to the same place, so the camera cannot detect the motion of the hose, even though it’s actually making a circle.

That’s why the water appears motionless. Imagine that instead of a continuous stream of water, it’s actually made up of droplets, shot out of the hose 240 times per second. That means every time the hose makes a little circle, it shoots out 10 drops. They come out in order, one after another, each at a slightly different angle around the circle from the one before it. If you connect the droplets you’d get a spiral pattern. The water itself is moving straight away from the hose, but the spiral pattern is what we see as we mentally connect the dots.

The camera frame rate matches the rate the hose is moving, and voilà! Motionless water.

I love how the video maker changes the vibration rate of the speaker a bit to change the water’s apparent motion. At 25 vibrations per second (called 25 Hertz, or 25 Hz for short) the water appears to slowly wind around, because each droplet from the hose is caught at a different position due to the unmatched camera and vibration rates. Then he switches it to 23 Hz and it looks like the water is moving up, backwards, into the hose! Very cool, and totally an illusion.

You’ve actually seen this trick before:  On TV and in movies, sometimes it looks like car or bicycle wheels are spinning backwards. It’s the same principle; the camera rate is mismatched a bit from the rate the wheel is spinning. In one frame you see a spoke that’s vertical, but in the next frame it’s come almost—but not quite—all the way around. It does that every time, and on the video it looks like the wheel is spinning backwards. It's not a trick, not really. It's physics.

Science! I love this stuff.

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