February 02, 2017

The Wile E. Coyote Slinky

There's a scene in every Wile E. Coyote cartoon where he scampers pell-mell off the edge of a cliff. Still running in mid-air, he hangs there in space for a couple of seconds with that "Oh, crap!" look on his face before plummeting to the ground.

Well, that same effect can be duplicated with a Slinky.

The "Newtonian illusion" here is that our brains treat the top and bottom of the Slinky as a single object, rather than as two separate parts of an "information system."

The information that the top end has been dropped can't propagate down the Slinky any faster than the speed of sound in the Slinky (the speed at which waves propagate down it), so there's a delay before the bottom end "knows" it's been dropped. But it's surprising to see how long the delay is.

My common sense tells me the top and bottom of the Slinky are accelerating towards the center of mass at the same time the center of mass is accelerating downward. The bottom of the Slinky won't move until the center of mass catches up with it.

Looking at the video, though, the "information theory" explanation makes sense (even if it doesn't make any common sense) because the bottom of the Slinky simply isn't moving.

Likewise, gymnastics wouldn't be so physically and aesthetically compelling if we only saw the gymnast's bouncing center of mass, and not the gymnast's body rotating around the center of mass.

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# posted by Blogger Joe
Cool video, but I'm skeptical of the explanation. Using that reasoning, if both ends were a ball tied together by a string and you let go of one end, the other end wouldn't drop. However, if you use a rubber band then you might see something similar--the point being that the lower end requires a counteracting force.

I further suspect that you have to have the right slinky for this to work as it does in the video.
2/03/2017 6:54 PM