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Submission + - The Physics of the 'Levitating' Slinky

Hugh Pickens writes writes: "Robert Krulwich writes at NPR that there's a new science video making the rounds that has people saying "somebody doctored this footage. This can't be." In the video Derek Muller from the Australian science video website Veritasium takes a Slinky and holds it from the top with his hand then releases the lower part so it will slink down to its full extension, elongating, and come to a dangling rest. Now the professor will release the top of the slinky while they record run the footage at 300 frames a second (video). "Now comes the miracle. If you keep your eye on the bottom of the Slinky, on the last curl at the very end, you will notice that as the top of the Slinky starts to fall, the bottom doesn't drop. It just hangs in the air, levitating, as if it had its own magic carpet. It will stay there, hovering quietly, until a wave, or signal, passing through the Slinky finally reaches it," writes Krulwich. "Apparently, the bottom doesn't know it's supposed to fall, so it sits there, seeming to defy gravity, until the very end." According to a paper by physicists R. C. Cross and M. S. Wheatland the fall of the slinky illustrates the physics of a tension spring, and more generally wave propagation in a spring where because of tension in the spring, the bottom of the slinky cannot begin to fall until the top collides with it (PDF)."
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The Physics of the 'Levitating' Slinky

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