Calling all physicists

Yesterday, I posted a slo-mo  video of a Slinkeys, which, when dropped while their springs were completely distended, appeared to levitate momentarily, until their springs had time to re-compress, whereupon they began their expected downward trajectory. My pal Peter Barss (who is descended from a real pirate, kids) has a question “for anyone who remembers their physics better than I do.”

During most of the its fall, the bottom of the Slinky remains absolutely motionless, which, to my mind, means the gravitational force acting on the slinky pulling it down is exactly balanced by the force compressing the bottom of the slinky upwards. How does this happen without calibrating the force exerted by the energy stored in the “spring” of the slinky to match the force of gravity? It seems to me that the two forces would have to be precisely balanced for the bottom of the slinky to remain motionless while the top collapses.

I wondered the same thing. Is that precise balance an inherent quality of a Slinky—the same quality that enables it to walk down stairs? Some hints in this article at Phys.Org, a web-based science, research and technology news service, and in this paper by UBC physics prof Bill Unrah. Turns out the late, great Martin Gardner kicked off the discussion a dozen years ago. [Animated gif via KnowledgeForDummies.]

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