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Falling slinky displays slow-motion causality

Sydney University boffin explains why dropped springs seem to defy gravity

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Vid Researchers from the University of Sydney have explained why a spring dropped from a height - in this case the toy “slinky” – appear to ignore the force of gravity for a time.

The very odd thing is that “if a slinky is hanging vertically under gravity from its top (at rest) and then released, the bottom of the slinky does not start to move downwards until the collapsing top section collides with the bottom.”

As you’ll see in the video below, that leaves the bottom of the slinky hanging mid-air very incongruously indeed.

The phenomenon is explained in a paper, Modelling a falling slinky (PDF), by Sydney University Associate Professors Mike Wheatland and Rod Cross. First published on arxiv.org last August and since accepted by The American Journal of Physics, the paper points out that “The behavior of a falling slinky is likely to be counter-intuitive.”

The explanation for slinky hang-time is quite simple, with the paper describing it as occurring because “the collapse of tension in the slinky occurs from the top down, and a finite time is required for a wave front to propagate down the slinky communicating the release of the top.”

In the video, Associate Professor Wheatland says the same thing happens in other falling objects, but because they are denser the signal to start falling travels more quickly.

Watch Video

Pressed on the notion that signals to obey the force of gravity are transmitted through matter, Wheatland says “you are changing something at the top and there is a finite amount of time to get that information to the bottom of the slinky. It’s a signal.”

“Whenever you do something physically to effect a change it is a signal. Causality means you do something. There is a cause and an effect. Between the two a signal has to propagate if they are not at the same location."

In the case of the slinky, around 0.3 of a second is required for the signal to pass from the top to the bottom.

The paper explains how that signal is transmitted with math it says is suitable for undergraduate physicists. The paper also points out its model is not universal, as it also takes into account the nature of the slinky and what happens when each of its loops hits the one below.

The paper seems not to be the only investigation of the topic: footnotes cite another dozen papers investigating other interesting qualities of the humble slinky, which can now take its place beneath Reg readers' Christmas trees for didactic, rather than purely pleasurable, reasons. ®

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