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.
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. ®
COMMENTS
Re: @John Latham / xyz Seems overcomplicated
"The center of mass still drops down according to our old pal Newton."
QFT.
The slinky is both falling and contracting at the same time. It has two forces acting on it; gravity and the tension within the spring. The speed it contracts up at the bottom end exactly equals the speed the centre of slinky mass is dropping down. Hence the far end does not move until the centre of mass reaches it.
The force *up* stored within the spring is exactly that obtain from gravity when the spring is first stretched out. So it is no surprise that it precisely balances the gravity force *down* when the spring is dropped. At least for the time it lasts and the spring is fully contracted.
If someone was to stretch the spring out further at the bottom end, and let go at the same time as it was dropped, the force up would be greater for a while, and we would see the bottom end *rise* at first. No-one would think this unusual. So why all this nonsense when the forces are balanced?
All this talk about "messages" and "waves" over complicates what is quite simple. And I'm not a physicist.
Seems overcomplicated
The bottom doesn't move because it's being held in place by spring tension, which is only released when the adjacent coils collapse.
Nice slo-mo though.
Re: Seems overcomplicated
No what they're talking about is similar to Wile E Coyote running off the edge of a cliff and only falling when he realises he's not on the ground anymore. Then instead of his head staring at the camera as his bottom half goes shooting down, the opposite happens. Thinking about it again, you might be right, the bottom end of the slinky isn't just sitting in the air, it's trying to collapse up towards its other end as the other end is collapsing towards it. I need a coffee.
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