Feeds

Falling slinky displays slow-motion causality

Sydney University boffin explains why dropped springs seem to defy gravity

Internet Security Threat Report 2014

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. ®

Top 5 reasons to deploy VMware with Tegile

More from The Register

next story
Antarctic ice THICKER than first feared – penguin-bot boffins
Robo-sub scans freezing waters, rocks warming models
I'll be back (and forward): Hollywood's time travel tribulations
Quick, call the Time Cops to sort out this paradox!
Your PHONE is slowly KILLING YOU
Doctors find new Digitillnesses - 'text neck' and 'telepressure'
Reuse the Force, Luke: SpaceX's Elon Musk reveals X-WING designs
And a floating carrier for recyclable rockets
NASA launches new climate model at SC14
75 days of supercomputing later ...
Britain's HUMAN DNA-strewing Moon mission rakes in £200k
3 days, and Kickstarter moves lander 37% nearer takeoff
prev story

Whitepapers

Designing and building an open ITOA architecture
Learn about a new IT data taxonomy defined by the four data sources of IT visibility: wire, machine, agent, and synthetic data sets.
5 critical considerations for enterprise cloud backup
Key considerations when evaluating cloud backup solutions to ensure adequate protection security and availability of enterprise data.
Getting started with customer-focused identity management
Learn why identity is a fundamental requirement to digital growth, and how without it there is no way to identify and engage customers in a meaningful way.
Reg Reader Research: SaaS based Email and Office Productivity Tools
Read this Reg reader report which provides advice and guidance for SMBs towards the use of SaaS based email and Office productivity tools.
Business security measures using SSL
Examines the major types of threats to information security that businesses face today and the techniques for mitigating those threats.