Re: What holds it up?

Autymn D. C. wrote:
Disregard the gibberers. The rope transmits hòrizòntal force tom
vertical force. Juro action comes later.

The Indian rope trick is better:
<http://google.com/search?q=%22rope+trick%22+%22flexible+rod%22>.

Fiiiinally, a fiend to find (happily in email arkiv):

http://www.nature.com/news/2002/021209/full/021209-8.html
Published online: 11 December 2002; | doi:10.1038/news021209-8

Wobbling wire defies gravity
John Whitfield
Engineers could learn from motions that stand curtain cable upright.

[image]
Rapid vibrations make the wire wave violently.
© Royal Society

"It shouldn't work, but it stands upside down," says Tom Mullin. The
vibrating piece of curtain wire in his laboratory has baffled
physicists for several years.

Now Mullin's team has explained the uncanny hardware. Their insights
might find a use in bridge building, or laser technology.

Held at one end, plastic-coated curtain wire stands upright if it is
moved rapidly up and down a little. When vibrating slowly, the wire
flops over. But if it is moved faster, it begins to wave violently.

The wire stays upright because waving and flopping cancel each other
out, says Mullin who works at the University of Manchester, UK. Once
waving takes over, the two types of instability amplify one another,
often leaving the wire bent out of shape.

"What used to be two fairly small effects suddenly become large
effects," explains Mullin's colleague Alan Champneys, a mathematician
at the University of Bristol, UK.

Amplified instabilities might occur elsewhere, he says, for example in
the cables of bridges. Champneys and his colleagues' mathematical
explanation of the levitating wire1might give engineers new a way to
work out whether their structures would be capable of such violent
behaviour.

The standing wire is an example of a phenomenon called parametric
oscillation, in which one aspect of a physical system - in this case
gravity - is changed rapidly. Some crystals can send laser beams into
parametric oscillations, splitting them in two and allowing their
colour to be varied.


The same equations govern the waves on a cup of coffee

Tom Mullin

University of Manchester


"Parametric oscillations stabilize things that normally you'd think of
as unstable," says Mullin. In theory, he says, an upside-down cup of
coffee could stay full if it was vibrated fast enough.

Parametric oscillations can also make pendulums stand vertically,
although the maths used to explain this is very different. To get a
rope to stand upright, as in the Indian rope trick, would require
impossibly fast vibrations.


References
Mullin, T. et al.The 'Indian wire trick' via parametric excitation: a
comparison between theory and experiment. Proceedings of the Royal
Society A, published online, doi:10.1098/rspa.2002.1056 (2002).

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