IN A stunt reminiscent of the legendary Indian rope trick, a short length of wire or stiff rope can defy gravity and stand upright when its end is shaken. The effect was mystifying but it has now been explained 鈥� at certain frequencies the wire tries to fall over in two ways at once and so, perversely, stays upright.
Four years ago, British-based researchers Tom Mullin, a physicist at the University of Manchester, and David Acheson from the University of Oxford, showed that they could balance a vertical chain of rigid rods by applying rapid up-and-down oscillations to the bottom end. They could even explain the physics 鈥� each downwards pull straightened the links and stopped the structure falling over (快猫短视频, 21 February 1998, p 12).
But the mathematics that described the effect predicted that a flexible rope should only be able to stand up if shaken infinitely fast 鈥� an impossibility. Recently, Mullin and Acheson tried it and found that their rope was too floppy. However, they did manage to keep a half-metre length of slightly stiffer curtain wire upright by oscillating its base through two centimetres about 20 times a second.
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If the wire was moved slower than this it fell to the side, any faster and it thrashed wildly. Now Alan Champneys and his team at the University of Bristol, colleagues of Mullin and Acheson, think they can explain the effect. The wire is steady at the boundary between the two modes because the 鈥渂uckling鈥� mode cancels the 鈥渨aggling鈥� one. The interaction between the two modes is surprising, says Champneys, but his equations agree well with the experiments (Proceedings of the Royal Society A, DOI: 10.1098/rspa.2002.1056).
Unfortunately the effect is not strong enough to explain the Indian rope trick. 鈥淭hat鈥檚 magic, not science,鈥� says Champneys. But the model might in future help to earthquake-proof buildings. A skyscraper shaken by seismic waves will wiggle in the same way as the curtain wire at high frequencies 鈥� but the building is more likely to break. Engineers should try to design buildings so that they stay in the stable zone when the ground shakes, says Champneys.
