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Physicists untangle rope's twists: twine, string, cord or cable, it all winds up the same way.

Researchers have unraveled the math that keeps ropes from unwinding.

The trick lies in the number of times each strand in a rope is twisted, say Jakob Bohr and Kasper Olsen, physicists at the Technical University of Denmark in Lyngby. Their paper was posted online April 6 at

In a traditional rope, each individual strand is twisted as much as possible in one direction. The twisted strands are then wound together in a spiral shape called a helix, which itself rotates in the opposite direction.

The interlocking of these twists and countertwists gives the rope strength so that when yanked, it does not unwind.

By plotting a rope's length against the number of times each strand is twisted, Bohr and Olsen discovered that there is a maximum number of twists--resulting in what is called the "zero-twist point" for the overall rope. A good rope--one that won't unravel when pulled or pushed--is always in the zero-twist configuration.

A triple-stranded rope in the zerotwist configuration, Bohr and Olsen found, is 68 percent the length of its untwisted component strands. That figure stays the same no matter what material the rope is made of, says Bohr.

"If you have an old Egyptian rope or one made by modern petrochemical industries, they all look the same," he says. "It is beyond material--it is geometry."


Physicist Henrik Flyvbjerg, also of the Technical University of Denmark, agrees: The rule of the zero-twist point is universal. "If there is life on other planets in other solar systems, their rope makers must follow the same rules," he says.

The work also explains why rope makers need to feed in the strands at a splayed-out angle, Bohr says; the tensile stress in the rope will automatically adjust the newly added portion to the zero-twist configuration.

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Title Annotation:Numbers
Author:Witze, Alexandra
Publication:Science News
Article Type:Brief article
Geographic Code:1USA
Date:May 8, 2010
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