Mystery Behind the Coiling Ropes
(figure from PRL 99, 154302)
If you carefully lower a rope onto the floor it will probably form a neat coil. While most people wouldn’t give this a second thought, an international team of physicists has done a series of experiments and numerical simulations to work out why. Their new insights into coiling could shed light on the behaviour of an important class of materials called “elastic ropes”, which includes DNA molecules and structural reinforcing rods in buildings (Phys. Rev. Lett. 99 154302)..
This team studied coiling over a wide range of descent speeds and the drop lengths. What surprising is that
the coiling always occurred at several different “frequencies” for fixed values of the feed rate and fall distance. These frequencies correspond to the vibrational modes of the nearly vertical upper part of the falling rope. They discovered that coiling occurs when any of these frequencies matches the angular frequency at which bottom end of the rope whirls into a coil.
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