Researchers discover why shoelaces come untied | Science| In-depth reporting on science and technology | DW | 12.04.2017
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Researchers discover why shoelaces come untied

Three US researchers have untangled a problem that has tripped us up for centuries: Why do we have to retie our shoes so often? The answer has to do with stomping and whipping.

It's simple knot mechanics, really - it's just not so easy to explain.

Over the course of 17 pages, Christopher Daily-Diamond, Christine Gregg and Oliver O'Reilly of the University of California at Berkeley have revealed the causes for - as they put it - "shoelace knot failure." It's a process that occurs in a matter of seconds.

"Your laces can be fine for a really long time, and it's not until you get one little bit of motion to cause loosening that starts this avalanche effect leading to knot failure," Christine Gregg said.

Christopher Daily-Diamond, Oliver O'Reilly and Christine Gregg. Copyright: UC Berkeley

They solved the shoelace problem: Christopher Daily-Diamond, Oliver O'Reilly and Christine Gregg

The long-sought answer to the mystery goes as follows:

When running, your foot hits the ground at seven times the force of gravity. The knot stretches in response to that force and relaxes again when the step is done.

With time, the knot loosens.

Then a second invisible force attacks: the inertial forces on the free ends of the laces. This occurs while the leg is swinging back.

"To untie my knots, I pull on the free end of a bow tie and it comes undone. The shoelace knot comes untied due to the same sort of motion," said Gregg.

Both forces together - an impulsive force right at the knot and a pulling force on the free ends (but also the loops) - finally does the trick.

The three researchers used slow-motion cameras to record the untying of shoelaces while one of the researchers - Christine Gregg - ran on a treadmill.

Shoelace knot on an impacting pendiculum (picture-alliance/dpa/O’Reilly/Daily-Diamond/Gregg/University of California at Berkeley/Proceedings of the Royal Society A)

This device investigates forces applied on shoelace knots

They also swung a shoelace knot around an impacting pendulum to investigate knot mechanics, all while adding additional weights to the free ends of the laces.

As expected, the more weight on the free ends, the sooner the knot comes untied.

Nor does it matter which way you tie your bow-tie knot, the researchers say - they will all open up sooner or later.

There is just one thing that you can do to postpone the inevitable: Tie your shoelaces as tightly as you can.

"Tightly tied laces can require more cycles of impact and leg-swinging to cause knot failure than one might experience in a day's worth of walking or running," read the UC Berkeley's press release.

If a 17-page study on shoelaces untying themselves seems a bit overboard, the researchers say their higher aim is to understand knots from a mechanical perspective. Knotted structures are widespread in nature.

"If you can start to understand the shoelace, then you can apply it to other things, like DNA or microstructures, that fail under dynamic forces," said Christopher Daily-Diamond.

The study appears in the journal "Proceedings of the Royal Society A."


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