Why your shoelaces come untied

Why your shoelaces come untied April 12, 2017

Shoelaces_02On April Fool’s Day, not at the time being able to think of anything better, tried the old gag on two of my granddaughters:  “Your shoe is untied!”

One said, “Nice try.  But I’m wearing sandals.”  With the other, her shoe really was untied.

I report my failed joke to introduce a fascinating bit of research.  Engineers have determined why and how people’s shoelaces become untied.

The action of the foot striking the ground loosens the knot and the swinging of the leg acts much like a hand pulling on the strings.

This discovery, detailed at a physics website after the jump, contributes to the field of “knot mechanics,” which turns out to be an important topic.

A new study by mechanical engineers at UC Berkeley finally shows why your shoelaces may keep coming untied. It’s a question that everyone asks, often after stopping to retie their shoes, yet one that nobody had investigated until now. The answer, the study suggests, is that a double whammy of stomping and whipping forces acts like an invisible hand, loosening the knot and then tugging on the free ends of your laces until the whole thing unravels.

 The study is more than an example of science answering a seemingly obvious question. A better understanding of knot mechanics is needed for sharper insight into how knotted structures fail under a variety of forces. Using a slow-motion camera and a series of experiments, the study shows that shoelace knot failure happens in a matter of seconds, triggered by a complex interaction of forces.

“When you talk about knotted structures, 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, study co-author and a graduate student at Berkeley. “This is the first step toward understanding why certain knots are better than others, which no one has really done.”

[Keep reading. . .]

Photo by Oto Zapletal (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons

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