Physics trips up efforts to keep shoelaces tied

Mix of inertia and pavement pounding loosens knots, sends ends flying

running screenshot

KNOT BAD  Shoelace knots don’t unravel at random. Instead, as a runner jogs, the ends of her laces swing due to inertia, and the knot deforms from the force of each footfall. Both effects cause the knot to untie.

C.A. Daily-Diamond, C.E. Gregg and O.M. O’Reilly/Proceedings of the Royal Society A 2017

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Blame physics — not kindergarten-level clumsiness — for perpetually untied shoelaces. The combined forces from legs swinging and feet pounding the pavement create a perfect lace-loosening storm, scientists report April 12 in Proceedings of the Royal Society A.

Mechanical engineer Oliver O’Reilly of the University of California, Berkeley was familiar with the infuriating phenomenon of undone laces. “It always happens to me, and I’ve always wondered why that happens.” He taught his daughter how to tie her shoelaces, but says, “I had no confidence in my own ability to do it properly.” 

So O’Reilly and colleagues set out to understand the physics of shoelaces. With a high-speed camera, they filmed a runner on a treadmill, catching loosening laces in the act. Using a pendulum with weighted laces tied on it, the researchers mimicked the forces of running or walking to study the phenomenon in a reproducible way.

Two effects contribute to shoelace knots unraveling, the scientists determined. Each time the runner’s heel hits the ground, the knot stretches and deforms, loosening it. Then, as the legs swing back and forth, the laces’ inertia causes them to whip around, pulling on the free ends. As a result, the knot slowly loosens over several minutes, before rapidly coming undone in one or two footsteps.

One version of the classic shoelace knot, the tried-and-true square knot, is known to be stronger than another, called the granny knot. In a square knot, the laces are first tied right over left, and then the bow is tied left over right. The granny knot is tied in the same direction both times, making it twist to one side instead of lying straight. In the researchers’ tests, the granny knot failed more easily than the square — it fell apart every time in the pendulum tests, whereas the square knot survived some tests.

“We do not have a very good, rigorous understanding behind the mechanics of even the most simple types of knots,” says mechanical engineer Khalid Jawed of Carnegie Mellon University in Pittsburgh, who was not involved with the research. He calls the result “a very good first step” toward understanding simple knots.

O’Reilly’s recommendation to keep those pesky laces in line is to use a square knot, not a granny one. But, either way, don’t be surprised if it fails.

Another option: Just don some slip-ons.

A SHOE LOOSE Two complementary effects tag team to untie a runner’s shoelace in this video, shot with a high-speed camera. Each time the runner pounds her feet into the treadmill, the impact causes the knot to stretch and begin to loosen. As she swings her legs, inertia pulls the shoelace’s ends forward, tugging them out of the knot, which reveals little sign of its impending doom until the last few strides. C.A. Daily-Diamond, C.E. Gregg and O.M. O’Reilly/Proceedings of the Royal Society A 2017

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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