Why do shoelaces come untied? Physics.

Science News investigates the forces at play when your shoes come untied:

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.

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.

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