Why shoelaces come untied? Study has an answer

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Shoelaces invariably come untied – an annoying thing to happen while you are on the move – but there is science behind it as a new study has shown what goes on while you are walking and what are the factors that cause the shoelaces to untie.

Researchers at UC Berkeley have revealed through an extensive investigation 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. Scientists used a slow-motion camera and a series of experiments to unravel the mysteries behind shoelaces untying and how the shoelace knot failure happens in a matter of seconds, triggered by a complex interaction of forces.

Scientists were actually after the reason why shoelaces come untied every now and then, but the goal of the new study was to develop a baseline understanding of the mechanics of how a shoelace bow tie knot comes untied under dynamic forces. Previous studies have described how knotted structures fail under sustained loads, but little research has shown how knotted structures fail under the dynamic pressures of changing forces and loads.

According to the team behind the study a shoelace knot unties like this: When running, your foot strikes the ground at seven times the force of gravity. The knot stretches and then relaxes in response to that force. As the knot loosens, the swinging leg applies an inertial force on the free ends of the laces, which rapidly leads to a failure of the knot in as few as two strides after inertia acts on the laces.

In addition to the dynamic interaction of forces on the knot, the footage also revealed a large magnitude of acceleration at the base of the knot. To dig deeper, the researchers then used an impacting pendulum to swing a shoelace knot and test knot mechanics using a variety of different laces.

The researchers also tested their theory that increasing inertial forces on the free ends would trigger runaway failure of the knot. They added weights to the free ends of the laces on a swinging knot and saw that knots failed at higher rates as the inertial forces on the free ends increased.