Why shoelaces become untied decoded

Los Angeles: The mystery behind shoelaces getting untied on their own has been solved by scientists. The study is more than an example of science answering a seemingly obvious question, researchers said. 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. Graduate student Christine Gregg, a runner, laced up a pair of running shoes and ran on a treadmill while her colleagues filmed her shoes. The researchers found that when running, the 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. “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. The footage also showed 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. The study appears in the journal