Los Angeles: The mystery behind shoelaces getting untied on their own has been solved by scientists who have found that a double whammy of stomping and whipping forces act like an invisible hand, loosening the knot and tugging on the laces until the whole thing unravels.
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, they said.
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, a graduate student at University of California, Berkeley.
"This is the first step towards understanding why certain knots are better than others, which no one has really done," said Daily-Diamond.
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.
The first step was to record the process of a shoelace knot untying in slow motion. 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 forces that cause this are not from a person pulling on the free end, but from the inertial forces of the leg swinging back and forth while the knot is loosened from the shoe repeatedly striking the ground," she said.
In addition to the dynamic interaction of forces on the knot, 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.
