Relaxing leads to loosening, and add the swinging leg, which causes a pulling force on the loose ends of the laces, and the laces can come undone in as few as two strides. While almost everybody has stopped many times to tie their shoes, no one has asked the relevant question why these pesky shoelaces become loose in the first place. If you’re wondering what the difference is, you can re-watch Terry Moore’s TED talk about it-but suffice it to say, if you tie your laces and they fall parallel to your foot, you’ve tied a weak knot, if they fall perpendicular, you’ve tied a strong knot. At the same time, the force of your swinging legs (called “inertial force” because it resists the change in velocity) acts upon the ends and loops of your laces, pulling them like a hand.
The mistake we’re making is tying the first knot with the left lace over the right and doing the same for the bow – left lace over right.
The forces at work here are inertial forces of your legs swinging back and forth while the repeated striking of the foot on the ground loosens the knot. The knot stretches and then relaxes in response to that force.
That is the conclusion of scientists who have studied the shoelace knot and found that it seems nearly uniquely ill-suited to survive a walking gait.
The researchers say the finding could have knock-on uses when applied to other intertwined structures, like DNA.
Stomping or whipping forces by themselves are not enough to untie the shoe, as both forces must take effect. They published a paper with their results in the Proceedings of the Royal Society A journal. The new paper examines how weak and strong knots react to the application of force during sustained periods of running, shedding light onto a common problem that has troubled humans for millennia. The weaker version is the “granny knot”: take a rope, cross both ends left over right, bring the left end under and out, and repeat.
This could help to create better surgeon’s knots and stronger fibres-and even unravel the reasons why deep-sea optic cables become tangled and break.
The footage showed that your foot often strikes the ground at seven times the force of gravity.
The researchers first recorded the process of knotted laces untying in slow motion by filming runner Christine Gregg on a treadmill while her colleagues filmed her shoes.
Of course, there are a variety of different knots, and some are better than the others.
“We deal with them coming untied all the time”, says Colin Adams, a mathematician at Williams College in Williamstown, Massachusetts, who was not involved with the study.
While some laces might be better than others for tying knots, they all suffered from the same fundamental cause of knot failure, the study found.
The team also tested their theory that increasing inertial forces on the ends of the shoelaces would cause runaway failure of the knot.