When your shoelaces comes untied, the thought that probably pops in your brain is: "Ugh, that's annoying. But to mechanical engineers, this tedious occurrence represents a scientific black hole. Why do shoelaces shake off their knots, and how? Why is one way of tying a shoelace stronger than the other? Engineers at the University of California, Berkeley, recently took these questions to the lab.
It is a skill mastered by the average six-year-old, but most of us are apparently doing it wrong. Mechanical engineers have found the way many people learn to tie their shoelaces makes them far more likely to come undone. The mistake is to tie the first knot left over right, then tie a bow in the same way. Instead, if the left lace is crossed over the right, the bow should be crossed the opposite way – right over left. The result will be a bow which sits correctly and horizontally across a brogue or deck shoe, rather than being pulled diagonally towards the ankle.
April 12, 2017 --Well, it happened again. You were walking down the road, minding your own business, when all of a sudden you looked down, only to realize that your shoes spontaneously untied themselves again. Muttering under your breath, you kneel down, retie the laces, and go on with your day. That scenario is familiar to anyone who has owned a pair of lace-up shoes. For many, retying one's shoes after they seemingly untie themselves is simply one of life's little inconveniences that has to be dealt with from time to time.
A combination of stomping and whipping explain why your shoelaces seem to come undone all by themselves. In 2015, MIT researchers came up with an equation for the simplest knots to describe the forces at work – tension, friction, and stiffness – and how they relate to the number of turns that make up the topology of the knot. But although there have been many studies of the durability of various knot configurations, nobody had really focused on the physics of why a knot comes undone on its own. Oliver O'Reilly at the University of California, Berkeley, decided to study spontaneous unknotting after noticing that his young daughter could never keep her shoelaces tied. He and two graduate students ran real-world experiments to investigate further.
You put on your shoes, tie them as firmly as possible, but soon after the laces come undone. Now scientists think they know what causes one of life's knotty problems. They found the force of a foot striking the ground stretches and then relaxes the knot, while a second force caused by the leg swinging acts on the ends of the laces, like an invisible hand. The researchers say an understanding of shoelaces can be applied to other structures, such as DNA. Using a slow-motion camera and a series of experiments, mechanical engineers at University California Berkley found "shoelace knot failure" happens in a matter of seconds, triggered by a complex interaction of forces.