IT's a question that has frustrated generations: why do shoelaces come untied?
And it has taken a group of mechanical engineers at one of the USA's top universities to crack the conundrum - it's the double stomp and whip whammy!
A study, Shoe-string theory: Science shows why shoe laces come untied, by researchers at UC Berkeley used slow-motion cameras and a series of experiments to show that shoelace knot failure happens in a matter of seconds triggered by a complex interaction of forces.
There are two ways to tie the common shoelace bowtie knot, and one is stronger than the other, but no one knows why. The strong version of the knot is based on a square knot: two lace crossings of opposite handedness on top of each other.
The weak version is based on a false knot: the two lace crossings have the same handedness, causing the knot to twist instead of lying flat.
The study shows that both knots fail in the same way.
The first step for the researchers was to record the process of a shoelace knot untying.
Study co-author Christine Gregg laced up a pair of running shoes and ran on a treadmill while her colleagues filmed her shoes.
The researchers found the shoelaces untied 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 lead to the knot failing.
However, when a person goes walking or running, their shoelaces don't always come untied. Tightly tied laces can require more cycles of impact and leg swinging to cause knot failure than one might experience in a day's worth of running or walking.
"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 study co-author Christopher Daily-Diamond.
