Why are my shoelaces always untied?

Gabriella Pedro
Mr. Gray
Physics .1 - Period G
5/2/17

"Hey, your shoe is untied!"

Have you ever wonder why your shoelaces are always untied? Well scientist are curious and after extensive research they now know why. With the combination of foot stomping and leg swinging it causes our laces to slip apart!

You may be thinking to yourself "seriously I could of guessed that," but there are important reasons why scientists have given knots a closer look behind the obvious.

What is so special about a shoelace? Why do scientists care about my shoelaces? These questions are all valid but I can assure you that there is indeed a reason for this research. The reality is that knots are everywhere, from strands of DNA to stitches used in surgery to steel cables used in construction. Taking a closer look on why knots come undone could help scientist better understand not only shoelaces but also other object that consist of knots. 

The University of California at Berkeley came together with a group of mechanical engineers to become better intrigued with knot strength after coming across a TED talk that explained the two different ways to tie a shoelace. There are two ways to tie the classic bow tie knot, one of which is stronger than the other. 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. The stronger version is the square knot: instead of repeating the first step, finish the knot by crossing the right end over the left.

“I wear dress shoes, and my shoelaces seem to come untied all the time,” says study lead author Christopher Daily-Diamond. “But when we looked into it, while people knew one of these knots was stronger than the other, the mechanics of why that was remained a mystery.”

In order for researcher to completely study shoelaces that must add sensors to the laces. The candidates wearing the sensors also repeatedly swung a pendulum arm with a shoelace knot tied on it to better analyze forces knots experience. This test provided great results that helped solve the puzzle. 

Study Showed:  
"Slow-motion videos of Gregg running on a treadmill showed the granny knot held together for many strides, but when it only slightly loosened, the knot typically failed catastrophically within as few as two strides. Intriguingly, the weak knot did not untie itself when Gregg’s leg was just swung back and forth, nor when the foot was only stomped repeatedly on the ground. This suggested that knot failure is based on some interplay between the swing and stomp."

Results shoes that the repeated impact of shoes on the floor during running loosens the knots. The sensors revealed that during running, feet strike the ground with seven times the force of gravity, causing knots to deform. While walking or running the whipping motions of the free ends of the laces caused by swinging legs then led the laces to slip, eventually leading to knots coming undone. 
                 

          

Researchers say: 

"In line with this theory, adding weights to the free ends of the laces, which increased the pulling those ends experienced as they swung, led knots to fail more often. Daily-Diamond, Gregg and senior author Oliver O’Reilly detailed their findings online April 12 in the journal Proceedings of the Royal Society A. "

So why study knots?
Daily-Diamond has noted that researchers today are trying to build microscopic structures of DNA and other molecules and to have success scientists must first fully understand knots and how they work. “These can be incredibly complicated knotted structures that are subject to a variety of forces, so if you want to start building these structures, you’ll want to know how they can become untied,” Daily-Diamond said.