Why do Your Shoelaces Become Untied?

Why is that whenever you start to move around your shoelaces become untied? No matter how tight you tie them or how many times you knot them they somehow always become untied. The combination of foot stomping and leg swinging cause the laces to slip apart but that's old news. Scientists decided to take a closer look at knots and how they become untied because knots are everywhere from shoes to stitches in surgery.

The medical engineers from the University of California at Berkeley decided to take that closer look at the physics of knot strength. The researchers knew that there are two ways to tie a knot with one being the square knot (stronger) and the other being the granny (weaker). Christopher Daily-Diamond says “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.” So in order to solve this mystery they attached sensors to shoelace knots as a fellow researcher walked and ran. In addition, they attached a pendulum arm to a shoelace knot in order to better analyze force knots experienced.

After many tests, slow-motion video showed the laces being held tightly for many strides but when the granny knot became loosened only slightly disaster struck. Within one to two strides the knot become untied completely. Researchers, though, noticed something interesting. The weak knot did not untie itself when the test subject's leg was swung back and forth or when the foot was only stomped repeatedly on the ground. This meant that the knot becoming untied is due to interplay between the swing and the stomp.

Researchers found that the repeated impact of shoes on the floor during running loosened the knots. The sensors then revealed that during running, feet strike the ground with seven times the force of gravity which caused the knots to deform. Once the knots deformed, the whipping motion of the laces caused by swinging legs then led the laces to slip which then led to unraveling. When weights were added to the free ends of the laces the knots failed more often which supported their theory.

Future research into how shoelaces move during walking and running may also explain why the square knot is stronger than the granny says Daily-Diamond. Computer simulations of knots could also unravel the complex role that friction plays. So until this mystery has been solved and you want your laces to remain tied stick to the square knot!

http://blogs.discovermagazine.com/d-brief/2017/04/11/physics-why-shoelaces-come-untied/#.WfD_5WhSzIV

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