The Invisible Hand of Physics Is Constantly Untying Your Shoelaces

Scientists have discovered an "invisible hand" constantly working against the knot in your shoelaces.

Mechanical engineers from the University of California, Berkeley, carried out a series of experiments to analyze the impact walking has on a tied shoelace. Their findings, published in the journal Royal Society A: Mathematical and Physical Sciences, reveals a series of events that cause the knot to become untied.

The team looked at two types of shoelace knot—one being characterized as weak, the other strong. The weak version is commonly known as the granny or false knot. The strong version is based on the square knot.

Scientists used slow-motion video observations of runners on a treadmill, looking at the events that unfolded before a shoelace became untied.

Their findings revealed a "complex interplay between impact-induced deformation of the center of the knot, dynamic swinging of the walking motion, and inertial forces of the laces and free ends of the knot".

Essentially, the researchers have shown there are three stages that lead to a shoelace becoming untied. "First, the repeated impact of the shoe on the floor during walking serves to loosen the knot. Then, the whipping motions of the free ends of the laces caused by the leg swing produce slipping of the laces. This leads to eventual runaway untangling of the knot," the researchers write.

As expected, the weak shoelace came untied far faster than the strong one. But the mechanism behind both was similar.

"As demonstrated using slow-motion video footage and a series of experiments, the failure of the knot happens in a matter of seconds, often without warning, and is catastrophic."

The scientists add that their work is "far from exhaustive". For example, they did not investigate differences in shoelace material or the surface on which the person was walking. They said further work will be needed to understand the complex relationship between walking, knot strength and the accidental untying of a shoelace.

But the study also has implications for other areas of science. Study co-author Christopher Daily-Diamond explains in a statement: "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. This is the first step toward understanding why certain knots are better than others, which no one has really done."