科学美国人60秒:Shoelace Study Untangles a Knotty Problem

You’re strolling down the street or maybe hauling that load of laundry down a flight of stairs when all of a sudden. [sound fx] Your laces come undone. If you’ve ever pondered what precipitates this pedestrian wardrobe malfunction, you might want to tie your shoes and beat a path to the Proceedings of the Royal Society A. [Christopher A. Daily-Diamond, Christine E. Gregg and Oliver M. O'Reilly, The roles of impact and inertia in the failure of a shoelace knot]

In that journal, researchers have trotted out data that show that a combination of whipping and stomping forces is what causes laces to unravel without warning.

The investigators noted that shoelace knots frequently fail when people are walking. But not when they are, say, sitting on the edge of a table and swinging their legs. Laces also stay tight when marching in place with no forward motion. 

That led the gumshoes to suspect that stepping and swinging somehow work together to foil footwear security. But how? To untangle this knotty problem, the researchers made a slo-mo video of a student running on a treadmill, thus recording the literal steps that lead to catastrophic knot failure.

Here’s what they slowly saw: When the foot strikes the ground, the force of that impact causes the knot at the center of the lace to deform and stretch. And when the foot swings forward, the ends of the shoelace fly forward. That whipping motion pulls the knot open a bit more, which allows the free end of the lace to slip through a tiny bit. With each cycle of impact and whip, the free end slides a little more, until enough of it has come through that the knot finally unravels.

The researchers confirmed that some knots are stronger than others. The granny knot that most of us use on our shoes is fairly weak and prone to failure, perhaps because the knot winds up twisted when pulled tight. A square knot, on the other hand, held up well in the treadmill test. To get that knot to buckle, the researchers had to attach weights to the free end to aid in the tugging of those whipping tips.

The researchers did not determine whether different types of laces are more likely to produce ties that bind. So though this initial study has made great strides in our understanding of shoelace mechanics, we still have a long way to go before this research is all tied up.

—Karen Hopkin