The mystery of why shoelaces come untied has been solved

Millions of shoelaces come unfurled every day, and yet the mechanics of that process has never been thoroughly examined. To tackle the enigma, a trio of mechanical engineers at the University of California, Berkeley, filmed a knot — on the shoe of a researcher running on a treadmill — coming undone in extreme slow motion. The images suggested a two-pronged attack on the knot's integrity. "When running, your foot strikes the ground at seven times the force of gravity," which stretches and relaxes the knot, said co-author Christine Gregg, a graduate student. And then — as the knot relaxes — the legs swing into motion, applying additional force.

A double whammy of stomping and whipping forces acts like an invisible hand, loosening the knot and then tugging on the free ends of your laces until the whole thing unravels.

University of California researchers

Follow-up tests with a mechanical foot and leg showed that some laces were better than others, but none were impervious to failure. Of the two most commonly used knots to tie shoes, one is "weak" and the other "strong," the study found. The strong version is based on a square knot, which is more symmetrical, while the so-called false knot twists when tightened rather than lying flat. Both fail in the same way, but one takes longer than the other.

We were able to show that the weak knot will always fail and the strong knot will fail at a certain time scale. ... But we still do not understand why there's a fundamental mechanical difference.

Professor Oliver O'Reilly, whose lab conducted the experiments Source: https://www.yahoo.com/digest/20170413-P0200/finally-mystery-shoelaces-come-untied-solved-00437822