Mechanical engineers at the University of California at Berkeley have found out why shoelaces come undone. Sound like bullshit to you? You are wrong, according to Christopher Daily-Diamond, one of the authors of the research: “We are talking about knotted structures. If you can understand how cords work, you can apply that knowledge to other intertwined things like DNA or certain microstructures that also unravel to cause of dynamic forces “.
Through a series of experiments and using slow-motion recordings, engineers have unraveled the keys to those forces that undo knots and can hit your bones on the ground if you step on them: with each step or stride, inertia pushes the extremes from the laces forward; then the impact of the foot on the ground – which has a force up to seven times that of gravity – loosens the knot. Repeating this two-step process ends up undoing the loop, something that happens within seconds if the loop is weak.
This video recorded by Berkeley scientists shows how it happens:
Tangled dynamics
Christine Gregg, one of the authors of the paper and a seasoned runner , ran on a treadmill while her colleagues recorded what happened to her shoelaces. They experimented with different types of knots, tied different weights to their ends, and Christine varied the speed and length of her strides. Weak knots always fell apart; the strongest held out much longer, but eventually there comes a time when the complex forces acting on them end up loosening them.
Of course, knots don’t always unravel. If they are well made and tight, they require more cycles of inertia and impact than we experience in a full day of walking or running. But this is the first research to look at what dynamic forces break knots (previous studies have looked at how they unravel when sustained forces such as weights are applied), which may lead to an understanding of what holds or breaks much more complex structures.
