If you've spent your entire life wondering why your shoes can't seem to stay tied, a team of mechanical engineering grad students at the University of California Berkeley have some answers for you. It turns out that whipping and pounding forces from moving your feet can loosen knots and tug on laces, acting "like an invisible hand."
Their method was simple: Co-author Christine Gregg ran on a treadmill until her shoes came untied while the rest of the team filmed it in slow-mo. The research team used the slow-motion footage to analyze the two different ways people tie their shoelaces into a classic bow tie knot. One of these methods, aka the "reef knot," is based on a square knot and involves crossing the laces over each other in opposite directions. The other, less-secure method is known as the "granny knot," done by crossing the two laces in the same direction; this makes the knot actually twist as opposed to laying flat across your shoe like a reef knot does.
What they discovered is that our feet hit the ground with a force seven times stronger than gravity and the knot stretches and relaxes in tandem with this force. As the knot is loosening, the swinging of our legs imposes an additional force on the ends of the laces. Together, these forces can undo laces in as little as two strides after a knot has loosened.
The video also showed that the knot's base underwent lots of acceleration as Gregg ran. According to study co-author Christopher Daily-Diamond, all three factors are essential to untying our shoes. Interestingly enough, this study tells us that both knot-tying methods will fail eventually, but granny knots untie much faster than reef knots.
Gregg said in a press release that "the interesting thing about this mechanism is that your laces can be fine for a really long time, and it's not until you get one little bit of motion to cause loosening that starts this avalanche effect leading to knot failure."
Why on earth is UC Berkeley having its bright young engineers look into this? Because it might help them understand how other structures come undone. As Daily-Diamond added via the release: "When you talk about knotted structures, if you can 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."
Previous studies have looked into why knots come undone under continuous stress, but this one is the first to research how they do when said stress is changing, like when you run and your laces are being pushed and pulled around. It will probably be awhile before they learn anything about how this applies to DNA, but in the meantime it's an explanation why your perfectly good-looking knot made you trip mid-run.
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