Everyday life can unravel knots
Alan Edwards, of Belleknowes, asked:
No matter how tightly I tie my ‘‘flat’’ shoelaces, they work loose after walking a few blocks, yet shoes with ‘‘round’’ laces will stay done up all day. There must be a logical physical reason for this?
Oliver O’Reilly, a mechanical engineer, at the University of California at Berkeley, responded:
The failure of shoelace knots is a phenomenon that has intrigued me for many years.
What we know is that the failure of the knot happens because of the combined action of the swinging of your leg and the impact of your foot on the ground. You can easily see this if you sit on a table and swing your leg back and forth — the flat shoelaces you describe won’t come undone. Similarly, if you stomp your foot on the ground repeatedly without swinging your leg back and forth your shoelaces won’t come undone, but your housemates might! You may see this process in slow motion in this video: youtu.be/_aiynIphTw
It’s also well known that there are two common types of shoelaces knots — the weak (or granny) knot and the strong (square or reef) knot. From what you describe, you are probably tying your shoelaces with a weak knot. As you are walking, the impact of your foot with the ground causes the knot to open up (loosen), then the swinging of your leg allows the free ends to come undone. The difference between the square laces and the round ones is probably the amount of friction that’s present. I suspect that the square laces don’t provide as much friction as the round laces and so it’s easier for the free ends (or loops) to come undone. Another way to say this is that you can tighten the round laces more than the square ones.
My shoelaces come undone, and I have flat shoelaces, because I always tie a weak knot — it’s subconscious. If I can think about it, then I tie a strong knot. This is simply a matter of switching the loop when you form the bow than what you naturally are inclined to do. The knot still fails, but I need to walk far more for that to happen. The clearest explanation that I’ve seen on how to tie the strong knot is a BBC Ideas video on ‘‘Why you are tying your shoelaces all wrong!’’.
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