Translated from: 鞋帶綁了又掉？科學家解釋鞋帶鬆脫背後的原因
Sitcom
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As you woke up this morning, you found that the weather seemed nice out there. The sun shone bright with the breeze blowing gently. What a nice day to go out, you thought. After brushing up yourself, you decided to try on that new pair of sneakers and give it a shot. Just as you tied up your shoelaces and were ready to set out, you suddenly remembered that your partner’s birthday is coming up. So, it is resolved that you head toward the mall first to pick up her present. Walking into Macy’s, you were stunned by the pungent smells of cosmetics and fragrances. You dashed ferociously toward the escalator to get away from all the nauseating odors as soon as possible. Finally when you were about to set foot on the moving staircase, hm…my shoelaces are undone! To prevent future mishaps, you stepped aside to let the people behind go first while you tied up your shoelaces.
After the brain-twisting gift-shopping experience, you got a ring from your best mate asking you out for lunch and perhaps a match afterwards. You were already starving to death so you said yes immediately. After resting a bit, you guys came to the basketball court and were glad to find that there was still one space left. You told your friend that you are going to crush him to the core today. He found a wide-open space—jumped, a three-point shot. He’s ready to knock you out, but unfortunately you spotted him before he did it and gave him a block shot. The ball’s in your court now. A quick crossover and you are ready for a slam dunk. Yes! I’m nailing it, you thought. But before you could fly up high, you tripped on something, and soon you were lying by the side. Then came a swish. Damn it! Who just tripped me? As you angrily raised up your head, what, it’s the shoelaces again? You wondered why your shoelaces keep coming untied. How should you tie your shoelaces so they won’t come undone?
Although the sitcom is merely an imagined scenario, we must have tons of experiences when we tie our shoelaces as strong as we can but they still keep coming untied. Some of us might even decide to eventually give up wearing shoes with shoelaces. This would be a pity since a lot of adorable and comfy shoes come with built-in shoelaces. To give shoelaces one more chance, in today’s article, we will be looking at how scientists have unraveled the causes of shoelaces that easily come undone as well as the ways to tie shoelaces that stay tied.
Professor Christopher A. Daily-Diamond from the Engineering Department in the University of California, Berkeley along with his two other graduate students in 2017 published a research article elaborating on the keys factors behind a shoelace that keeps coming undone. Some of the determinants include the way shoelace knots are tied, whether the shoelaces experience a repeated cycle of movements, the impact magnitude incurred upon the tip or head of the shoelace knot, and etc. (Daily-Diamond, Gregg, & O’Reilly, 2017).
In their pilot study, they invited a female participant to tie two types of knots on various kinds of shoes (e.g. “running shoes, hiking boots, casual sneakers, barefoot shoes”) (Daily-Diamond, Gregg, & O’Reilly, 2017, p. 6). The two knots are called the granny knot and the square knot respectively (Daily-Diamond et al., 2017), in which the difference between them are the sequential order of knot formation. To illustrate, the granny knot is tied by having the two trefoils crossed in the same direction or order, that is, the left is always crossed on the right or vice versa. The square knot, which is also the stronger knot version, on the other hand, is tied by two trefoils running in opposite direction/order, meaning that if the first trefoil is done by crossing the left aglet over the right aglet, then the second trefoil would be tied in a right-over-left manner.
To capture the exact movements and impact scales of swinging shoelaces, the researchers placed a LORD Microstrain wireless accelerometer underneath the center of the shoelace knot. Through this wireless device, the whipping motions as well as the velocity of the free ends of the shoestrings may be detected and passed onto a receptor through voltage signals which will then be collected for further analyses. Once the test began, the female runner was required to run or walk on a treadmill following the cadence of a metronome (Daily-Diamond et al., 2017). As can be noted in the following video, the shoelaces loosened at about the second time the feet alternated.
Makasdijian, R. & McNally, S. [UC Berkeley]. (2017). Why do your shoelaces come untied? [Youtube]. Retrieved from https://youtu.be/_-aiynIphTw
As the result shows, the type of shoes does not in any way affect the likelihood of an unfastened shoelace. More specifically, once the knots are tied wrongly, the shoelaces will always come off easily no matter what shoes are worn. On top of that, square knots have been concluded to hold stronger and longer than granny knots (Daily-Diamond et al., 2017). Following the pilot study, the research team brought the study into their engineering lab where they attached the shoelaces to an intricately designed gadget in order to investigate the “effects of impact magnitude, impact direction, and free-end inertial force magnitude on knot failure” (Daily-Diamond et al., 2017, p. 6). (Please watch the footage below as an example of the study)
Daily-Diamond, C. A., Gregg, C. E., & O’Reilly, O. M. (2017). Supplemental video 2 from “The roles of impact and inertia in the failure of a shoelace knot” [figshare]. Retrieved from https:// shorturl.at/eLX49
Using the actuated pendulum, the investigators have accurately emulated the oscillating motions of shoelaces during a run or a walk. At the same time, they purposefully changed the weights (e.g. 1 gram, 2 grams, 3 grams) on the free ends of the fastened shoelaces. In result, setting aside all possible factors (e.g. shoelaces that are insecure), there is a positive linear correlation between the masses of the free ends and the rate shoelaces come off. That is to say, whenever the sizes of the free ends are magnified, the time that the shoelaces come unbound speeds up as well (Daily-Diamond et al., 2017).
One interesting thing revealed from the study tells us that simply moving your feet forward and backward will not make a difference to the shoelaces. According to Christopher Daily-Diamond and his colleagues, the untying of shoelaces will follow through a cycle in which the force impact of taking off and landing of shoes as well as the inertial chaffing of shoelaces all affect the chances that they come undone (Daily-Diamond et al., 2017).
This study by Prof. Daily-Diamond and his colleagues certainly showed us that there is a cause for every effect. Thanks to this group of scientists who may seem a bit silly to conduct such a study on the movements and untying theories of shoelaces, we now know clearly how to avoid tying weak knots. Why not replace your granny knots with the square knots before you head out today?
References:
- Daily-Diamond, C. A., Gregg, C. E., & O’Reilly, O. M. (2017). The roles of impact and inertia in the failure of a shoelace knot. Proceedings of Royal Society, 473, 1-16. doi: 10.1098/rspa.2016.0770
- Daily-Diamond, C. A., Gregg, C. E., & O’Reilly, O. M. (2017). Supplemental video 2 from “The roles of impact and inertia in the failure of a shoelace knot” [figshare]. Retrieved from https:// shorturl.at/eLX49
- Makasdijian, R. & McNally, S. [UC Berkeley]. (2017). Why do your shoelaces come untied? [Youtube]. Retrieved from https://youtu.be/_-aiynIphTw
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