囧研究：系紧的鞋带为什么会松？（双语）

　　You’re strolling down the street or maybe hauling that load of laundry down a flight of stairs when all of a sudden. Your laces come undone. If you’ve ever pondered what precipitates this pedestrian wardrobe malfunction, you might want to tie your shoes and beat a path to the Proceedings of the Royal Society A.

　　有一天你在街上漫步，或是搬着一大堆衣服下楼，这时突然……你的鞋带松了。如果你也曾好奇过这种“行人服装故障”的原因，那你可以系好鞋带，然后在英国《皇家学会会刊.A辑》中寻找答案。

　　In that journal, researchers have trotted out data that show that a combination of whipping and stomping forces is what causes laces to unravel without warning.

　　在该期刊中，研究人员提出的数据显示，甩动和跺脚这两种力的结合是导致鞋带突然松开的原因。

　　The investigators noted that shoelace knots frequently fail when people are walking. But not when they are, say, sitting on the edge of a table and swinging their legs. Laces also stay tight when marching in place with no forward motion.

　　研究人员注意到，鞋带结经常在人们走路时散开。但是当人们坐在桌边晃腿时，鞋带却不会松。同样，当人们原地踏步，不向前运动时，鞋带也不会松开。

　　That led the gumshoes to suspect that stepping and swinging somehow work together to foil footwear security. But how? To untangle this knotty problem, the researchers made a slo-mo video of a student running on a treadmill, thus recording the literal steps that lead to catastrophic knot failure.

　　这使研究人员怀疑，踏步和摆动以某种方式结合会破坏鞋带的稳定性。但这是怎么做到的?为了解开这个难题，研究人员制作了一个学生在跑步机上跑步的慢动作视频，来记录导致鞋带松开的步数。

　　Here’s what they slowly saw: When the foot strikes the ground, the force of that impact causes the knot at the center of the lace to deform and stretch. And when the foot swings forward, the ends of the shoelace fly forward. That whipping motion pulls the knot open a bit more, which allows the free end of the lace to slip through a tiny bit. With each cycle of impact and whip, the free end slides a little more, until enough of it has come through that the knot finally unravels.

　　这是他们逐渐发现的现象：当脚着地的时候，这种冲击力会导致鞋带中间的结变形和拉伸。当脚向前迈的时候，鞋带末端会向前飞起。这种甩动的动作让鞋带结稍稍松动，进而导致鞋带自由端向外滑动一点点。随着“冲击和甩动”的一次次循环，鞋带自由端持续滑动，直到鞋带滑出，鞋带结最终松开。

　　The researchers confirmed that some knots are stronger than others. The granny knot that most of us use on our shoes is fairly weak and prone to failure, perhaps because the knot winds up twisted when pulled tight. A square knot, on the other hand, held up well in the treadmill test. To get that knot to buckle, the researchers had to attach weights to the free ends to aid in the tugging of those whipping tips.

　　研究人员证实，有些结会更结实。我们大多数人在系鞋带时常用的是“奶奶结”，这种结其实不太结实，很容易松开，可能是因为我们拉紧鞋带时导致鞋带结变形。另一方面，平结在跑步机测试中表现良好。为了让鞋带结系牢，研究人员不得不增加鞋带自由端的重量，帮助鞋带头找到牵引感。

　　The researchers did not determine whether different types of laces are more likely to produce ties that bind. So though this initial study has made great strides in our understanding of shoelace mechanics, we still have a long way to go before this research is all tied up.

　　研究人员还没有确定哪种鞋带打结更结实。虽然初步研究在帮助我们了解鞋带机理上取得了巨大进展，但是，在研究全部完成之前，我们还有很长的路要走。