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Keep Rolling Luggage Upright With Physics 物理学帮你稳住拉杆箱

If you've ever pulled a rolling suitcase through the airport, chances are you've also been annoyed by this common occurrence: the suitcase starts rocking back and forth¹ on each wheel and finally tips over.

如果你也曾在机场拉过滚轮行李箱，那你很有可能也被这种常见的情况惹恼过：行李箱的轮子开始不停地来回摇晃，最后翻倒了。

"It's a very classic phenomenon in physics." Sylvain Courrech du Pont, a physicist² at the University of Paris Diderot. "So if you perturb³ the system it goes very easily unstable⁴. This is shared by many, many phenomena⁵ in nature."

“这是一个非常经典的物理现象。”来自巴黎狄德罗大学的物理学家，Sylvain Courrech du Pont说道。“所以如果你扰乱了这个系统，它就会很容易不稳定。这是许多自然现象所共有的特点。”

\Courrech du Pont and his students wanted to get to the bottom of why luggage tips. So they built what looks like the skeleton of a wheeled suitcase, and then filmed it rolling on a treadmill⁶. Then they modeled what they saw mathematically.

Courrech du Pont和他的学生想要弄清行李箱翻倒的真相。因此，他们建造了一个滚轮行李箱的骨架（看起来很像），然后拍摄它在跑步机上滚动。然后他们运用数学方式模拟了所见现象。

And here's what they found. Under normal rolling conditions, the forward motion of the suitcase is perpendicular⁷ to the wheels' axis⁸ of rotation⁹. All good. But when you hit a bump, or jerk the handle, one wheel lifts up. All of a sudden the pulling motion is no longer perpendicular to the still-rolling wheel's rotation, and the wheel corrects that by drifting towards the center. But by that time the other wheel's coming down, same thing happens, and now the suitcase is really rocking.

他们发现，在正常的滚动条件下，手提箱的前进运动垂直于车轮的转动轴。一切看起来都很好。但是当你撞到一个凸起，或者猛拉手柄，某个轮子就会被抬起。突然间，拉力运动不再垂直于仍在转动的滚轮的转动轴。而车轮通过向中心漂移来进行修正。但此时，那个轮子落到地面，同样的事情又发生了，因而箱子便厉害地晃动。

"And the first reaction you will have maybe is to slow down. But actually if you slow down you'll experience very big rocking oscillations." The way back to a smooth ride, he says, is to keep up your speed, or move even faster, once rocking begins. The study is in the Proceedings¹⁰ of the Royal Society A.

“你的第一反应可能是减速。但如果你慢下来，就会经历非常大的震荡。”他说，回到平稳行驶状态的方法，就是保持你的行走速度不变，或者一旦开始摇晃，就加快速度。这项研究发表在《英国皇家学会学报》A辑上。

There are other solutions to suitcase instability—like doing a better packing job. "If you put heavy stuff close to the axis of symmetry, or close to the wheel axis, then it's stable too."

解决滚轮不稳还有其他的办法，比如做好打包工作。“如果你把重物放在接近对称轴线上，或者靠近车轮轴线的地方，那么它也会很稳定。”

And Courrech du Pont has one more piece of advice to avoid rocking in the first place: "If you go slow enough it will always be stable." Keep that in mind, next time you're running to your gate.

Courrech du Pont还有一个可以从一开始就避免晃动出现的建议:“如果你走得够慢，那它就会一直平稳。”下次你急匆匆奔向登机口时，不防记住这一点。