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2019年10月20日 01:14:09来源:爱问典范

When you turn a cars steering wheel,whether you go forwards or backwards.So it seems like it should be just as easy to steer going backwards as going forwards.But clearly,its not.Suprisingly,the difficulty of driving backwards has almost nothing to do with the direction youre facing and everything to do with the fact that you steer using the front wheels.当你扭转方向盘,车子都会随相同的弯曲度运动。所以看起来倒车似乎和前进一样简单。但是显然,事实并非如此。奇怪的是,倒车的困难之处在它几乎与你对方向盘的控制无关问题在于你所控制的是车子的前轮。When driving forwards,a car goes in the direction the front wheels are pointed.And when driving backwards,the car goes where the back wheels are pointed.Simple!But the difference is that going forwards,the direction of travel is determined by the same wheels you steer with,so you just point them where you want to go,and thats where you go,the back wheels follow automatically.向前行驶时,车子顺着前轮的方向前行。而倒车时,车子顺应的是后轮的方向。很简单,问题在于,前进时决定方向的车轮与你控制的车轮相同,所以你只要打好方向盘车子就朝你,预期的方向前进了,后轮会跟着前轮前进。In physics,we call this a ;stable;system.like dangling a pencil from your fingers-move your hand around and the pencil follows,no thought required.However,when driving backwards,the direction of travel is determined by the back wheels but you steer with the front wheels.在物理中,我们称之为“稳定”系统,就像挂在指尖的铅笔移动手指它就会跟着动,无需自具想法。但是,在倒车时,决定车子运动方向的是后轮,而你控制的却是前轮。So rather than just pointing the front wheels where you want to go,you have to point them in the direction that will get the back wheelsto point in the direction you want to go-its an additional level of separation before the car does what you want.与只需控制前轮运动不同,你需要通过控制前轮引导后轮向着你预期的方向运动。这是控制好车子的附加操作。And in fact,if you dont make any corrections,then over time the back wheels point farther and farther away from the direction you want to go:instead of following the front wheels,they want to run away!This is an unstable system,like trying to balance a pencil on your fingertip.Its incredibly finicky and requires a ton of coordinated hand-eye feedback to maintain.事实上,如果你不采取修正措施,随着时间流失,后轮就会越来越偏离你预期的方向:不但不随前轮运动,而且还会失控!这是一个不稳定系统,就像尝试平衡立在指尖上的铅笔一样。这相当讲究,需要不停的进行手眼反馈协调来完成。Thats why you constantly have to just your steering when driving backwards-mathematically,your car is like an upside-down pencil! Of course,there are some successful rear-steering wheeled vehicles-forklifts,for example,only drive slowly and choose rear-steering for greater control of their forks.这就是倒车时你需要不断调整轮胎方向的原因-数学说法就是,你的车就像东歪西倒的铅笔!当然,也有一些很棒的后轮控制转向的车子-例如 叉车,行驶缓慢,并且选择后轮主控叉子。But lets go in the other direction and add a TRAILER to your car steering backwards has gained ANOTHER level of difficulty:you use your front wheels to get the back wheels to point in the direction needed to get the TRAILER to point in the direction you want to go.Not complicated enough?It becomes worse the more trailers you add each one adds another level of separation between the direction of your steering and the actual direction of travel.现在我们加点难度,给车子加个拖车,这下倒车难度又更上一层楼了:你通过控制前轮引导后轮的方向,同时还需要使拖车向预定方向运动。难度还不够?增加更多拖车后情况就更糟糕了,每增加一辆拖车就会使实际运动方向更加偏离预期的运动方向。Which is why its nearly impossible to push on one end of a chain or a rope and have the other end go where you want it to,and why you will never,ever blance a flexible rope or chain upright on your finger.which makes backwards driving seem downright easy!这就是为何将链条或绳索的一端置于指尖时你永远无法控制另一端的运动方向,也是你为何永远无法令灵活的锁链直立于指尖上的原因。相比之下,倒车真是简单爆了!201504/367542。

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  • That means whats normal is critical,这意味着多少才是正常至关重要and the way that figure is arrived at is causing some concerns.然而获得正常值的方法却令人担忧;The ;Normal; Level is one that 25-year-olds have.所谓正常 就是指25岁人的血脂水平This became the ;Normal; Level on the称这个为正常水平basis that people with lipid levels such as 25-year-olds have是建立在25岁的人不会心脏病发dont go on to have a heart或是中风attack and dont go on to have a stroke.这一假设前提上的Setting the bar so low年龄标准设置如此低 means most people fall into that category.意味着大部分人都不在正常范围内Almost anyone over the age of几乎所有年龄超过30岁的人30 will have what appears to be raised lipid levels,都会有血脂上升的趋势will have what appears to be abnormal lipid levels,也就是血脂水平异常will have a lipid level that成为需要医生治疗的becomes a problem for the doctor treating them血脂异常患者for which the pill is the answer.而治疗用药就是抑制素This would not be a problem这原本也没有什么问题but for the fact that statins, like all drugs,问题在于 抑制素和所有其他药物一样can have side effects.都有副作用Theres a huge number of people that go on these pills有大量用这种药物的病人whose lipid levels come down on the pills他们的血脂水平降了下来but whose quality of life gets worse.但生活质量却变得更糟201503/365577。
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