y sinking a bit. If her timing is just right, she'll seem to float sideways, instead of rising and falling. The effect is not only beautiful; it also makes the jump seem bigger by "stretching out" the peak.
Of course, what goes up must come down. During a typical grand jete, a dancer's center of gravity rises 2 feet off the ground. Pulled by gravity from such a height, the dancer's body falls very fast-roughly 3.4 meters per second-by the time it reaches the floor. As it falls, body carries with it momentum. Momentum is the weight of the body multiplied by its speed. The bigger the body is and the faster it falls, the greater its momentum.
The only way a dancer can stop dropping through the air is by stopping the body's momentum, which requires an opposing force-the ground. Landing can be very jarring to a dancer and can cause injuries. The dancer can ease the landing by needing her knees and letting her arms fall, but she also gets help from an unexpected source: the floor. Wooden dance floors are designed to act like shock absorbers. They are springy and can recoil as mush as an 9ch under extreme pressure. That little bit of give makes a big difference. Landing on a springy floor, the dancer undergoes a slower change in momentum than she would hitting a rigid floor. The give in the floor allows the decrease in momentum to happen more gradually with less force and less chance of injury. [理工类A]
纵身一跃
在许多不同风格的舞蹈表演中，跳跃都是很关键的一项。一般说来，让一个跳跃动作精彩的因素不在于它的停留时间，即舞者在空中停留的时间。
追求更长的空中停留时间实际上是和重力的一场较量，重力是地球对你往下拉的力量，要整个身子都脱离地面 。舞者必须用腿部肌肉的力量创造出一个比重力更大的向上的冲力。但是最后最高的高度只取决于一件事，即舞者脱离地面向上跳时的速度大小。
一种用来达到跳得更高，空中停留时间更长的很明显的做法是通过加强肌肉以使它能更有强度地推动身体往上跳。但是芭蕾舞蹈演员也用一种小小的又很简单的技巧来制造一种停留空中很长时间的假象，但实际上他们并没停留多长时间。
在一种大幅度的侧身跳跃动作（小跳）中，一个技艺精湛的芭蕾舞演员好像在空中可以停上一段让人难以置信的时间。当然，一个舞者不可能真正地在空中悬着的。物理学中的定理规定了在任何一种跳跃中，一个舞者的重心是循着一条抛物线的轨迹走的。一条抛物线就如同一个你随手扔去的球所经过的路线。但是一个舞者如何做到使他们看起来像是在空中悬着的呢？
一个舞者在她达到跳跃过程中最高处时伸展她的双腿和双臂，从而创造了一个停在空中的幻觉。她身体的其他部分如躯干和头部则相应地下沉一点。如果时间控制得好，她就会看起来像是侧面浮在空中，而不是上升或下降，所产生的效果则不仅仅是漂亮的动作，而且也使这跳跃看起来更大，因为它在跳得最高的时候“伸展”开了。
当然，不可否认的是，跳上去就必定得落下来。在一个标准的芭蕾舞小跳动作中，舞者的重心跃到离地2英尺的高处。在这样的高度受重力拉动，她的身体将以每秒约3.4米的速度往下坠。在下落的过程当中，身体是带有动量的。一个人的动量的大小是体重和速度的乘积值表示的。体重越大，下落的速度越快，动量就越大。
舞者要阻止从空中掉落的惟一方法便是中止自己的动量，而这需要一个反面的来自一个平面的阻力。对舞蹈演员来说，落地会产生很大的反弹力，也有可能因此受伤。舞蹈演员可能以通过屈膝和放下手臂来对落地来一下缓冲，但她同时也可以得到地面这意想不到的帮助。木制的舞蹈地板被设计成像是震动吸收者。它们有弹性，可以后缩，在非常大的压力之下可以后缩到一英寸。这么一个小小的弹性产生的作用可不小。舞蹈演员如果落在有弹性的地板上将使动量变化过程比落在一块硬地上的要慢。这个弹力使动量减少的过程更为缓慢，因而只带有较少的冲击力和受伤的几率。[理工类A]
16 Crashed Cars to Text
Message for Help
There is no good place to have a car crash — but some places are worse than others. In a foreign country, for instance, trying to explain via cellphone that you are upside down in a ditch when you cannot speak the local language can fatally delay the arrival of the emergency services.
But an answer may be at hand. Researchers funded by the European Commission are beginning tests of a system called E-merge that automatically senses when a car has crashed and sends a text message telling emergency services in the local language that the accident has taken place.
The system was developed by ERTICO, a transport research organization based in Brussels, Belgium. Cars are fitted with a cellphone-sized device attached to the underside of the dashboard which is activated by the same sensor that triggers the airbag in a crash. The device includes a cellphone circuit, a GPS positioning unit, and a microphone and loudspeaker.
It registers the severity of the crash by reading the deceleration data from the airbag's sensor. Using Gps information, it works out which country the car is in, and from this it determines at which language to compose an alert message detailing precise location of the accident.
The device then automatically makes a call to the local emergency services operator. If the car's occupants are conscious, they can communicate with the operator via the speaker and microphone.
E-merge also transmits the vehicles make, model, color and license number, and its heading when it crashed, which in turn indicates on which side of a multi-lane highway it ended up.
This helps the emergency services find the vehicle as soon as they arrive on the scene, "We can waste a large amount of time searching for an incident," says Jim Hammond, a (an) expert in vehicle technology at the Association of Chief Police Officers in the UK. Tests will begin soon with police car fleets in the UK. Trails have already started in Germany, Sweden, Spain, the Netherlands and Italy.
In-car systems that summon the emergency services after a crash have already been fitted in some premium cars. ERTICO says that if EU states are willing to fund the necessary infrastructure, Emerge could be working by 2008.
A study by French car maker Renault concluded that the system could save up to 6,000 of the 40,000 lives lost each year on Europe's roads, and prevent a similar number of serious injuries.
The Renault study estimates that fitting E-merge to every car in Europe would eventually save around 150 billion per year in terms of reduced costs to health services and insurance companies, and fewer lost working days. [理工类A]
撞毁汽车发送短信求助
没有什么地方是适于撞车的，但是如果在有些地方发生事故却会更为糟糕。例如在国外，如果你不会当地的语言却想通过无绳电话和对方解释你的车头下尾上地翻到了沟里，会大大延迟紧急事件处理部门救援的到来。
但现在人们可能找到了解决的办法。欧洲贸易委员会资助的研究员们正在开始测试一个叫做E-merge的系统。这个系统可以自动察觉到车祸并向紧急事件处理部门用当地语言发送文本信息告知事故地点。
这个系统是由一个位于比利时布鲁塞尔的名叫ERTICO交通研究机构开发的。汽车可以在仪表板后面安装一个无绳电话大小的装置，然后由撞车后放出保护气囊的感应器一并带动工作。这个装置包含了一个无线电话电路，全球定位系统单元，一个扬声器和一个喇叭。
这个装置可以通过安全气囊感应
