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时间：2015-12-04 08:22:33

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（单词翻译）

Last time we imagined a tunnel1 through the center of the earth.   If such a thing existed, you could step into one end, fall all the way to the other side, and step out. Gravity2 would cause you to pick up almost exactly as much speed on the way down as would be needed to get you back up the other side. Once you passed the center of the earth, your speed would gradually3 decrease back to zero just at the point where you stepped out.
The trick comes in the "almost" part. It couldn't really be the case that you'd pick up exactly as much speed as would be needed to reach the same spot on the other side. Some energy always finds a way to escape.
Think of swinging on a swing. If you just push off once and then sit still, the swing doesn't keep going back and forth4 forever. Gradually it slows down.
It's the same situation when you step into that hole. Down you go, and when you come out the other side, your feet will stop at slightly less than ground-level. If you miss your chance to grab5 onto something, you will fall though the entire planet6 again. Back at your starting place you rise slightly less than last time, and so on.
See the trouble? It would be a fast way to travel, but miss your step and you will wind up yo-yoing inside the tunnel, each time rising slightly less than the time before. Eventually you will stop moving altogether and be stuck in the middle -- with a long climb home.  

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1 tunnel qHHyz     
n.隧道，地下道；vi.挖隧道；vt.掘隧道于
参考例句：
  • They will have to block up the entrance to the tunnel.他们将不得不封闭通往隧道的入口。
  • The train passed through a tunnel.火车通过了一条隧道。
2 gravity Felz2     
n.重力，引力，地心引力
参考例句：
  • An apple falls down because of gravity.由于重力的关系，苹果往下掉。
  • The stone rolled down the mountain by gravity.这块石头由于重力作用而滚下山。
3 gradually UrNzxS     
adv.逐渐地
参考例句：
  • The roaring sea gradually calmed down.咆哮的大海逐渐平静下来。
  • Gradually he became silent.渐渐地他变得沉默寡言了。
4 forth Hzdz2     
adv.向前；向外，往外
参考例句：
  • The wind moved the trees gently back and forth.风吹得树轻轻地来回摇晃。
  • He gave forth a series of works in rapid succession.他很快连续发表了一系列的作品。
5 grab ef0xd     
vt./n.攫取，抓取；vi.攫取，抓住(at)
参考例句：
  • It is rude to grab a seat.抢占座位是不礼貌的。
  • The thief made a grab at my bag but I pushed him away.贼想抢我的手提包，但被我推开了。
6 planet A26z1     
n.行星
参考例句：
  • Neptune is the furthest planet from the sun. 海王星是离太阳最远的行星。
  • Rubbish, however, is only part of the problem of polluting our planet. 然而, 垃圾只是我们这个星球的污染问题的一个方面。

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