2. 外语
  3. The Earth has a force that pulls things toward itself. We call this force gravity(地心引力). This is something we live with all the

The Earth has a force that pulls things toward itself. We call this force gravity(地心引力). This is something we live with all the

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问题     The Earth has a force that pulls things toward itself. We call this force gravity(地心引力). This is something we live with all the time, and we take it for granted and hardly ever think about it. But it is a most important factor in rocket operation and must be overcome if we are to get anywhere in space, or off the ground at all.
    Take the throwing of a ball as an example. The harder the ball is thrown, the faster and higher it will go. What is the secret? It’s speed. If we could throw the ball hard enough it would go up and up forever and never come down. The speed at which it would have to be thrown to do this is known as escape speed. Of course, we cannot throw a ball hard enough because the speed required to escape completely from’ the Earth’s gravity is seven miles per second, or over twenty-five thousand miles per hour.
    Once escape speed has been reached by a spacecraft(宇宙飞船), no further power is needed. A rocket aimed at the Moon, for instance, will "coast"(滑行) the rest of the way because the Earth’s gravity cannot then pull it back, and there is no air resistance(阻力) in space to slow it down. This "coasting" is known as "free fall". That does not mean the rocket is falling down towards the Earth but that it is traveling freely in space without the aid of power, like a bicycle coasting downhill.
    Free fall is an important feature of space travel: it would be impossible to carry enough fuel to provide powered flight all the time.
选项 A、To travel as fast as it can.
B、To overcome the Earth gravity.
C、To get away from the air resistance.
D、To have strong power in its operation.
答案B
解析 从文章的第一段最后一句"地心引力是火箭飞行中的一个重要因素，如果我们想到达太空或者离开地球，我们就必须脱离地心引力"，可以得出本题的答案应为B。
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