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  3. When you watch a football game on a Saturday afternoon, you feel secure in your knowledge of what will happen when a player boot

When you watch a football game on a Saturday afternoon, you feel secure in your knowledge of what will happen when a player boot

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问题     When you watch a football game on a Saturday afternoon, you feel secure in your knowledge of what will happen when a player boots the ball.  It first goes up and then it comes down. That’s how it is, was, and will be--unless.... if the kicker someday should kick the ball such that at the instant it left his toe it were travelling upward at a rate of 11.2 kilometers per second (25 000 mhr). We would find that we would not have to worry about a punt return, because the ball simply would not come down. This particular speed is the escape velocity of an object from the earth. At this speed, any object, large or small, will escape from the earth to soar forever upward until captured by the gravitational attraction of some other planet or heavenly body. At first thought it might seem that a heavy object might require a greater initial speed to escape from the surface of the earth than a lighter one, remember that the way objects fall (or move upward against gravity) is independent of the mass of the object. As a result, if a velocity of 11.2 kilometers per second is enough to cause a football to escape from the earth, it is also enough to send a bowling ball (or a freight car) on its way.
    The concept of escape velocity applies to objects as small as atoms and molecules. The earth’s atmosphere is composed primarily of oxygen and nitrogen, and it contains practically no trace of hydrogen or helium, the lightest elements.  All the atoms of a gas travel in random motion at very high speeds, and the less massive atoms and molecules travel at the greatest speeds. In fact, the lighter gases go so fast that their speeds are greater than the critical speed of 11.2 kilometers per second. Thus they have long ago left our comfortable home and are now wandering through space in search of a more hospitable gravitational field.
    The moon has a weaker gravitation field than the earth. The escape velocity of an object on the moon is about 2.4 kilometers per second, and even the heavier gases have enough speed to escape the moon. This explains why we find no atmosphere there; any gases that may be released near the moon quickly escape.
Helium is not found in the earth’s atmosphere because ______.
选项 A、its moving speed well exceeds 11. 2 km per second
B、it left the atmosphere long ago in search of another planet
C、its moving speed is not great enough to reach the atmosphere
D、it is lighter and cannot be thrown upwards
答案A
解析
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