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  3. Weight on and off the Earth We are so used to our life on the surface of the earth that it can be quite an effort for our mi

Weight on and off the Earth We are so used to our life on the surface of the earth that it can be quite an effort for our mi

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问题                     Weight on and off the Earth
    We are so used to our life on the surface of the earth that it can be quite an effort for our mind to break free of all the ideas that we take for granted. Because we can feel that things are heavy, we think of "weight" as being a fixed quality in an object, but it is not really fixed at all. If you could take a one-pound packet of butter 4, 000 miles out from the earth, it would weigh only a quarter of a pound.
    Why would things weigh only a quarter as much as they do at the surface of the earth if we took them 4, 000 miles out into space? The reason is this; all objects have a natural attraction for all other objects; this is called gravitational attraction. But this power of attraction between two objects gets weaker as they get farther apart. When the butter was at the surface of the earth, it was 4, 000 miles from the center. When we took the butter 4, 000 miles out, it was 8, 000 miles from the center, which is twice the distance. If you double the distance between two objects, their gravitational attraction decreases two times two. If you treble the distance, it gets nine times weaker(three times three)and so on.
    So this is one of the first things we need to remember that the weight of an object in space is not the same as its weight on the surface of the earth.
    What about the weight of our pound of butter on the surface of the moon? At the distance the pull of the earth is about 4, 000 times smaller than it is here on the surface, so we can forget all about the earth-pull on our butter.
    On the other hand, on the moon there will be an attraction between the butter and the moon, but the butter will weigh only about one-sixth as much as it does on the earth. This is because the moon is so much smaller than the earth. The amount of gravitational pull that a body produces depends on the amount of material in it. A packet of butter has a gravitational pull of its own; but this is very small in relation to the pull of something as large as the moon, or the earth, or the sun.
According to the passage, "weight" should be understood in the sense that _
选项 A、it is fixed if it is outside the earth’ s gravitational pull
B、it decreases four times when it is 4, 000 miles from the earth’ s center
C、it varies with the change of the gravitational attraction between two objects
D、things increase in amount as they are closer to the earth’ s surface
答案C
解析 第一段第二句表明，物体的重量并非固定不变。第二段解释说，物体的重量源于物体间的引力。也就是说，物体的重量随着物体间引力的改变而改变。
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