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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

admin2014-09-13  96
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.
We do not feel the gravitational pull of a packet of butter because______
选项 A、it is too small to have a gravitational pull of its own
B、its pull is so small that we tend to ignore it
C、its pull disappears in the presence of the earth’ s gravitation
D、it tends to melt and loses its gravitational pull
答案B
解析 最后一段第三句说明一个物体产生的引力与物体本身的重量有关。最后一段第四句又表明由于本身重量较小，一包黄油产牛的引力很有限，相对于月球、地球、太阳等产生的强劲引力来说，比较容易被我们忽略。
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