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　　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 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 Hat 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.

　　41 How much would four pounds of tea weigh if it was taken 4,000 miles out from the surface of the earth?

　　A 1 pound.

　　B 2 pounds.

　　C 3 pounds.

　　D 4 pounds.

　　42 We find it hard to break free from ideas connected with living on the earth because___________

　　A it demands too great an effort for us to do so.

　　B we are too familiar with the way things are to question the ideas.

　　C we have proved that those ideas are correct.

　　D we are so sure of ourselves that we never doubt anything on the earth.

　　43 According to the passage, "weight" should be understood in the sense that___________

　　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.

　　44 We do not feel the gravitational pull of a packet of butter because___________

　　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.

　　45 The main idea of this passage is___________

　　A different weight systems in different places.

　　B freedom from traditional ideas.

　　C traveling from the earth to the moon.

　　D the effect of gravitation on weight.

　　答案：31. A　　32. B　　33. C　　34. B　　35. D