第4部分：阅读理解—第三篇
eight on and off the Earth
e 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 e take for granted. Because e can feel that things are heavy, e think of "eight" 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 ould eigh only a quarter of a pound.医,学全,在线.搜集.整理www.lindalemus.com
hy ould things eigh only a quarter as much as they do at the surface of the earth if e 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 poer of attraction beteen to objects gets eaker as they get farther apart. hen the butter as at the surface of the earth, it as 4,000 miles from the center. hen e took the butter 4,000 miles out, it as 8,000 from the center, hich is tice the distance. If you double the distance beteen to objects, their gravitational attraction decreases to times to. If you treble the distance, it gets nine times eaker (three times three) and so on.
So this is one of the first things e need to remember: that the eight of an object in space is not the same as its eight on the surface of the earth. hat about the eight 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 e can forget all about the earth-pull on our butter.
On the other hand, on the moon there ill be an attraction beteen the butter and the moon: but the butter ill eigh 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 on; but this is very small in relation to the pull of something as large as the moon, or the earth, or the sun.
41 Ho much ould four pounds of tea eigh if it as taken 4,000 miles out from the surface of the earth?
A 1 pound.
B 2 pounds.
C 3 pounds
D 4 pounds.医 学全,在线.搜集.整理www.lindalemus.com
42 e find it hard to break free from ideas connected ith living on the earth because___________
A it demands too great an effort for us to do so.
B e are too familiar ith the ay things are to question the ideas.
C e have proved that those ideas are correct.
D e are so sure of ourselves that e never doubt anything on the earth.
43 According to the passage, "eight" 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 hen it is 4,000 miles from the earth's center.
C it varies ith the change of the gravitational attraction beteen to objects.
D things increase in amount as they are closer to the earth's surface.
44 e do not feel the gravitational pull of a packet of butter because___________
A it is too small to have a gravitational pull of its on.
B its pull is so small that e 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 eight systems in different places.
B freedom from traditional ideas.
C traveling from the earth to the moon.
D the effect of gravitation on eight.
