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 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.
单选题
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.
【正确答案】
A
【答案解析】
单选题
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
【正确答案】
B
【答案解析】
单选题
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
【正确答案】
C
【答案解析】
单选题
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