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问题 问答题

月球的质量约为地球的1/81，半径约为地球半径的1/4，地球上第一宇宙速度约为7.9km/s，则月球上第一宇宙速度为多少？（计算结果保留2位有效数字）

答案

设地球的质量和半径分别为M1、R1，月球的质量和半径分别为M2、R2

根据题意，则有：M1：M2=81：1   R1：R2=4：1，

物体绕星体表面做匀速圆周运动的速度为第一宇宙速度，

有：G

Mm
R2
=m
v2
R

可得，第一宇宙速度为   v=

GM
R

故地球与月球的第一宇宙速度之比为v1：v2=

GM1
R1
?
R2
GM2
=
M1R2
M2R1
=
81
1
?
1
4
=9：2

又地球第一宇宙速度v1=7.9km/s

故月球第一宇宙速度v2=1.8km/s

答：月球第一宇宙速度是1.8km/s．

单项选择题

应用总流能量方程时，渐变流断面的压强（）。

A.必须用绝对压强

B.必须用相对压强

C.必须用压强水头

D.相对压强和绝对压强都可以
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阅读理解

阅读理解

    We can see how the product life cycle works by looking at the introduction of instant coffee.When

it was introduced,most people did not like it as well as "regular" coffee and it took several years to gain

general acceptance (introduction stage). At one point,though,instant coffee grew rapidly in popularity

and many brands were introduced (stage of rapid growth)．. After a while people became attached to

one brand,sales leveled off (stage of maturity). Sales went into a slight decline (衰退) when freeze

dried coffees were introduced (stage of decline).

     The importance of the product life cycle to marketers is this:Different stages in the product life cycle

call for different strategies.The goal is to extend product life so that sales and profits do not decline.One

strategy is called market modification.It means that marketing managers look for new users and market

sections.Did you know,for example,that the backpacks that so many students carry today were originally

designed for the military?

    Market modification also means searching for increased usage among present customers or going for

a different market,such as senior citizens.A marketer may re－position the product to appeal to new

market sections.

    Another product extension strategy is called product modification.It involves changing product quality,

features,or style to attract new users or more usage from present users.American auto manufacturers are

using quality improvement as one way to recapture (夺回) world markets.Note,also,how auto

manufacturers once changed styles dramatically from year to year to keep demand from falling.

1. According to the passage,when people grow fond of one particular brand of a product,its

    sales will________.

A. decrease gradually  

B. become unstable

C. improve enormously  

D. remain at the same level

2. The first paragraph tells us that a new product is________.

A. usually introduced to satisfy different tastes

B. often more expensive than old ones

C. often inferior to old ones at first

D. not easily accepted by the public

3. Marketers need to know which of the four stages a product is in so as to________.

A. work out marketing policies

B. promote its production

C. increase its popularity

D. speed up its life cycle

4. The author mentions the example of "backpacks" (Line7,Para.2) to show the importance of _____.

A. increasing usage among students

B. pleasing the young as well as the old

C. exploring new market sections

D. serving both military and civil needs

5. In order to recover their share of the world market,US auto makers are________.

A. improving product quality

B. re－positioning their product in the market

C. modernizing product style

D. increasing product features

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