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宇宙间存在一些离其它恒星较远的、由质量相等的四颗星组成的四星系统，通常可忽略其他星体对它们的引力作用．已观测到的四星系统存在着一种基本的构成形式是：三颗星位于等边三角形的三个顶点上，并沿外接于等边三角形的圆形轨道运行，第四颗星位于圆形轨道的圆心处，已知圆形轨道的半径为R，每颗星体的质量均为m．求：

（1）中心星体受到其余三颗星体的引力的大小；

（2）三颗星体沿圆形轨道运动的线速度和周期．

答案

（1）中心星体受到其余三颗星体的引力大小相等，方向互成1200．

据力的合成法则，中心星体受到其他三颗星体的引力的合力为零．

（2）对圆形轨道上任意一颗星，根据万有引力定律和牛顿第二定律有：

G

m?m
R2
+2G
m?m
r2
?cos300=m
v2
R

又因为r=2R?cos30°

由以上两式可得三颗星体运动的线速度为：v=

(1+
3
)Gm
3
R

三颗星运动的周期为：T=

2πR
v
=2πR
3
R
(1+
3
)Gm

答：（1）中心星体受到其余三颗星体的引力的大小为零；

（2）三颗星体沿圆形轨道运动的线速度为

(1+
3
)Gm
3
R
，周期为2πR
3
R
(1+
3
)Gm
．

填空题
阅读下面短文，根据文中意思，从方框中选出恰当的词，并在下面对应题号的横线上写出各单词的完整、正确的形式使短文意义完整。

They,   careful,   be,   fact,   take,   would,   other,   stand,   what,   decide

    I am eighteen years old this year. I   1   a pupil before and now I'm a university student. My mother was my
first teacher when I was very little. Then, I had three   2   teachers in different schools. Now Miss Williams is
one of my university teachers.
    I have never expected that I will be a teacher one day. I am a shy girl and I am afraid of   3   before many
people. I do not know   4   to say. But I will be a teacher tomorrow! Tomorrow morning!
    I made this   5   just two weeks ago. Miss Williams, my English teacher, told me there was a teacher job, just
for two weeks in this summer. She asked if I   6   be interested. I wanted to make some money, so I said "yes"
to her. At once I regretted but it was too late to   7   my word back.
    There are about twenty foreign boys and girls in the class. They know very little English. I have   8   read the
book that Williams gave me. Four of the lessons are very simple, in   9   too simple. I do not know what to do
with these few simple words and sentences. I will read the lesson to them, and ask them to read after me, and
then ask them to read it  10  . That will be about ten minutes. What shall I do next?
1._________   2._________   3. _________    4. __________    5.__________
6._________   7._________   8. _________    9. __________   10.__________
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单项选择题

2.0LMondeo以下关于EATC模块的描述正确的是（）

A.EATC模块温度显示单位能改变，但必须用WDS

B.EATC模块能用WDS进行自检，也可通过面板自检

C.EATC模块控制三个风门步进电机，不控制再循环风门马达

D.使用WDS不能查看EATC模块数据流
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