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问题 问答题

如图所示，小球P一边贴着水面每秒振动5次，一边沿x轴正方向匀速移动，x=0点

是它的初始位置，图示为恰经10个周期时观察到的水面波，此时小球运动到x=20cm处．问：

（1）在x=-20cm处的浮子的振动频率比波源频率大还是小？

（2）小球P匀速移动的速度及此水面波的传播速度分别多大？

答案

（1）由图可知，质点在向右移动，故在x=-20cm处的浮子的振动频率比波源频率小

（2）小球P振动的周期T=

1
5
s=0.2s

小球P振动10个周期所用时间t=10T=2s

小球P的速度v1=

s1
t
=
0.2
2
m/s=0.1m/s

水面波的传播速度v2=

s2
t
=
0.4
2
m/s=0.2m/s；

答：x=-20cm处的浮子的振动频率比波源频率小；（2）小球P的速度为0.1m/s，水面波的传播速度为0.2m/s．

判断题

全焊无缝道岔的尖轨与导轨焊连时，必须在设计锁定轨温范围内进行，并使限位器子母块居中。
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单项选择题 案例分析题

Shopping habits in the United States have changed greatly in the last quarter of the 20th century. （1） in the 1900s most American towns and cities had a Main Street. Main Street was always in the heart of a town. This street was （2） on both sides with many （3） businesses. Here, shoppers walked into stores to look at all sorts of merchandise: clothing, furniture, hardware, groceries. （4）,some shops offered （5） .These shops included drugstores, restaurants, shoe-repair stores, and barber or hairdressing shops. （6） in the 1950s, a change began to （7） .Too many automobiles had crowded into Main Street （8） too few parking places were （9） shoppers. Because the streets were crowded, merchants began to look with interest at the open spaces （10） the city limits. Open space is what their car-driving customers needed. And open space is what they got （11） the first shopping centre was built. Shopping centres, or rather malls, （12） as a collection of small new stores （13） crowded city centres. 14 by hundreds of free parking space, customers were drawn away from （14） areas to outlying malls. And the growing （16） of shopping centres led （17） to the building of bigger and betterstocked stores. （18） the late 1970s,many shopping malls had almost developed into small cities themselves. In addition to providing the （19） of one stop shopping, malls were transformed into landscaped parks, （20）benches, fountains, and outdoor entertainment.

请在15处填上正确答案（）

A.inner

B.central

C.shopping

D.downtown
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