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问题 问答题

如图所示，固定的光滑圆弧轨道与水平地面平滑相接，在光滑的水平地面上有两个滑块A、B，其质量分别为mA=2kg，mB=4kg，A、B中间夹一被压缩的弹簧，弹簧与A、B不连接，开始处于静止状态．当弹簧被释放后，滑块A、B被弹开，其中滑块B能滑至圆弧轨道最大高度H处，H=0.2m，求弹簧开始具有的弹性势能？g=10m/s2

答案

设mA、mB被弹簧弹开时的速度大小分别为vA、vB，则根据能量守恒有：mAvA-mBvB=0

B从弹开到圆弧轨道的最高点的过程中机械能守恒，可得：mBgH=

1
2
m
v2B

弹簧将两个物体弹开的过程中机械能守恒，得：EP=

1
2
mA
v2A
+
1
2
mB
v2B

联立以上三式，代入数据得：EP=24J

答：弹簧开始具有的弹性势能是24J．

单项选择题

Everybody dreams. Some people think dreams can tell us about the future. Other people think dreams can tell us about ourselves. It’s like our (11) is talking to us.

Why are dreams (12) strange and hard to understand Some people think our brain uses (13) to talk to us. When we fly, swim, or fall down in our dreams, it has a (14) meaning. Lots of doctors help people (15) their dreams. They find that dreams tell us about our (16) and fears.

These days, many scientists (17) that dreams are very important. During the day, we have many (18) , and our brain receives a lot of information. When we dream, our brain (19) information that is not important, and puts the most important information into our (20) . As we learn more about the brain, we may find answers to our questions about dreams.

13().

A.symbols 

B.examples 

C.numbers 

D.pictures
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单项选择题

在计算机中采用二进制，是因为( )。

A．电路简单
B．工作可靠并稳定
C．二进制的运算法则简单
D．上述三个原因
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