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问题 计算题

如图17，光滑水平轨道上有三个木块A、B、C，质量分别为、，开始时B、C均静止，A以初速度v0向右运动，A与B相撞后分开，B又与C发生碰撞并粘在一起，此后A与B间的距离保持不变。求B与C碰撞前B的速度大小。

答案

设A与B碰撞后，A的速度为vA，B与C碰撞前B的速度为vB，B与C碰撞后粘在一起的速度为v，由动量守恒定律得

对A、B木块：        

对B、C木块：          

由A与B间的距离保持不变可知

                  

联立123式，代入数据得

                    

本题考查动量守恒定律，找到碰撞前后两个状态列式求解

阅读理解




What are the most successful movies?
American movies are the most popular around the world. The most successful movies of the twentieth century wad Titanic. People around the world paid almost $2 billion, and they saw the famous ship sink again. Titanic was more popular than Star wars. This Star Wars film made almost $1billion in the late 1990s. Close behind this was Jurassic Park, a film about dinosaurs. And it’s interesting that Titanic was also the most expensive film: it cost $200 million to make.
Which is the world’s most popular sport?
The world most popular sport is or soccer．It’s more popular than football，basketball，or baseball．FIFA has 4.1 million teams，and 200 million players in l90 countries around the world．More than one billion people watch the World Cup Final on TV every four years. In the United States，soccer is not the most popular for spectators(观众)，but it is the most popular sport for kids and teens．About 3 million Americans under the age of nineteen play soccer．
What are the largest shopping malls?
The largest shopping mall in the world is the West Edmonton Mall in Edmonton，Canada．The mall has more than 800 retail stores，including eight department stores．There is also a 335‘room hotel·It’8 much；larger than the 8econd largest mall in the world：this is the mall of America in Bloomington，Minnesota．The US mall has 400 stores. Its amusement attractions include a roller coaster. Some people spend their vacation at this mall．But in the future，the biggest stores probably are going to be on the computer：some online malls already say that they have stores in l00 countries·
小题1:What’s the most successful movie of the twentieth century?
A．Titanic．B．Star Wars．C．Jurassic Park．D．Avatar．
小题2:How much did the movie Star Wars make in the late l990s?
A．Almost 8 2 billion．B．Almost $l billion．
C．Almost$200 million．D．Almost$3 million．
小题3:Which of the following descriptions about soccer is TRUE?
A．In the US，soccer is the most popular for both spectators and kids．
B．FIFA has 2,000 million players in l90 countries around the world．
C．Football is more popular than soccer，basketball，or baseball．
D．About 3 million Americans under the age of nineteen play soccer．
小题4:Where is the world’s largest mall?
A．In Canada．B．In America．C．In England．D．On the computer．
小题5:What CANNOT people do in the West Edmonton Mall according to the passage?
A．Enjoy a roller coaster．B．Spend vacations．
C．Go shoppin9．D．See Mickey Mouse．
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直线y=3x+2与y=
1
2
x+2的______，相同，所以这两条直线______，同一点，且交点坐标______；直线y=5x-1与y=5x-4的______相同，所以这两条直线______．
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