问题
不定项选择
如图所示，位于介质I和II分界面上的波源S，产生两列分别沿x轴负方向与正方向传播的机械波。若在两种介质中波的频率及传播速度分别为f1、f2和v1、v2，则
A．f1＝2f2，v1＝v2
B．f1＝f2，v1＝0.5v2
C．f1＝f2，v1＝2v2
D．f1＝0.5f2，v1＝v2
答案
答案：C
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如图所示，位于介质I和II分界面上的波源S，产生两列分别沿x轴负方向与正方向传播的机械波。若在两种介质中波的频率及传播速度分别为f1、f2和v1、v2，则
A．f1＝2f2，v1＝v2
B．f1＝f2，v1＝0.5v2
C．f1＝f2，v1＝2v2
D．f1＝0.5f2，v1＝v2
答案：C
Who designed the first helicopter(直升机)? Who __1___some of most famous pictures in the world? Who knew more about the human body than most ___2___of his time? There is an answer ___3___ all these questions --Leonardo da Vinci (达•芬奇) Leonard may have been the greatest genius ___4___ have ever known. He lived in Italy around the year 1500, but many of his inventions seem modern to us today. For example, one of his notebooks has drawings of a helicopter. Of course, he couldn’t ____5___ a helicopter with the things he had. But scientists say his idea would have worked. But Leonard ___6___ an inventor. He was one of the greatest artists of his day. By the time he was twenty years old, he was called a master(大师) painter, and as he got older he became ___7__ more famous. Sometimes he drew a hand in ten different ways ____8___he was ready to paint. Many of Leonardo’s wonderful paintings are still with ____9___today. You may know one of his most famous works-- the ___10___ woman known as the Mona Lisa.
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