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在“探究小车速度随时间变化的规律”的实验中,打点计时器使用的交变电流的频率为50HZ,记录小车运动的纸带如图所示,在纸带上选择0、1、2、3、4、5的6个计数点,相邻两计数点之间还有四个点未画出,纸带旁并排放着带有分度值为毫米的刻度尺,零点跟“0”计数点对齐.

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    American country music is also known as(被称为)country Western music.It has a very long history.It comes from the folk songs of immigrants(移民)from Britain.
    Country music uses simple music and words to express everyday feelings such as loneliness,love,and sadness.That is to say,country music describes life.It talks about friends and enemies,trucks and roads,farms and crops.People in many parts of the world like country music,because everyone can understand what the music is about.
    Country Western music became popular first among cowboys(牛仔)in the American West courttryside.Cowboys had to take care of cows day and night.When they were alone with the cows,they often sang beautiful and peaceful songs to calm the animals and to overcome their own fear.They were not well—educated and so they sang about their daily life in very simpie words.They played guitars,violins,and other instruments.  In the south of America,many people added instruments from their homes,like bottles and spoons.  When cowboys visited their friends and families on holidays like Thanksgiving,they usually sang and played country Western music.
    In recent years,many musicians have made country Western music a little different from the one in the past.These changes make the songs sound even better.Today singers such as Garth Brooks, Reba McEntire,Emmylou Harris,Lyle Lovett,Eddie Rabbitt,LeAnn Rimes,Randy Travis,and a group called Alabama are singing and playing in different styles.They have brought country Western music more and more fans from all over the world.