物体沿直线作匀变速运动, 它的位移与时间的关系是x = 24t －6t2, (x的单位是m, t的单位是s)，则它的速度为零的时刻是（ ）
A．
s
B．4s
C．2s
D．24s
答案：C
根据匀变速位移时间关系：
和
对比
可知
由此可以看出物体再做匀减速直线运动，所以速度为零的时间：
故答案选C。
直接运用匀变速直线运动公式对比求出初速度和加速度，然后根据
求减速的时间。
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| Dear mom, I'm having a great time in Hong Kong, although I have to be honest and say that I prefer Shanghai. Stil  l, it's a great place to visit and I'm lucky to be here for my six-month English course. My accommodation is with an English family. They are friendly to me. We are getting on well with each other. By the way, my host family will take me to an Indian film festival next weekend. There are so many things to see and to do here. Every day I have many activities which take place after class. I can take trips to the beaches, Disne  yland and soon I can also go to various shopping centres, sports centres, cinemas and concerts. There are lots of different kinds of food here. Last night I went for Italian food before the concert. Do you like it? I don't know what to try next. Recently I have made rapid progress with my speaking and listening skills. My host family also teaches me something about English culture. In general, I'm having a good time, but I miss you very much. Yours, Li Feng | ||||||||||||
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