如图为某工厂生产流水线上水平传输装置的俯视图，它由传送带和转盘组成。物品从处无初速放到传送带上，运动到处后进入匀速转动的转盘，设物品进入转盘时速度大小不发生变化，此后随转盘一起运动（无相对滑动）到处被取走装箱。已知A、B两处的距离，传送带的传输速度，物品在转盘上与轴的距离，物品与传送带间的动摩擦因数。取
（1）求物品从A处运动到Ｂ处的时间t？
（2）若物品在转盘上的最大静摩擦力可视为与滑动摩擦力大小相等，则物品与转盘间的动摩擦因数至少为多大？
解：（1）设物品质量为
，物品先在传送带上做初速度为零的匀加速直线运动，其位移大小为
。
由
=
，
得
。
之后，物品和传送带一起以速度做匀速运动
匀加速运动的时间
匀速运动的时间
所以
（2）物品在转盘上所受的静摩擦力提供向心力，当物品在转盘上恰好无相对滑动时，
有
得
。
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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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