题目

It’s Sunday. It’s fine. Tom does not go to school today. But he is not at home. He’s now sitting on a Chinese bus. He is going to the cinema. All his classmates will go to see a movie this afternoon. His teacher Mr Wang will go there, too. They will meet at 2:00 p.m. at the gate of the cinema. An old man gets on the bus. Tom stands up and says to the old man, ‘Come and sit here, please.’ The old man goes over and sits down. He says, ‘Thank you very much, my boy. You are a good student.’ Tom answers, ‘You’re welcome.’ ‘What school are you in?’ the old man asks Tom.‘I a17.如图所示,一同学用双手(手未画出)水平对称地用力将两长方体课本夹紧,且同时以加速度a竖直向上匀加速捧起.已知课本A质量为m,课本B质量为2m,手的作用力大小为F,书本A、B之间动摩擦因数为μ.用整体法与隔离法可分析出此过程中,书A受到书B施加的摩擦力大小为(  )A.μFB.2μFC.$\frac{1}{2}$m(g+a)D.m(g+a)
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