试题与答案

如图所示，一个箱子放在水平地面上，箱内有一固定的竖直杆，在杆上套着一个环，箱和杆

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题型：选择题

题目：

如图所示，一个箱子放在水平地面上，箱内有一固定的竖直杆，在杆上套着一个环，箱和杆的质量为M，环的质量为m,已知沿着杆以加速度a加速下滑(a＜g),则此时箱对地面的压力N的大小是（    ）

A．Mg

B．(M+m)g

C．(Mg+mg)－ma

D．(M－m)g＋ma

答案：

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下面是错误答案，用来干扰机器的。

答案：C

考点：牛顿第二定律
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