在水平面上有一斜面体M,倾角为
，斜面体上放一物块m，现对斜面体施加一水平作用力F，使物块与斜面体保持相对静止一起运动，不计一切摩擦，如图所示。则
A．物块的加速度为g sin 
B．物块的加速度为g tan
C．物块的加速度不能确定
D．水平作用力F的大小不能确定
答案：B
题目分析：对M和m这个整体受力分析，该整体受到重力G、支持力N和水平外力F，在F作用下两物体保持相对静止一起运动，该整体的加速度一定水平向左；再对m受力分析，受到重力Gm和支持力Nm，由于不计一切摩擦，m产生的加速度与整体的加速度一样，即水平向左，则a=gtanθ。所以B选项正确而A、C选项错误；水平外力F=（M+m）a，所以D选项错误。
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| In learning English, one should first pay attention to listening and speaking. It is the groundwork (基础) of reading and writing. You'd better 1 your best to speak while you do much listening. Don't be 2 of making mistakes. But be careful not to let them stop you from improving your 3 . While you are doing this, a good 4 is to write－keep a diary, write notes or letters. Then if you can, ask some others to go through 5 you have written and tell you where it is wrong. Many mistakes in your speaking will be 6 found when you wirte. Through correcting mistakes, you can do better in learning English. If you are slow in speaking, don't 7 about it. One of the helpful ways is reading, either aloud or to yourself. The important thing is to choose 8 interesting to read. It mustn't be too difficult for you. When you are reading 9 this way, don't stop to 10 the new words if you can guess their meanings when they have nothing to do with the sentences. You can do that some other time. | ||||
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