题目内容
如图所示,mA>mB,设地面对A的支持力为FN,绳子对A的拉力为F1,地面对A的摩擦力为F2,若水平方向用力F拉A使B匀速上升,则:( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241518251101175.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241518251101175.png)
A.FN增大,F2减小,F1增大 |
B.FN增大,F2增大,F1不变 |
C.FN减小,F2减小,F1增大 |
D.FN减小,F2减小,F1不变 |
B
试题分析:本题对A进行受力分析列出平衡表达式,分析随θ变化各个力的变化情况。
如图对物体A进行受力分析,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151825282574.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151825375901.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151825453411.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151825656586.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151825453411.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151826140353.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241518263433902.jpg)
故选B
点评:定滑轮之改变力的方向,不改变力的大小。找出不变量和影响各个力的因素。
![](http://thumb2018.1010pic.com/images/loading.gif)
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