题目内容
重为G的两个完全相同的小球,与水平面的动摩擦均为
。竖直向上的较小的力F作用在连接两球轻绳的中点,绳间的夹角
,如图所示。缓慢增大F,到两球刚要运动的过程中,下列说法正确的是
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241540528432004.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154052719266.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154052781412.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241540528432004.jpg)
A.地面对球的支持力变大,摩擦力变大 |
B.地面对球的支持力变小,摩擦力变小 |
C.球刚开始运动时,地面对球没有支持力 |
D.球刚开始运动是,球受到的摩擦力最大 |
D
试题分析:以轻绳的中点为研究对象有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154052859812.png)
对小球进行受力分析如图
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241540534364598.png)
竖直方向
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154053514959.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154053623699.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154053670411.png)
水平方向
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154053779816.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154054138785.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154054201375.png)
当球刚开始运动时,小球的摩擦力达到最大静摩擦力,此后摩擦力为滑动摩擦力,随着F增大,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154053670411.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154054201375.png)
故选D
点评:当物体在外力作用小刚要滑动时的摩擦力为最大静摩擦力,最大静摩擦力稍大于滑动摩擦力,一般认为最大静摩擦力近似等于滑动摩擦力,本题还要注意正压力一直在减小,滑动摩擦力也是减小的。
![](http://thumb2018.1010pic.com/images/loading.gif)
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