题目内容
一根水平粗糙的直横杆上,套有两个质量均为m的小铁环,两铁环上系着两条等长的细线,共同拴住一个质量为M的球,两铁环和球均处于静止状态,如图所示,现使两铁环间距稍许增大后系统仍处于静止状态,则水平横杆对铁环的支持力N和摩擦力f的变化是( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241140401501665.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241140401501665.jpg)
A.N不变,f不变 | B.N不变,f变大 |
C.N变大,f变大 | D.N变大,f不变 |
B
对两小铁环和球进行受力分析如图:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241140401813964.jpg)
设绳与水平面的夹角为
。把两铁环和球看成一个整体,在竖直方向上合力为零即
,N和
无关故N不变;对M由平衡条件
,
减小,T增大;
,
减小,f增大,故选B。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241140401813964.jpg)
设绳与水平面的夹角为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114040196200.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114040212579.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114040196200.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114040228544.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114040196200.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114040415347.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114040196200.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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