题目内容
如图所示,A、B两个异种电荷的等质量小球,分别被两根绝缘细绳系在木盒内的同一竖直线上。静止时,木盒对地面的压力为FN,细绳对A的拉力为F1,细绳对B的拉力为F2,若将系B的细绳和系A的细绳同时断开,则两细绳刚断开时( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241547436391978.jpg)
A.木盒对地面的压力增大
B.木盒对地面的压力减小
C.木盒对地面的压力为FN+F2-F1
D.木盒对地面的压力为FN+F1-F2
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241547436391978.jpg)
A.木盒对地面的压力增大
B.木盒对地面的压力减小
C.木盒对地面的压力为FN+F2-F1
D.木盒对地面的压力为FN+F1-F2
BC
试题分析:断开前,将三者看作一个整体,所以有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154743654858.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154743826858.png)
设断开前两小球之间的吸引力为F,则对A有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154743888663.png)
对B有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154744013704.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154744091678.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241547443251492.png)
点评:需要对断开前用整体法分析,断开后用隔离法分析,结合力的平衡条件分析
![](http://thumb2018.1010pic.com/images/loading.gif)
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