题目内容
如图,一条长为L的细线上端固定,下端拴一个质量为m的带电为q的小球,将它置于一方向水平向右的匀强电场中,使细线竖直拉直时小球在A点静止释放,当细线离开竖直位置偏角α=60°时,小球速度为0。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250022569963950.jpg)
(1)求:①小球带电性质②电场强度E
(2)要小球恰好完成竖直圆周运动,求A点释放小球时应有初速度vA(可含根式)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250022569963950.jpg)
(1)求:①小球带电性质②电场强度E
(2)要小球恰好完成竖直圆周运动,求A点释放小球时应有初速度vA(可含根式)
(1)正电
(2)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257043856.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257012691.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257043856.png)
试题分析:(1)①小球带正电(1分)
动能定理:0 ="EqLsinα-mg" L (1-cosα)(2分)
∴ E =
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257012691.png)
(2分)(2)等效重力=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257074688.png)
与竖直夹角30° (1分)
恰好过等效最高点:m
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257090437.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257074688.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257121339.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257121339.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257074688.png)
∴vA=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002257043856.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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