题目内容
一根长为L的丝线吊着一质量为m的带电量为q的小球静止在水平向右的匀强电场中,如图所示,丝线与竖直方向成37o角,现突然将该电场方向变为向下且大小不变,不考虑因电场的改变而带来的其他影响,重力加速度为g,sin370=0.6,cos370=0.8,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020131181387.png)
(1)匀强电场的电场强度的大小;
(2)求小球经过最低点时丝线拉力的大小。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020131181387.png)
(1)匀强电场的电场强度的大小;
(2)求小球经过最低点时丝线拉力的大小。
(1)
;(2)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002013149784.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002013134766.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002013149784.png)
试题分析:(1)由平衡条件得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002013165846.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002013134766.png)
(2)电场方向变成向下后,小球开始摆动做圆周运动
由动能定理得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020131961450.png)
在最低点时根据牛顿第二定律有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020132121080.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002013149784.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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