题目内容
质量为
、带电量为
的小球用一绝缘细线悬于
点,开始时它在
之间来回摆动,
、
与竖直方向
的夹角均为
,如图所示。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241139579362489.jpg)
(1)如果当它摆动到
点时突然施加一竖直向上的、大小为
的匀强电场,则此时线中的拉力
。
(2)如果这一电场是在小球从
点摆到最低点
时突然加上去的,则当小球运动到
点时线中的拉力
= 。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957780204.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957827219.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957842187.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957858241.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957874238.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957905242.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957920241.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957936200.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241139579362489.jpg)
(1)如果当它摆动到
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957952209.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957983442.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957998224.gif)
(2)如果这一电场是在小球从
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958014202.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958092205.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957952209.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958123211.gif)
(1)0
(2)![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958154544.gif)
(2)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958154544.gif)
(1)当小球摆动到
点时,速度为零,向心加速度为零,此时合外力便为零,因为电场力与重力已抵消,故拉力
。
(2)从A到C点由动能定理可得:
,在最低点C点:
,联立可得:
。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113957952209.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958217264.gif)
(2)从A到C点由动能定理可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958217788.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958232468.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113958264593.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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