题目内容
质量为
的带正电小球由空中某点
无初速度自由下落,在
秒末加上竖直向上、范围足够大的匀强电场,再经过
秒小球又回到
点。不计空气阻力且小球从未落地,则( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161843225298.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161843349292.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161843490257.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161843490257.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161843349292.png)
A.整个过程中小球电势能变化了![]() |
B.整个过程中小球机械能增量的大小为![]() |
C.从加电场开始到小球运动到最低点时小球动能减小了![]() |
D.从A点到最低点小球重力势能减小了![]() |
BD
试题分析:根据运动学公式,抓住位移关系求出电场力和重力的大小关系.根据电场力做功判断电势能的变化,根据动能定理求出从加电场开始到小球运动到最低点时小球动能的变化,根据重力做功判断从A点到最低点小球重力势能变化.
A、自由落体运动的位移
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161845019640.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161845175938.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161845315576.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161845502833.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161845877540.png)
C、加电场时,物体的速度v=gt,末速度为零,则动能的减小了
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161846048611.png)
D、ts末加上电场匀减速运动到零的位移
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161846345992.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161846579951.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824161846719644.png)
故选BD.
点评:解决本题的关键知道电场力做功与电势能的关系,重力做功与重力势能的关系以及合力功与动能的关系.
![](http://thumb2018.1010pic.com/images/loading.gif)
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