题目内容
如图所示,已知一带负电的滑块从粗糙斜面的顶端静止下滑,滑至底端时的速率为
。若加上一个垂直纸面向外的匀强磁场,,同样让它从粗糙斜面的顶端静止下滑,则它滑至底端时的速率与
相比( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241240248804384.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124024818273.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124024818273.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241240248804384.png)
A.变大 | B.变小 |
C.不变 | D.条件不足,无法判断 |
B
本题考查动能定理的应用、共点力平衡条件
未加磁场时,根据动能定理,有mgh-Wf=.加磁场后,多了洛伦兹力,洛伦兹力不做功,但正压力变大,摩擦力变大,根据动能定理,有mgh-Wf′=
,Wf′>Wf,所以v′<v.故B正确,A、C、D错误.故选B.
未加磁场时,根据动能定理,有mgh-Wf=.加磁场后,多了洛伦兹力,洛伦兹力不做功,但正压力变大,摩擦力变大,根据动能定理,有mgh-Wf′=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124024911643.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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