题目内容
如图(甲)所示,一对足够长平行光滑轨道放置在水平面上,两轨道间距l=0.5m,左侧接一阻值为R=1?的电阻;有一金属棒静止地放在轨道上,与两轨道垂直,金属棒及轨道的电阻皆可忽略不计,整个装置处于垂直轨道平面竖直向下的匀强磁场中。t=0时,用一外力F沿轨道方向拉金属棒,使棒以加速度a=0.2 m/s2做匀加速运动,外力F与时间t的关系如图(乙)所示。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082412315773911134.jpg)
(1)求金属棒的质量m
(2)求磁感强度B
(3)当力F达到某一值时,保持F不再变化,金属棒继续运动3秒钟,速度达到1.6m/s且不再变化,测得在这3秒内金属棒的位移s=4.7m,求这段时间内电阻R消耗的电能。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082412315773911134.jpg)
(1)求金属棒的质量m
(2)求磁感强度B
(3)当力F达到某一值时,保持F不再变化,金属棒继续运动3秒钟,速度达到1.6m/s且不再变化,测得在这3秒内金属棒的位移s=4.7m,求这段时间内电阻R消耗的电能。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241231577548470.jpg)
由牛顿第二定律得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157770752.gif)
(2)3分。 棒做匀加速运动,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241231577851681.gif)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157801620.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241231578171097.gif)
(3)9分。 F变为恒力后,金属棒做加速度逐渐减小的变加速运动,经过3秒钟,速度达到最大
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157832468.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157832468.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157863382.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241231578791030.gif)
将
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157895422.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157910383.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157910256.gif)
该时刻金属棒的速度为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123157926664.gif)
这段时间内电阻R消耗的电能:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241231579411772.gif)
略
![](http://thumb2018.1010pic.com/images/loading.gif)
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