题目内容
两个大人和一个小孩一起拉船前进,两个大人的作用力如图所示,为了使船在河中间行驶,小孩施加的最小力为( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241508093152726.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241508093152726.jpg)
A.30N | B.40N | C.50N | D.70N |
D
试题分析:若保证船在河中行驶,沿垂直河岸方向的各个力的分量的矢量和为零即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241508094241022.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150809549761.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150809627450.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824150809705661.png)
故选D
点评:中等难度。先对物体进行受力分析,然后根据平衡条件进行分析,明确题目中的物理量是在什么条件下取极值,或在出现极值时有何物理特征,根据这些条件或特征去寻找极值.
![](http://thumb2018.1010pic.com/images/loading.gif)
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