题目内容
如图所示,在竖直平面内有一个半径为R的圆弧轨道。半径OA水平、OB竖直,一个质量为m的小球自A正上方P点由静止开始自由下落,小球沿轨道到达最高点B时恰好对轨道没有压力,已知PA=2R,重力加速度为g,则小球 ( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250000439437674.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250000439437674.png)
A.从B点飞出后恰能落到A点 |
B.从P到B的运动过程中机械能守恒 |
C.从P到B的运动过程中合外力做功![]() |
D.从P到B的运动过程中克服摩擦力做功mgR |
C
试题分析:A、小球能通过C点,在C点最小速度v满足:mg=m
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043974453.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000044021434.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000044036547.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000044052314.png)
B、小球沿轨道到达最高点B时恰好对轨道没有压力,根据牛顿第二定律,有mg=m
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000044052512.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000044021434.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043958338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000044099402.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043958338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043958338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043958338.png)
C、从P到B过程,合外力做功等于动能增加量,故为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043958338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000044099402.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043958338.png)
D、从P到B过程,克服摩擦力做功等于机械能减小量,故为mgR-
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043958338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000043958338.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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