题目内容
一细绳一端固定在竖直放置的光滑圆环上的B点(B点在O点的正上方),另一端系一质量为m的小球于A点,小球穿过圆环,细绳与竖直方向的夹角为30°,如图所示,则细绳的拉力和环对小球的弹力为( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250022239872142.jpg)
A、
mg B、 mg
C、
mg/2 D mg/2 ![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002224080657.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250022239872142.jpg)
A、
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002224002528.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002224002528.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002224080657.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002224080657.png)
A
试题分析:假设圆环半径为R,由几何关系可知,AB对应的弦长为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002224127442.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250022241435570.jpg)
由受力平衡可知,重力的两个分量分别与细绳的拉力和环对小球的弹力等大反向,根据相似三角形可知,
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002224158567.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002224252903.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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