题目内容
如图所示,一根轻弹簧和一根细绳共同拉住一个重物,平衡时细绳恰处水平,此时弹簧的弹力大小为80N ,若烧断细绳,测出小球运动到悬点正下方时弹簧的长度正好等于未烧断细绳时弹簧的长度,试求:小球运动到悬点正下方时向心力的大小。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241729331711411.jpg)
40N
试题分析:设弹簧原长为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172933202332.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172933499280.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172933717297.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172934029893.png)
水平细线未烧断时弹簧的弹性势能为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172934216378.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172934700406.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172934965483.png)
由机械能守恒得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241729352151324.png)
故②带入③可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241729365251087.png)
故可得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172936790820.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172933717297.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172937321649.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172937570654.png)
所以有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172937804920.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172938085671.png)
在最低点有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824172938304907.png)
点评:本题比较难,是力学一道综合应用题,对学生的能力要求比较高,
![](http://thumb2018.1010pic.com/images/loading.gif)
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