题目内容
(10分)重为100N的物体在细线OA、OB共同作用下处于静止状态,细线OA、OB与水平天花板的夹角分别为30°、60°,如图所示,求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250009571042195.jpg)
(1)细线OA的拉力FOA、细线OB的拉力FOB
(2)若细线OA、OB所承受的最大拉力分别为200N、300N,为了使细线OA、OB不会被拉断,则物体的重力不能超过多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250009571042195.jpg)
(1)细线OA的拉力FOA、细线OB的拉力FOB
(2)若细线OA、OB所承受的最大拉力分别为200N、300N,为了使细线OA、OB不会被拉断,则物体的重力不能超过多少?
(1)FOA=50N,FOB=100
N;(2)200
N。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957120344.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957120344.png)
试题分析:(1)对重物受力分析,如图所示,由于重物静止,所以F=G=100N,
由几何关系:FOA=Fcos60°=50N,FOB=Fsin60°=100
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957120344.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250009571511717.jpg)
(2)解法一:假设OB细线拉力达到300N时,
由几何关系得:FOA=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957167714.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957120344.png)
当OB细线拉力达到300N时,物体最重;
所以,重物最大重量为G=F=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957198721.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957120344.png)
解法二:假设OA细线拉力达到200N时,由几何关系得:FOB=FOAtan60°=200
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957120344.png)
因为FOB>300N,所以当OA拉力达到200N时,OB细线早已断,
当OB拉力最大时,物体最重,所以重物最大重量为G=F=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957198721.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000957120344.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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