题目内容
如图所示,将一根不可伸长柔软的轻绳左右两端分别系于AB两点上,一物体用动滑轮悬挂在轻绳上,达到平衡时两段绳子间的夹角为θ1,此时绳子张力为F1,将绳的右端移到C点,待系统达到平衡时,两段绳子间的夹角为θ2,此时绳子张力为F2,再将绳子的右端再由C点移到D点,待系统达到平衡时,两段绳子间的夹角为θ3,此时绳子张力为F3,不计一切摩擦,且BC为竖直线,则( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241444378552934.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241444378552934.png)
A.θ1=θ2<θ3 | B.θ1=θ2=θ3 |
C.F1>F2>F3 | D.F1=F2<F3 |
AD
本题考查受力平衡、力的合成与分解,滑轮和绳上都无摩擦,所以两边绳子中的拉力相等,故两绳与竖直方向的夹角也相等,如图所示,设动滑轮和物体的总质量为m,A、B或A、C两点的水平距离为d, A、D两点的水平距离为d,线总长为
。当绳系B点时,将BO延长至与竖直壁的延长线交于C点,由几何知识可知OA=OC,则绳与竖直方向的夹角为
,则![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438183847.png)
同理,当绳系C 点时,绳与竖直方向的夹角
,即
,![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438370444.png)
当绳系D点时,绳与竖直方向的夹角为
,因
,则
,即
。由
得F1=F2<F3,所以选A D。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241444388851801.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241444389011372.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241444389631244.png)
点评:对于动态变化的平衡问题,找到几个特殊问题比较也是一个方法,本题中两倾斜绳子的拉力大小相等,方向对称是解决本题的关键
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144437918299.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438043962.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438183847.png)
同理,当绳系C 点时,绳与竖直方向的夹角
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438261450.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438323443.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438370444.png)
当绳系D点时,绳与竖直方向的夹角为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438433980.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438479421.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438542572.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438589561.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824144438823881.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241444388851801.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241444389011372.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241444389631244.png)
点评:对于动态变化的平衡问题,找到几个特殊问题比较也是一个方法,本题中两倾斜绳子的拉力大小相等,方向对称是解决本题的关键
![](http://thumb2018.1010pic.com/images/loading.gif)
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