题目内容
下图中弹簧秤、绳和滑轮的重力均不计,绳与滑轮间的摩擦力不计,物体的重力都是G,在图甲、乙、丙三种情况下,弹簧秤的读数分别是F1、F2、F3则( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514251514459.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514251514459.jpg)
A.F3>F1=F2 | B.F3=F1>F2 |
C.F1=F2=F3 | D.F1>F2=F3 |
B
试题分析:弹簧称的读数等于弹簧受到的拉力.甲图、乙图分别以物体为研究对象由平衡条件求解.丙图以动滑轮为研究对象分析受力情况,根据平衡条件求解.
对于甲图,物体静止,弹簧的拉力
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151425307538.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514254162658.jpg)
由平衡条件得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514254311158.png)
丙图:以动滑轮为研究对象,受力如下图.由几何知识得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151425572561.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151425650553.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241514258992297.jpg)
故选B
点评:本题中分析物体的受力情况是关键,作出受力分析图,互成120º等大的两个力的合力与分力大小相等.
![](http://thumb2018.1010pic.com/images/loading.gif)
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