题目内容
如右图所示,
=
粗糙斜面的倾角为30°轻绳通过两个滑轮与A相连,轻绳的另一端固定于天花板上,不计轻绳与滑轮的摩擦.物块A的质量为m不计滑轮的质量,挂上物块B后,当滑轮两边轻绳的夹角为90°时,A、B恰能保持静止且A所受摩擦力向下,则物块B的质量为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241320393902621.png)
A.
B.
C.m D.2m
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132039296306.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132039343464.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241320393902621.png)
A.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132039546510.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132039577584.png)
C.m D.2m
B
因为A恰好能静止在斜面上,所以所受摩擦力为
,A物体在沿斜面方向上
,所以
,定滑轮两边的绳子的拉力是相等的,B保持静止,所以
,故
,B正确。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241320396241047.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132039671831.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132039717658.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132039764771.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824132039827728.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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