题目内容
物块A、B的重量分别50N和200N,用细绳连接如图所示,
,不计滑轮与绳间摩擦,物块B与地面间的动摩擦因数
,若整个装置静止。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153026877830.png)
求:
(1)绳子的拉力;
(2)画出物块B的受力示意图;
(3)地面对B的支持力和摩擦力的大小。(
)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153025769452.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153025909512.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153026877830.png)
求:
(1)绳子的拉力;
(2)画出物块B的受力示意图;
(3)地面对B的支持力和摩擦力的大小。(
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153026986792.png)
(1)50N
(2)物块B受到四个力的作用,如图所示:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241530270642063.jpg)
(3)160N、30N
(2)物块B受到四个力的作用,如图所示:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241530270642063.jpg)
(3)160N、30N
试题分析:(1)以A为研究对象,受力分析可知,A受重力和绳子的拉力,由平衡条件可知
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153027391735.png)
(2)绳对B的拉力不为零,而B又处于静止状态,根据共点力的平衡条件,水平面对B一定有向左的静摩擦力.由于摩擦力的存在,水平面对B一定有支持力.综上所述,B物体一定会受到四个力的作用(重力、绳子的拉力、水平面的支持力和摩擦力),则物块B受到四个力的作用,如图所示:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241530274852065.jpg)
(3)对B受力分析:水平方向:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153027891707.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241530279692216.png)
竖直方向:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153028499852.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153028561647.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824153028655578.png)
点评:此题关键是绳子拉力不为零,B静止,则一定受摩擦力,有摩擦力一定有弹力。
![](http://thumb2018.1010pic.com/images/loading.gif)
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