题目内容
一个截面是直角三角形的木块放在水平地面上,在斜面上放一个光滑球,球的一侧靠在竖直墙上,木块处于静止,如图所示, 若在光滑球的最高点再施加一个竖直向下的力,木块仍处于静止,则木块对地面的压力FN和摩擦力Ff的变化情况是 ( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241510142402457.jpg)
A FN增大,Ff增大
B FN增大,Ff不变
C FN不变,Ff增大
D FN不变,Ff不变
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241510142402457.jpg)
A FN增大,Ff增大
B FN增大,Ff不变
C FN不变,Ff增大
D FN不变,Ff不变
A
试题分析:受力分析须严密,外部作用看整体,互相作用要隔离.找施力物体防“添力”,顺序分析防“漏力”;分力和合力避免重复,性质力、效果力避免重复。
以小球为研究对象,分析受力,作出力图如图1所示:小球受到重力
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014287445.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014396333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014521353.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014396333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014833411.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014396333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014957411.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151015082375.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151015145459.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151015285671.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014396333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151015082375.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824151014957411.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241510157843521.jpg)
故选A
点评:稍难。本题属于共点力平衡问题,采用隔离法和整体法交叉使用的方法研究,也可以只用隔离法处理,分析受力是基础.
![](http://thumb2018.1010pic.com/images/loading.gif)
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