题目内容
如下图所示,把一重为G的物体,用一水平方向的推力F=kt(k为恒量,t为时间)压在竖直的足够高的平整墙上,从t=0开始物体所受的摩擦力f随t的变化关系是下图中的( ).![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154421213999.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241544212294014.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154421213999.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241544212294014.jpg)
B
试题分析:当刚开始时,根据F=kt可知推力很小,最大静摩擦力很小,重力大于最大静摩擦力,物体开始下滑,根据
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154421275597.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154421385595.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154421494579.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154421853375.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154421977494.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154422024522.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824154421977494.png)
故选B
点评:本题难点是当摩擦力等于重力时为什么摩擦力还会增大,因为此时物体有速度,物体还受滑动摩擦力,滑动摩擦力随压力的增大而增大。
![](http://thumb2018.1010pic.com/images/loading.gif)
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