题目内容
如图所示,质量为m的物体靠在粗糙的竖直墙上,物体与墙之间的动摩擦因数为μ.若要使物体沿着墙匀速运动,则与水平方向成α角的外力F的大小如何?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241136138572004.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241136138572004.jpg)
F2=
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613888600.gif)
当物体沿墙匀速下滑时,受力如图(a)所示,建立如图所示的坐标系,由平衡条件得F1sinα+F
=mg ①
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241136139204259.jpg)
=F1cosα ②
又有F
=μ
③
由①②③解得F1=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613998608.gif)
当物体匀速上滑时,受力如图(b)所示,建立如图所示的坐标系,由平衡条件得
F2sinα=F
+mg ④
=F2cosα ⑤
又有F
=μ
⑥
由④⑤⑥解得F2=
.
答案:
或![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613998608.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613904188.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241136139204259.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613951228.gif)
又有F
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613904188.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613951228.gif)
由①②③解得F1=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613998608.gif)
当物体匀速上滑时,受力如图(b)所示,建立如图所示的坐标系,由平衡条件得
F2sinα=F
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113614013193.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113614044235.gif)
又有F
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113614013193.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113614044235.gif)
由④⑤⑥解得F2=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613888600.gif)
答案:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613888600.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824113613998608.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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