题目内容
如图,倾角θ= 30o的斜面固定在水平地面上,物块A重力G = 20N,物块与斜面间的动摩擦因数μ= 0.4,物块放在斜面上。试求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250024242324185.png)
(1)当物块在斜面上静止时,物块受到的摩擦力f1的大小和方向;
(2)若用平行斜面向下的力F1拉物块沿斜面向下匀速运动,F1为多大?
(3)若用平行斜面向上的力F2拉物块沿斜面向上匀速运动,F2为多大?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250024242324185.png)
(1)当物块在斜面上静止时,物块受到的摩擦力f1的大小和方向;
(2)若用平行斜面向下的力F1拉物块沿斜面向下匀速运动,F1为多大?
(3)若用平行斜面向上的力F2拉物块沿斜面向上匀速运动,F2为多大?
(1)
,方向沿斜面向上;(2)2N;(3)22N
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002424263692.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250024242792641.png)
试题分析:物体受力分析如图所示,受到G、FN、Ff、重力沿斜面方向的分力与摩擦力大小相等,摩擦力大小为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002424263692.png)
(2)若用平行斜面向下的力F1拉物块沿斜面向下匀速运动,则此时摩擦力是滑动摩擦力,又弹力
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002424310415.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250024243251130.png)
此时,沿斜面方向的平衡力可表示为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002424357785.png)
得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002424372540.png)
(3)当沿斜面向上拉时,摩擦力方向沿斜面向下,则平衡力可表示为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002424388815.png)
得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002424435593.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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