题目内容
如图所示,轻杆AC、AD长均为2 m,轻杆AB长为3 m,A端由活络头铰在一起,B、C、D均支于水平地面上,AB^AC,AB^AD,ÐCAD = 60°,在A端铰链上挂一重为G的物体,则AB杆所受压力的大小为 ,AC杆所受压力的大小为 .![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108489751.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108489751.png)
G/2(得2分),G/2(得2分).
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/2014082412310850514686.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241231085361923.png)
根据题意,由于AB^AC,AB^AD,ÐCAD = 60°,过A点作DC的垂线,垂点为E,连结B点和E点,作图如图1所示。
①设ÐABE =α,因为物体悬挂于A点,可先把物体的重力G沿AB、AE方向进行力的分解得G1=Gsinα, G2 =Gcosα,而三角形ADC中AD=AC=2m,ÐCAD = 60°,可知三角形ADC是等边三角形,由于AE^DC,AB=3m,所以可得:AE=AC·cos60°=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108692337.png)
又因为AB^AC,AB^AD,AD、AC是三角形ADC的两边,所以可知AB垂直于三角形ADC,
而AE是三角形中的一条边,所以AB^AE,则有三角形ABE中的边BE=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108708680.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108692337.png)
sinα=AE/BE=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108786338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108801482.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108786338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108801482.png)
②设ÐCAE=θ,三角形AEC是直角三角形,E是DC的中点,ÐCAD = 60°,则
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108864538.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108879414.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108895424.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108911509.png)
然后分别过C、D两点作AC和AD的平行辅助线相交于点H,有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108926718.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241231089571322.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108911509.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108989396.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123108801482.png)
化简的
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123109347470.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824123109550515.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241231095971096.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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