![](http://forum.eduzhixin.com/uploadfiles/20160823/14719530612ace7e0ddc324f941d9408346ec00132.JPG)
三力平衡。对单位长度的橡皮筋分析,对应的圆心角Δθ很小,Δθ/2=sin(Δθ/2)用。Δθ=1/r2
![](http://forum.eduzhixin.com/uploadfiles/20160825/147206209603ee3bf3f8c4a2cc7ef37be0af2c8b51.jpg)