∑M(F)=0,-P2+(71-72)2=0 ∑M(F)=0,-P150+F2350=0 ∑F Py-FBy -FAy =0 ∑M,(F)=0,-P150+F2+(71+72)500=0 ∑F=0. T+72-P2=0 An=38.1N,FA=142N =286N 418N 例 已知:物重P0kN,CB=BB=DE,的=30y 求:杆受力及绳拉力 解:画受力图如图,列平衡方程 B B F F1sn45°-Fsn45=0 ∑F FAsn30°-Fcos45cs30°-F2cos45cos30°=0例 已知：物重 P=10kN，CE=EB=DE； 求：杆受力及绳拉力 解：画受力图如图，列平衡方程 28.6 N, 418N 38.1N, 142 N, 71N, = = − = = = By Bz Ay Az F F F F P 0, 0 ( ) 0, 150 ( ) 500 0 0, 0 ( ) 0, 150 350 0 0 2 ( ) 2 ( ) 0, 1 2 1 2 1 1 2 = + + + − = = −  + + +  = = − − = = −  +  = = −  + − =      z Az Bz z y z Bz y y By Ay z y By x y F F F T T P M F P F T T F P F F M F P F D T T D M F P 0  = 300  = 30 Fx = 0 1 sin 45 − 2 sin 45 = 0   F F Fy = 0 sin 30 − 1 cos45 cos30 − 2 cos45 cos30 = 0      FA F F