⊙空间力偶系的平衡 ∑M,=0 i=1 2 M=0,∑M,=0,∑M,:=0 i=1 i=1 i=1空间力偶系的平衡 1 1 1 0, 0, 0 n n n i x i y i z i i i M M M = = =    = = = 0 1  = = n i Mi 