I heard that the moment of F about point a is (r-r(a))xF but i can't understand what r and r(a) are.And when there are 2 forces act though different point,how can I find the total moment. e.g: Four points:O(0,0,0) P(9,-3,5) Q(6,4,7) R(3,2,3.5). OPQ is a triangular plane and R is a point on this plane. Two forces, F(1)=i-2j+k and F(2)=3i+3j+2k act through the points P and Q respectively. If the triangle OPQ is fixed at the point R, what is the total moment of the two forces about it?? I do it like that:(OP-OR)xF(1)+(OQ-OR)xF(2) Is that right?