點計centre of mass?

Centre of mass means that it is the point that all the masses of a rigid body can be construed as concentrated at that point.
Suppose, for simplicity, an object consists of three individual masses m1, m2 and m3 respectively at distances d1, d2 and d3 from an arbituary point A.
The moment by these three masses about point A is thus,
Moment = (m1)(d1)+(m2)(d2)+(m3)(d3)
From definition of centre of mass, we can imagine that if all masses are concentrated at the position where the centre of mass is, the moment should not be changed, hence
(m1)(d1)+(m2)(d2)+(m3)(d3) = (M)(D) = moment about point A
where D is the distance of the centre of mass from point A and M = m1+m2+m3, is the mass of the object.
This gives, D = [(m1)(d1)+(m2)(d2)+(m3)(d3)]/M