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These two systems are equivalent as far as the net gravitational pull is concerned, but the moment of the gravitational forces of the two systems about point A is different.
L11 : Center of mass and Center of Gravity

Consider a uniform rod of length L. When the rod is placed horizontally, each particle of the rod is pulled by the gravitional force of the earth. The sum of all three forces will be equal to the total weight of the rod.

Consider two systems: One is the rod AB of weight W and other is a concentrated weight W, at a distance 1/3 L from point A.