9.4 Static Equilibrium
Center of Gravity

 

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Explanation:
As explained in Section 9.4, individual blocks or sets of blocks will behave as if their mass were concentrated at their center of mass. A block is in stable equilibrium when the torque due to its weight is zero. This is accomplished when its center of gravity (here equivalent to center of mass) is above the supporting surface. This is true for a single block and for several blocks.
 
Source:
Fu-Kwun Hwang