phy moment小題一問

Consider the following hints for max value of x,
(i) the centre of mass of the upper box is at the edge of the lower box;
(ii) the centre of mass of the two-box system is at the edge of the table.

By taking moment about the edge of the table,
clockwise moment due to the upper box
= (x - L/2)w, where w is the weight of the box
counterclockwise moment due to the lower box
= [L/2 - (x - L/2)]w
= (L - x)w

Hence, (x - L/2)w = (L - x)w
x - L/2 = L - x
2x = 3L/2
x = 3L/4