PHY AL MC08#1

The system involves numerous internal forces. So, the simpliest way to do is to take moment about the edge of the table of the whole system. Denote this point by A.

Let W be the weight of one block. R be the reaction force acting on P by the table, which acts in a distance l from A.

The two blocks are identical and uniform, so their centre of gravity lies on the geometrical centre. That is 12 cm from the edge of one block.

Then take moment about A for the whole system.

Clockwise moment = Anti-clockwise moment

(x - 8)W + Rl = (12 - 4)W

R >= 0, l >= 0

So, Rl >= 0

Therefore, 8W >= (x - 8)W

x <= 16 cm

So, the answer is D.


Note that the two blocks are adhered to each other. So, we only have to focus about the point A. If they are not adhered, we also need to consider whether Q will topple about the end point of P.