Question

Consider the situation shown in the figure below. The wall is smooth but the surfaces of A and B in contact are rough. Can the system remain in equilibrium?

Answer 1

Let's suppose the system is in equilibrium. The weight of block A will be balanced by friction acting on it in vertically upward direction. This, in turn, implies that friction will act in the vertically downward direction on block B. As both weight and friction are acting vertically downwards, B cannot be in equilibrium. This is a contradiction. Hence, the system cannot remain in equilibrium.