A ladder is resting on a smooth ground and leaning against a roug

A ladder is resting on a smooth ground and leaning against a rough vertical wall. The force of friction will act

A.  towards the wall at its upper end

B. away from the wall at its upper end

C. downward at its upper end

D. upward at its upper end 

Please scroll down to see the correct answer and solution guide.

Explanation:

When a ladder rests on a smooth horizontal surface and leans against a rough vertical wall.
As seen from the diagram, the lower end of the ladder will start moving horizontally due to the gravitational pull of earth towards the ladder.
As the surface is smooth, there would be no frictional force on the smooth horizontal surface.
But as the ladder moves horizontally, the upper end of the ladder starts moving down and this motion of ladder down will be opposed by a frictional force in the upward direction as the wall is rough.