Question

Suppose all the surface in the previous problem are rough. The direction of friction of B due to A
(a) is upward
(b) is downward
(c) is zero
(d) depends on the masses of A and B.

Answer 1

(a) is upward


The normal reaction force on the system (comprising of wall and contact surface of A and B) is provided by F. As can be seen from the figure, the weight of A and B is in the downward direction. Therefore, the frictional force f_A and f_BA (friction on B due to A) is in upward direction.