Question

Consider the situation shown in figure The wall is smooth but the surface of A and B in contact are rough. The friction on B due to A in equilibrium-
(a) is upward
(b) is downward
(c) is zero
(d) the system can't remain in equilibrium

• A. only a
• B. only b
• C. only d
• D. c and d both

Answer 1

Answer: (D)

c and d both

As the given surface between wall and block A is smooth, so here no friction is acting, so blocks will fall down.
Therefore blocks can't be in equilibrium.

Friction force acts only when there is relative motion or tendency of relative motion between blocks. But here due to gravity both are going downward with same acceleration g. There is no tendency of relative motion as both of them accelerated by external force with same acceleration. So friction force between them is zero.