A system of two bodies and wall

[Steve4Physics]

There have to be relative motion between A and B for the friction to occur here.

No. That's only true for kinetic friction. I will add to @haruspex's comment...

Suppose a small parachute is attached to block B. Since A and B are pressed together, there could be enough static friction to prevent them sliding relative to each other. A and B would then fall together (in the same way as if they were stuck together).

Of course If you use a sufficiently big parachute and A is very heavy, then A will fall faster than B. Sliding occurs between A and B and the friction is kinetic.

 

[rudransh verma]

No. That's only true for kinetic friction. I will add to @haruspex's comment...

O yeah! But there is also no static friction here because of the absence of any upward force. Both will fall as one system.

 

[Steve4Physics]

O yeah! But there is also no static friction here because of the absence of any upward force. Both will fall as one system.

Yes indeed! If you read through the previous posts, you will see that this is the message we have been trying to convey!