Sliding blocks and a string problem

[Anatalbo]

Homework Statement

Block A, with weight 3w, slides down an inclined plane S of slope angle 36.9 degrees at a constant speed while plank B, with weight w, rests on top of A and is attached by a cord to the wall. If the coefficient of kinetic friction is the same between A and B and between S and A, determine its value.

Homework Equations

The Attempt at a Solution

I have some trouble doing the free body diagram for A, I assume the forces acting on said block are: NB, wA, static friction between S and A, but I am not sure about the friction between A and B.

 

[haruspex]

the forces acting on said block are: NB, wA, static friction between S and A, but I am not sure about the friction between A and B.

You mention static friction between S and A, but not the normal force there.
Why do you doubt friction between A and B, yet have listed "NB", which I take to be the normal force between A and B?