Weight of 2 Blocks on an Incline

[Sean Smith]

Homework Statement

Block B rests on Block A. Block B is restrained with a string, while Block A slides down the incline. During the time Block A is between Block B and the inclined plane, Block A slides with constant velocity. If the weight of Blocks A and B are both w, and the coefficient of friction is the same between Block A and B and between Block A and the plane, find the value of the coefficient of friction u. [The angle of the incline is 45].

Homework Equations

The professor's solution has a free body diagram for Block A, with two frictional forces and a normal force. This I understand, but he draws weight straight down with magnitude w+w. I thought this was incorrect, as Block A only experiences a Normal force from Block B.

The Attempt at a Solution

The rest of the problem is inconsequential, but my proposed diagram for Block A has 2 frictional forces up the slope, Normal A normal to the slope, Normal B perpendicular to the surface between Blocks A and B, and weight of w.

 

[Stephen Tashi]

This I understand, but he draws weight straight down with magnitude w+w. I thought this was incorrect, as Block A only experiences a Normal force from Block B.

If block B wasn't there, wouldn't you still draw a force of w acting on block A ?

 

[Sean Smith]

If block B wasn't there, wouldn't you still draw a force of w acting on block A ?

My bad wording, I thought that Block A would have Weight w and Normal force from B. I drew this in the picture I attached to the original post if you'd like to take a look. (:

 

[Stephen Tashi]

I can't tell anything from that picture.

 

[haruspex]

The attached picture looks fine, but I presume this is your picture, not your professor's. Without an exact reproduction of what your professor said/drew it will be impossible to tell whether you simply misunderstood. Do you arrive at different answers?