Problem involving Newton's second law

[Clever_name]

Homework Statement

see here for a situation of the problem http://i40.tinypic.com/6fbblk.jpg

Homework Equations

F=ma

The Attempt at a Solution

Ok well I've solved this problem but one thing about the solution process troubles me.
Namely when trying to find the force in the bar.

Say for instance i have the acceleration of the block B.
I don't understand why i can't do a force balance such that,

-Mb*g -Tsin(a) = Mb * Ab (1),

instead the solution is

-Tsin(a) = Mb*Ab (2)

I don't understand why the weight of the object is not accounted for in equation 2. Can someone please clear this up for me, thanks!

 

[consciousness]

The whole setup is stated to the horizontal! The gravitational force is balanced by the normal force.

 

[Clever_name]

thanks for the prompt reply, however i don't understand what you mean stated to the horizontal can you please elaborate for a slow poke like me.
to me it looks like B is hanging from the vertical.

 

[consciousness]

What you are seeing is the TOP VIEW. Think that the whole system is on a table with you looking at it from above it. The gravitational force acts toward the legs of the table but is balanced by the reaction force offered by the table. So we don't need to worry about these forces. And everyone gets confused sometimes. (never think low of yourself).

 

[Clever_name]

Ah i see now - its stated in the problem that it is to the horizontal! thanks for that.