1. The problem statement, all variables and given/known data see here for a situation of the problem http://i40.tinypic.com/6fbblk.jpg 2. Relevant equations F=ma 3. 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!