Here you should draw individual FBD's of blocks A and B. Since the link is massless, it is a two-force member (this tells you the line of action of the force of the link on the two blocks).
The kinematics is a significant part of the effort on this problem. You need to use kinematics to relate the accelerations of A and B. There are a couple ways to do this:
- You could use the rigid body kinematics equations to relate the accelerations of blocks A and B. This process is a little lengthy, but doable.
- The simpler approach is to write down the constraint equation relating the positions of A and B: xA^2 + yB^2 = L^2 , where L is the length of the link. Differentiate this expression once (using the chain rule of differentiation) to find the velocity of B. Differentiate again to find the relationship between the accelerations of A and B.
Regardless of the kinematics approach that you take, you will use this relationship between aA and aB in your Newton's equations. Altogether, you will have three equations and three unknowns (aA, aB and the force in the link).
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