My first post here, so I apologize if this is duplicated elsewhere. It IS a "homework" problem, but it's public domain, a posted exam with answers...
Here is the diagram in question
And here is the FBD for each block. (question a is to find the tension in the rope and question b is to find the acceleration of the system)
I am getting confused when the situation has , for example, two blocks simply sliding over each other. In those cases, the force of friction on the top block, by Newton's laws, produces a "reactive" force in the opposite direction that makes the lower block move.
In this case, the force of friction OPPOSES the motion of both blocks. I would value some help with understanding this.