the sol'n i'm posting is correct but i'm just having a hard time grasping it original question: block B with mass 5kg, rests on block A, with mass 8kg, which in turn is on a frictionless tabletop. the coefficient of static friction between block A & B is 0.750. what mass must block C have so that blocks A and B slide together when the system is at rest? my question: since A is accelerating to the right due to the string, IF there was friction, it would go opposite of acceleration (correct?). now if there was friction, it would go opposite, and since there is static friction at the top of block A or bottom of block B ... on my force diagram B, wouldn't it go opposite of block B's acceleration? or shouldn't acceleration be going the other direction?