Hello! I'm working on a problem right now, and I'm a bit stuck on the direction of the frictional force. Basically, there are two blocks A and B, with B sitting on A and they're connected by pulleys. Diagram of Setup I think I've got most of the forces covered, but I'm just wondering - in what direction does the friction between A and B exert a force? FBD of Block A (on the bottom) FBD of Block B The force in question is labelled FfB. Should it be in the opposite direction of the friction force on B when it's drawn on A? Do I need to consider it, even though I've considered the normal force of B on A, which changed the value of NA (normal force on A)?