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. http://img23.photobucket.com/albums/v68/AngelOfMusic/0cda86e7.jpg [Broken] 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? http://img23.photobucket.com/albums/v68/AngelOfMusic/190e01ee.jpg [Broken] http://img23.photobucket.com/albums/v68/AngelOfMusic/f174435d.jpg [Broken] 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)?