# Analyzing the Forces & direction of friction

1. Feb 17, 2004

### AngelofMusic

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)?

Last edited by a moderator: May 1, 2017
2. Feb 17, 2004

### Staff: Mentor

Yes. The frictional force that A exerts on B is equal and opposite to the frictional force that B exerts on A.
Of course. If you are treating A and B as separate bodies (which you are) you must consider all the forces on them.

The friction acts to oppose slipping between the blocks. If A moves down the plane, then B moves up. F(A on B) acts down, opposing B's sliding; F(B on A) acts up, opposing A's sliding.