Analyzing the Forces & direction of friction

[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

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

http://img23.photobucket.com/albums/v68/AngelOfMusic/f174435d.jpg

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)?

 

[Doc Al]

Originally posted by AngelofMusic
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?

Yes. The frictional force that A exerts on B is equal and opposite to the frictional force that B exerts 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)?

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.

 

[M98Ranger]

Thank you for your question! The direction of the frictional force between blocks A and B will depend on the direction of motion and the type of friction present. First, let's consider the direction of motion. If block B is sliding down on block A, then the frictional force will be in the direction opposite to the motion, so it will be pulling block B back towards block A. On the other hand, if block B is moving up on block A, then the frictional force will be in the direction of motion, helping to pull block B up.

Next, let's consider the type of friction present. There are two types of friction - static and kinetic. Static friction is present when the surfaces are not moving relative to each other, while kinetic friction is present when the surfaces are sliding against each other. In your diagram, the friction force is labelled as FfB, so I will assume it is kinetic friction. In this case, the direction of the frictional force will always be opposite to the direction of motion, regardless of the normal force or any other forces present.

In summary, the direction of the frictional force between blocks A and B will depend on the direction of motion and the type of friction present. It is important to carefully consider these factors when analyzing forces and their directions in a problem. I hope this helps clarify the concept of frictional forces for you!