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Analyzing the Forces & direction of friction

  1. Feb 17, 2004 #1
    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)?
     
  2. jcsd
  3. Feb 17, 2004 #2

    Doc Al

    User Avatar

    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.
     
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