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Determining the direction of friction

  1. Nov 30, 2015 #1
    1. The problem statement, all variables and given/known data
    Given the following diagram:
    When examining the FBD for member AB how does one determine the direction of friction at B? If surfaces B and C exhibit friction.

    2. Relevant equations

    3. The attempt at a solution
    I know that impending motion for post BC is in the direction of P so friction acts opposite but I am unsure as to how one determines the impending motion for Member AB.
    The answer provided is that the friction for member AB at surface B acts horizontally in the same direction as P.
  2. jcsd
  3. Nov 30, 2015 #2


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    The motion of member BC produced by the force P will create a friction force at B which opposes the motion of BC. That's the force which is shown on the FBD for member AB at point B.
  4. Nov 30, 2015 #3
    Would the force at of friction on member BC not be in this direction <----- so why is Member AB's force of friction in this direction ------> ?
  5. Nov 30, 2015 #4


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    As long as member AB is not moving, the net force between AB and BC must equal zero. When you draw a FBD of AB, a force opposite the friction force on BC must be included to bring the net force on this member to zero.
  6. Nov 30, 2015 #5
    Alright thank you sir!
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