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Conceptual Problem with Friction FBD.

  1. May 17, 2012 #1
    here is the scenario:

    there are 2 blocks A & B, both have the same mass 'm'.
    block A is kept on an endless surface.
    there is no friction between the block A and the surface.
    now the other block, B, is kept on top of the block A.
    the coeff of friction between A & B is say 'u'.
    the limiting friction between A & B is therefore = umg = 'f'.
    now say i apply a force 'F', equal to 'f', on the block A, i.e F = f.

    What will be the FBD's for A and B.

    I did the FBD for B & A and i am getting f = 0 = F , which is not possible since f = umg.
    So I think i have made some mistake.

    And I am not saying that A and B are necessarily stationary initially, they could be moving with a uniform velocity.

    Is this suggesting that since there is no relative motion between A & B therefore there is no friction existing. If this is the case then isnt this contradicting the fact that B moves with A
    (with a relative velocity = 0) due to friction when a force is applied to A and that if friction was not present A would just slide past underneath B and B would just fall to the ground.
    kindly help me out with this!

    Thanks.
     
    Last edited: May 17, 2012
  2. jcsd
  3. May 17, 2012 #2

    Doc Al

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    Staff: Mentor

    Realize that the limiting friction is not necessarily equal to the actual friction force.

    If you push A with a force equal to μmg, then the friction acting between the masses will be only half that.
     
  4. May 17, 2012 #3
    not sure i understand what you're saying.
     
  5. May 17, 2012 #4

    Doc Al

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    Staff: Mentor

    Try this: Assume that you push block A with a force equal to μmg. What will be the acceleration of the two-block system? Use that to figure out the friction force.
     
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