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Homework Help: Newton's Three Laws (moving a box)

  1. Sep 25, 2008 #1
    1. The problem statement, all variables and given/known data
    Block A weighs 4 N and block B weighs 8N. The coefficient of sliding friction between all surfaces is 0.25. Find the force P necessary to drag block B to the left at a constant speed if
    A) if A rests on B and moves with it
    B) if A is held at rest
    C) if A and B are connected by a light flexible cord passing around a fixed frictionless pulley.


    3. The attempt at a solution
    I have drawn the free body diagrams.

    A) Ff=coefficient x Fn
    = .25 (8 + 4)
    = 3
    Is that right?

    B) I know the fsB = (8 + 4) coefficient of static friction
    fsA = coefficient of static friction (4)

    I also know that these have to be equal in order to move.
    So...
    fsA = fsB
    But where do I put the P? On the A side or the B side?

    C) I know that P must be equal to all the forces in the x-direction in order for the block to move.

    The sum of the forces in the x-direction = T + fsA + fsB

    so...
    P = T + fsB + fsA
    and
    P = T + (.25)(4) + (.25)(12)
    P = T + 4

    How do I find T?
    I know that the sum of all the forces = ma
    And the acceleration in this case is 0.

    But 0 = T + 4 would give T= -4.
    And P =0.

    I don't think that can be right.
     
  2. jcsd
  3. Sep 26, 2008 #2

    Chi Meson

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    Science Advisor
    Homework Helper

    A) correct

    b) I think that block A is supposed to be held in place as B is dragged out from under it.
    In this case there will be two forces of friction opposing force P

    C) are the two weights hanging on either side of the pulley? is one on a surface while the other is hanging? I can't "see" what this one is supposed to look like.
     
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