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Homework Help: Block held against wall, force problem

  1. Oct 3, 2008 #1
    1. The problem statement, all variables and given/known data

    A 12 N horizontal force F pushes a block weighing 5.0 N against a vertical wall. The coefficient of static friction between the wall and the block is 0.60, and the coefficient of kinetic friction is 0.40. Assume that the block is not moving initially. (a) will the block move? (b) in unit-vector notation, what is the force on the block from the wall?

    2. Relevant equations

    f = (mu) * N

    3. The attempt at a solution

    For (a), I just calculated fk, max and since it is greater than the weight of the block, the block will not move.

    I'm confused about part (b). The answer at the back of the book is (-12i + 5j) newtons. I understand the 12i part. That's just the normal force of the wall on the block. But where does the 5j come from? I thought the 5 newtons upwards were due to the friction and not the wall exerting a force on the block?
  2. jcsd
  3. Oct 3, 2008 #2

    If this is so, then if the wall were removed then would the 5 newtons still be present?
    Yes the 5 newtons is due to the friction, however the friction is due to the normal force.
    So your -12i and your 5j are both caused by the same entity essentially.
  4. Oct 3, 2008 #3
    Oh, duh! The friction is also a force from the wall. Thanks for your help. =)
  5. Oct 25, 2008 #4
    fk, max = uk*N = .4*12 = 4.8 N which is less than the weight of the block, no?
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