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Homework Help: Finding the possible values of a force pushing a block

  1. Sep 27, 2012 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    I believe the only necessary equations would be those utilized to find the X and Y components of a certain force as well as the formula used to find μs

    µ = ffriction/fnormal

    3. The attempt at a solution


    I'm not really sure where to start. If someone could just give me a push in the right direction, it'd be much appreciated. :redface:
  2. jcsd
  3. Sep 27, 2012 #2


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    In all cases, the Normal Reaction Force [to be used in the Friction = μN] is the horizontal component of the applied Force, which they called P.

    Also in all cases, the vertical forces acting on the block are: mg, down, Pvert, up plus friction.

    If you push with too small a force, the block will slip down, so friction will act up. When the force becomes big enough to stop the slip, Pvert, up + friction, up will equal mg, down.

    If you push too hard, the block will slip up, so friction will act down. At the point of slipping

    Pvert, up will equal mg, down plus friction, down

    Note: Pvert, up means the vertical component of the applied force P
  4. Sep 27, 2012 #3

    Simon Bridge

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    The applied force P presses into the surface as well as along the surface.
    Since the block has no acceleration into (or away from) the surface, there must be another force pointing away from the surface to oppose this.

    Some people call this the "normal" force, and some the "reaction" force, at the surface.
    PeterO has a slightly different picture - where the applied force is divided into normal and parallel components - which works just as well for this situation.

    The starting point is to draw a free body diagram.
    It will have force arrows for weight (W), the applied force (P), and the normal/reaction (N) force and static friction (f).
    [I like to give friction a lower-case "f", and I don't like subscripts.]
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