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Homework Help: Force of block pushed against wall

  1. Oct 8, 2012 #1
    1. The problem statement, all variables and given/known data
    A block of mass 20 kg is pushed against a vertical surface (see picture). The coefficient of friction, μ, between the surface and the block is 0.2. If θ = 30°, what is the minimum magnitude of P to hold the block still?

    2. Relevant equations

    f = force of friction = μN

    3. The attempt at a solution
    I drew a force body diagram and got equations for x and y.
    x: N - Psinθ = 0
    Y: -pcosθ + mg + f = may

    N = psinθ
    so f = μ psinθ
    I put f = μ psinθ into the y equation

    so i get
    -pcosθ + mg + μpsinθ = may

    I'm stuck here. I want to find P but I don't know the acceleration.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 8, 2012 #2


    User Avatar
    Gold Member

    The problem states that the block is still, which would mean that ay = ?
  4. Oct 9, 2012 #3

    Simon Bridge

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    Science Advisor
    Homework Helper

    The question asks:
    Therefore the magnitude of the velocity is.....?
    The change in velocity is ....?
    The acceleration is ....?

    Why "minimum"? What kind of friction is this?
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