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Homework Help: Vertical Force Value Needed

  1. Jun 11, 2017 #1
    1. The problem statement, all variables and given/known data
    A block of mass 7 kg is pushed against a wall by force P. The coefficient of static friction between the block and the wall is 0.42. Determine an expression for the force P as a function of θ such that the block will not slide up the wall or fall down the wall.
    a.) Draw a free-body diagram for this problem
    b.) Determine the force value needed if θ = 13°
    c.) What is the minimum force required such that the block will remain stationary, and at what angle should this minimum force be applied?

    2. Relevant equations
    mg - f = Psinθ
    n = Pcosθ
    f = μscosθ

    3. The attempt at a solution
    I honestly wasn't sure where to start out since we haven't done any vertical force problems in class (I'm in a summer mini term, so my teacher tends to just rush through everything). So far, this is what I have, but I know that I am probably completely off.

    After doing my FBD, I decided that I could do part C to find the minimum force needed, but I just kept the angle at 13°, so I'm not sure I did that correctly, either.

    mg - μscosθ = Psinθ
    P(sinθ + μscosθ) = mg
    P = mg / sinθ + μscosθ
    P = 7(9.81) / sin13° + (.42)(cos13°)
    P = 108.28 N

    Thank you so much for helping! All of us in the class are just drowning right now.

    Attached Files:

  2. jcsd
  3. Jun 11, 2017 #2


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    Science Advisor
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    Gold Member

    Consequently you have found the answer to part b, at least on the assumption that the force direction is above the horizontal.
    Along the way, there are a couple of typos. You missed a P in the first equation and you should use parentheses to make the order of operations right.
    The very first part of the question is to find expressions in terms of a general theta for bounds on P such that the block will neither slide up nor slide down. How does that affect your general equation (before you plugged in 13 degrees)?

    Part c requires you to find the least P over all possible angles.
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