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Frictional forces question

  1. Jun 18, 2017 #1
    having major issue with this question.

    1. The problem statement, all variables and given/known data
    a wooden crate of mass m= 60 kg sits on the horizontal surface. the coefficient of static friction between the block and the surface is u=0.5 . an external force P = 220 N, is also applied to the block in a direction parallel to the surface. Also an external force F is applied to the block in a direction perpendicular downwards to the surface.
    A) if the force F = 40 N, what is the magnitude and direction of the frictional force that the surface exerts on the block?
    B) what is the smallest force F, necessary to hold the block stationary on the surface?
    upload_2017-6-19_9-35-5.png

    variables
    m = 60kg
    P = 220N
    U = 0.5
    2. Relevant equations
    equations
    Fs<= U*Fn
    F = ma
    Fn = mg

    3. The attempt at a solution
    not sure on where to start for part a in this question. but was thinking for part b you let Ff = P so that
    Ff<= U*Fn+f
    Ff<= U*mg + f
    220<= U*mg + f
    and then rearrange to solve for f
    am i on the right track for part b at least???
     
    Last edited by a moderator: Jun 18, 2017
  2. jcsd
  3. Jun 19, 2017 #2

    haruspex

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    Not always, and not in this case.
    Is f the same as the given F?
    What you have written means Ff<=(U*mg)+f. Is that what you intended?
     
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