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The minimum applied force before an object slips off wall

  1. Apr 9, 2016 #1
    1. The problem statement, all variables and given/known data
    A horizontal force F is applied to hold a block, mass m = 1.7 kg against a vertical wall. The coefficients of kinetic and static friction between the block and wall are 0.81 and 0.87 respectively. F is gradually reduced until the block falls. Just before it falls, what is the magnitude of the total contact force Fc exerted by the block on the wall, and what is the angle θ it makes with horizontal? (+ve for above, -ve for below the horizontal).

    2. Relevant equations
    μsN=Ff
    F=mg

    3. The attempt at a solution

    So, if weight surpasses friction, it slips.

    μsN<W

    N<W/μs

    N=1.7*9.8/0.87=19N approximately

    I have no clue what it is referring to when it asks me to find the angle. The contact force seems to be horizontal so angle is 0?
     
  2. jcsd
  3. Apr 9, 2016 #2

    haruspex

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    The total contact force consists of the frictional force and the normal force.
     
  4. Apr 9, 2016 #3
    Is this directed at the 2nd or 1st part of the question? I assume the first part is correct then?

    If the contact force is vertical, then the angle it makes with the horizontal is 90 degrees?
     
  5. Apr 9, 2016 #4
    You need to find the angle of the net force. 2 Forces are considered contact forces: Friction and normal force here that is what he meant
     
  6. Apr 9, 2016 #5

    haruspex

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    Both parts. You have to think of the normal force and frictional force as adding up, vectorially, to a single contact force. The question is aksing for its magnitude snd direction.
     
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