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Homework Help: Finding the force to break static friction

  1. Apr 19, 2015 #1
    1. The problem statement, all variables and given/known data
    Samantha and Rebekah each push on a stack of doughnuts equally. If the coefficient of static friction is .28 and the box has a mass of 18kg, what force does each push individually to break static friction? Once it's moving, the coefficient of kinetic friction is .17, how much will the doughnuts be accelerated onto the floor?

    mass of object = 18kg
    coefficient of friction (static) = .28
    coefficient of friction (kinetic) = .17

    2. Relevant equations

    The only one I know that is obvious to use is f = ma, not sure how to apply it to this context

    3. The attempt at a solution

    On the x and y plane, I know the force diagram reads normal force traveling up, force of gravity traveling down, Applied force traveling right, and force of friction traveling left.
    Last edited: Apr 19, 2015
  2. jcsd
  3. Apr 19, 2015 #2
    Do you know the formula for static and kinetic friction?
  4. Apr 19, 2015 #3
    I was never taught the formulas for each type of friction no
  5. Apr 19, 2015 #4
  6. Apr 19, 2015 #5
    So after trying this is how I attempt it
    (I assume gravity as 10m/s/s)

    I know normal force = mg so fn = (18)(10) fn = 180N
    f = (.28)x(180)
    f = 50.4N
    then I assume you divide the answer in two as it asks for the individual force, so
    f = 25.2N

    to find the acceleration you just use f = ma substituting your found force
    25.2 = (18)a
    a = 1.4m/s/s

    am I correct?
  7. Apr 19, 2015 #6
    Not quite. Is the box sliding or not?
  8. Apr 19, 2015 #7
    they both push on the stack of doughnuts, so I believe it is sliding. Really though you are just trying to find the force that was needed to break the static force, or when it wasn't in motion. Then the second part of the question just wanted the acceleration of their push
  9. Apr 19, 2015 #8
    Yes, so when you found the first answer did you assume the box was sliding or not?
  10. Apr 19, 2015 #9
    yes I did assume the box was sliding as there was force being emitted on it
  11. Apr 19, 2015 #10
    Well, you are contradicting yourself because you also said there was no motion. Sliding= motion.
  12. Apr 19, 2015 #11


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    Science Advisor

    That looks correct.

    A couple problems with this part. You've ignore kinetic friction force and only used one of the applied forces.
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