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Static friction probelm, help needed

  1. Apr 20, 2009 #1
    1. The problem statement, all variables and given/known data
    The coefficient of static friction between your hand and a pie is 0.70. if you want to put a cream pie pie in someone's face, what is the minimum acceleration needed to keep it from sliding down your vertical hand?


    3. The attempt at a solution
    Im not sure how to go about this.. but im prttty sure that is related to F=ma..
    Help would be greatly appreciated, thanks alot..
     
  2. jcsd
  3. Apr 20, 2009 #2
    draw a free body diagram of the PIE-mass, what forces acting downwards what forceas acting upwards? what force is the hand "pushing the" pie-what is the normal force?

    answer those questions, and see if it helps , answer them and I'll help You from there.
     
  4. Apr 20, 2009 #3
    ok.. i know the normal force is acting upwards, and weight is acting downwards, and there is also the force of the hand pushing on the pie.. but they dont give me a mass fro the pie.. so wouldnt that mean the normal froce and the wight cancel each other..
     
  5. Apr 20, 2009 #4
    1)Normal force= perpendicular to the surface(the hand) look at this in this way:
    hand pushes on Pie, Pie pushes same force on hand(third law of...)=Normal force direction=Left.
    2)upwards-friction. moment before it slides fs=Us * N
    N=normal force=(this is the key for the problem, try to figure out by your own).
    Write forces in the Y direction, and in equilibrium Fy=0

    Good Luck!

    P.S
    Mass of Pie will cancel out
    and the solution is for vertical hand:
    |=hand
    X=Pie
    _
    X|
    X|
    X|
     
    Last edited: Apr 20, 2009
  6. Apr 20, 2009 #5
    Don't forget to experimentally verify the result of the calculation.
     
  7. Apr 20, 2009 #6
    ok, i think im starting to figure it out... you have to take Us/g and that equals Fs
     
  8. Apr 20, 2009 #7
    "Don't forget to experimentally verify the result of the calculation."
    xD I would've preferred eating the pie :D
    You're close to the answer :}
    Write Your equations
    btw there was a mistake in my last post
    i wrote fs=us*m*n , its us*N

    I'm off now, good luck solving, other ppl can help you for now :
    good luck.
     
    Last edited: Apr 20, 2009
  9. Apr 20, 2009 #8
    Fs= Us * M*N
    Fs= Us *M/mg
    Fs=Us/g
    Fs=.0713

    no i take the Fs and plug it back in right
    so now i do this:
    Fs=Us*n
    Fs/Us=n
    .0713/.70=.101
    n=.101
    so that would mean since N=.101
    acceleration is also equal .101n
     
    Last edited: Apr 20, 2009
  10. Apr 20, 2009 #9
    Is anyone elsehere that can help me..
     
  11. Apr 20, 2009 #10
    check my last post.
    fs=us*N.
    N=m*a.
    What You've done is wrong Imo.
     
  12. Apr 20, 2009 #11
    ok if what ur saying is true, then that would mean Fs and Us are equal, and my acceleration would be 1.. but i think that wouldn't make sense... corret me if im wrong.
     
  13. Apr 20, 2009 #12
    I really wonna sleep xD
    fs=static friction=MG(in equilibrium state)=us*N
    how did yo get that they're equal (fs and us).

    Hope You see this:
    mg down, friction up which depends on the normal force, normal force acting on hand (and by hand on body).
    good luck.
     
  14. Apr 20, 2009 #13
    i still cant get it... anyone else can help me cause im stuck...
     
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