1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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!

    Mass of Pie will cancel out
    and the solution is for vertical hand:
    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

    no i take the Fs and plug it back in right
    so now i do this:
    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.
    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...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook