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Static friction probelm, help needed

  • Thread starter Jericho_15
  • Start date
  • #1

Homework Statement


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?


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..
 

Answers and Replies

  • #2
174
0
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.
 
  • #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..
 
  • #4
174
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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:
  • #5
1,838
7
Don't forget to experimentally verify the result of the calculation.
 
  • #6
ok, i think im starting to figure it out... you have to take Us/g and that equals Fs
 
  • #7
174
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"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:
  • #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:
  • #9
Is anyone elsehere that can help me..
 
  • #10
174
0
check my last post.
fs=us*N.
N=m*a.
What You've done is wrong Imo.
 
  • #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.
 
  • #12
174
0
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.
 
  • #13
i still cant get it... anyone else can help me cause im stuck...
 

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