Minimum Acceleration for Sliding Cream Pie: Solving Static Friction Problem

[Jericho_15]

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 I am prttty sure that is related to F=ma..
Help would be greatly appreciated, thanks a lot..

 

[Dweirdo]

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.

 

[Jericho_15]

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 don't give me a mass fro the pie.. so wouldn't that mean the normal froce and the wight cancel each other..

 

[Dweirdo]

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|

 

[Count Iblis]

Don't forget to experimentally verify the result of the calculation.

 

[Jericho_15]

ok, i think I am starting to figure it out... you have to take Us/g and that equals Fs

 

[Dweirdo]

"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.

 

[Jericho_15]

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

 

[Jericho_15]

Is anyone elsehere that can help me..

 

[Dweirdo]

check my last post.
fs=us*N.
N=m*a.
What You've done is wrong Imo.

 

[Jericho_15]

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 I am wrong.

 

[Dweirdo]

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.

 

[Jericho_15]

i still can't get it... anyone else can help me cause I am stuck...