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Old Sep 26th 2009, 04:55 PM   #1
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trouble in physics, need hw help

I'm in a college credit physics class, and i'm struggling. i did fine all last year, but this year its ridiculous. we are working on dynamics right now, and i'm attempting to do the hw, but i'm stuck on the first problem. its asking me to find the coefficent of static friction for a coffee cup.

"a coffee cup on the dashboard of a car slides forward on the dash when the driver decelerates from 40km/h to rest in 3.5 s or less, but not if he decelerates in a longer time. what is the coefficient of static friction between the cup and the dash?"

i started with finding the deceleration of the car which is the same as the cup. i changed 40km/h to 11.1 m/s and used v=vknot + at to get -3.17m/s2 as the acceleration. my deal is i have no clue how where to go from here. i know all forces = ma, and that Ff = (coeffecient of static friction)(Fnormal) but i can't find Fn because i am given no masses. i'm really just stuck and have no clue where to go from where I am. Help would be greatly appreciated, and thanks already for your time for just reading this.
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Old Sep 26th 2009, 09:18 PM   #2
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Frictional force= net force
(Normal force)(Coefficient of static friction)=(m)(acceleration)

Considering the vertical equilibrium, Normal force=mg
therefore,

mg(coefficient of static friction)=(m)(acceleration)
g(coefficient of static friction)=acceleration

For problems you think that you are not given one of the variables, usually it can be cancelled out.
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