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Magnitude of kinetic friction and static friction! 
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#1
Sep2911, 04:51 PM

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1. The problem statement, all variables and given/known data
A 12g coin slides upward on a surface that is inclined at an angle of 14degrees above the horizontal. The coefficient of kinetic friction between the coin and the surface is 0.25; the coefficient of static friction is 0.40. Find the magnitude of the force of friction when the coin is sliding. Find the magnitude of the force of friction when the coin comes to rest. 2. Relevant equations I found the magnitude of friction when the coin was sliding using the equation N=mgcostheta, and then plugging N into fs=muk*N. Would the same equation not tell me my magnitude of friction after the coin has stopped? 3. The attempt at a solution 


#2
Sep2911, 05:25 PM

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#3
Sep2911, 06:18 PM

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I meant to say the force of kinetic friction is muk*N. So with what you are essentially saying is that my force of static friction is mus*N, where N is equal to just m*g, and not m*gcostheta?



#4
Sep2911, 06:39 PM

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Magnitude of kinetic friction and static friction!
But when you ask 


#5
Sep2911, 06:47 PM

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I'm not quite sure I understand what you are saying. You do realize this is on an inclined plane right?
You are telling me in your example that the force N is equal to 50, but the force of static friction in not 25N. So is the equation of static friction not, fs=mu*N? 


#6
Sep2911, 07:02 PM

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#7
Sep2911, 07:19 PM

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It says if the net force of an object is = to 0, the velocity must be constant. I am starting to confuse myself. So according to this, fs=F, where F=ma. Can someone just tell me how to answer this.



#8
Sep2911, 07:36 PM

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To avoid more confusion, the force of static friction is
f_{s} is less than or equal to u_{s}N. it is equal to us*N only when relative motion between the 2 surfaces is just on the verge of taking pace. Otherwise, it is less than that value, and you must use newton's first law to solve for it. back to your problem on the incline, when the coin is at rest, there are 2 forces acting on it in the direction parallel to the incline. Identify those forces, use Newton 1, and solve for the static friction force (you seem to have done that correctly in an earlier problem you posted about a backpack at rest ,on a surface with friction, being pulled by a spring ). 


#9
Sep2911, 07:46 PM

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I hate to keep being a burden ... what I took out of that was that fs=mgsintheta?



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