static friction and kinetic friction, solve for acceleration and force.by tebes Tags: acceleration, force, friction, kinetic coefficient, static 

#1
Oct2811, 05:22 PM

P: 39

1. The problem statement, all variables and given/known data
A 62 kg crate is dragged across a floor by pulling on a rope attached to the crate and inclined 16° above the horizontal. (a) If the coefficient of static friction is 0.57, what minimum force magnitude is required from the rope to start the crate moving? (b) If μk = 0.31, what is the magnitude of the initial acceleration (m/s^2) of the crate? 2. Relevant equations 3. The attempt at a solution Attempt of number (a) fs=Fn*Ms Let Fn + Tsin15=mg Fn = mg  Tsin15 Attempt of number (b) ma= Tcos15  fk (Do I need to subtract fs in this equation?) fs=magnitude of static friction. In this case, it's the maximum value of static friction. Fn=normal force T= tension of the rope fk=magnitude of kinetic friction. Are the equations of my attempts correct ? How do find tension ? or is there anyway to cancel it out ? 



#2
Oct2811, 07:23 PM

P: 39

Double checked my question. No mistake.




#3
Oct2811, 08:01 PM

P: 39

Apparently, I made some mistakes on the equations above.
On the question, it states the minimum force to move the crate. As I can imagine, at the very small instant, "the bond is broke". Therefore, for (a), the equation would look like this T cos 16  fs,max = 0 (the minimum force to " break the bond") solve for tension. (fs,max= maximum value of static friction) Then, substitute back into the equation of the (b), to solve for acceleration. If I make any mistake, pls correct me. Thank you. 



#4
Oct2811, 08:11 PM

Sci Advisor
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PF Gold
P: 5,959

static friction and kinetic friction, solve for acceleration and force. 



#5
Oct2811, 08:14 PM

P: 39

Thank you. One more question, if a block is at rest, then start to move, does the force need to overcome the static friction ? 



#6
Oct2811, 08:26 PM

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P: 5,959





#7
Oct2811, 08:31 PM

P: 39

Thank you. 


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