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 Homework Statement
 If a pen accelerates down a book at 3.28 m/s/s, what is the coefficient of kinetic friction between the pen and book?
 Homework Equations

from earlier part: Normal force = 0.161 N
mass of pen = 18.7 g
perpendicular component of weight = 0.161 N
parallel component of weight = 0.0869 N
angle of book = 28.3 degrees
I tried using coefficient of friction = friction / Normal force, but needed a value for friction. I then tried to find the friction using a = f/m, but was unsure of which value to plug in for force. Simply finding the force given a and m will not yield the correct answer; the net force must be a difference of two forces.
Any suggestions?
Any suggestions?