A tradesman sharpens a knife by pushing it against the rim of a grindstone. The

Question

A tradesman sharpens a knife by pushing it against the rim of a grindstone. The 35cmdiameter stone is spinning at 200 rpm and has a mass of 20kg . The coefficient of kinetic friction between the knife and the stone is 0.20. If the stone loses 10% of its speed in 10 s

of grinding, what is the force with which the man presses the knife against the stone?

Somehow when I was trying this problem I got an answer of 1.3N. That wasnt the right answer, and I am very confused as to how you would solve this.

Explanation / Answer

W1= 200rpm = 20.93 rasd/s

W2= 200-0.1*200 = 180rpm = 18.84 rad/s

alpha = W2-W1/t = ?0.209 rad/s^2

Torque Q = F=mue*F*R= I*alpha

mue*F*R= 0.5*M*R^2*alpha

0.2*F*0.175 = 0.5*20*0.175^2*0.209

F = 1.82875 N

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