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Eject "NO NAME" before disconnecting or turning it off MasteringPhysics: CH Secure https session. asteringphysics com yct temview?offset next&assignmentProblemlD; 7.71 rest) « previous | 23 of 23 I return to assignment Problem 7.71 Part A n sharpens a knife by pushing it against rim of a grindstone. The 40 cm diameter stone is spinning at 200 rpm and has a mass of 26 kg. 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? The and the stone is 0.20 coefficient of kinetic friction between the knife Express your answer to two significant figures and include the appropriate units. HA Value Units Submit My Answers Give Up Continue

Explanation / Answer

initial angular speed =w = 200 rpm

1rpm = 1rpm * 2pi rad/1rev * 1/60s

200 rpm = 20.9 rad/s

after 10 s angular velocity loses 10% of original = wf =0.9w

wf = 18.8 rad/s  

angular deceleration = dw/t = (18.8-20.9)/10 = 0.21 rad/s^2

now we know moment of inertia = MR^2/2

M = 26 kg

R = 40/2 = 20 cm = 0.2 m

I = 0.52 kg-m^2

Torque = I*alpha  

t = 0.52 * 0.21 = 0.1092 N-m

torque = F*r

here friction force is working  

0.1092 = mu*F*r

F = 0.1092/(0.20 * 0.20)

F = 2.73 N = 2.7 N

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