A firm wants to determine the amount of frictional torque in their current line of grindstones, so they can redesign them to be more energy efficient. To do this, they ask you to test the best-selling model, which is basically a diskshaped grindstone of mass 1.6 kg and radius 8.90 cm which operates at 740 rev/min. When the power is shut off, you time the grindstone and find it takes 27.5 s for it to stop rotating.

Question

anonymous · Answer

What is the frictional torque exerted on the grindstone?

anonymous · Answer

What I did at first is torque=m*g*R, but I got the wrong answer. Can someone please help me explaining why my way is wrong

anonymous · Answer

average Torque = change in angular momentum / time
time = 27.5 seconds
change in angular momentum = 0 - I omega 
I = moment of inertia of solid  disk of mass 1.6 kg and radius .089 meters
= (1/2) m r^2
omega = 740 rev/min (1 min/60s)(2 pi radians/rev)