A firm wants to determine the amount of frictional torque in their current line

Question

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 1.8 kg and radius 9.90 cm which operates at 730 rev/min. When the power is shut off, you time the grindstone and find it takes 29.1 s for it to stop rotating.

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 1.8 kg and radius 9.90 cm which operates at 730 rev/min. When the power is shut off, you time the grindstone and find it takes 29.1 s for it to stop rotating. What is the angular acceleration of the grindstone? (Assume constant angular acceleration.) What is the frictional torque exerted on the grindstone?

Explanation / Answer

a )

angular acceleration: ? = ?/t

w=730 rev/min = 730*2*3.14/60 =76.4 rad/sec
acceleration=change of speed / time
Angular acceleration =76.4 rad/sec )/29.1 = 2.62rad/s^2

b)

moment of inertia for a disk: I = (1/2)mr

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