Physics help needed?

Question

A 750 gram grinding wheel 25 cm in diameter is in the shape of a uniform solid disk. When it is in use, it turns constant 220 rpm about an axle perpendicular to its face through its center. When the power switch is turned off, you observe that the wheel stops in 45 sec with constant angular acceleration due to friction at the axle. What torque does friction exert while this wheel is slowing down?

Answer 1

The angular motion of the wheel will follow the equation
w(t)=wi+a*t
where a is the angular acceleration

First, convert 220 rpm to radians per second by multiplying the rpm by 0.10472

=220*0.10472
=23 rad/sec

Compute the acceration

0=23+a*45
a=-23/45

Torque = I*a
for a uniform disk, I=.5*M*R^2
T=-.5*.75*.25^2*23/45
T=-1/2*3/4*1/16*23/45
=-0.012

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Answer 2

http://www.irintech.com/x1/images/jean/elephantintheway.jpg