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**1. Homework Statement**

A 750 gram grinding wheel 25.0 cm in diameter is in the shape of a uniform solid disc. When it is in use it turns at a constant 220 rpm about an axis perpendicular to its face through its center. When the power is switched off the wheel stops in 45.0 s with constant angular acceleration due to friction

A) What is rotational intertia of the wheel?

B) What is the initial angular velocity in rad/s?

C) What is the constant angular acceleration in rad/s^2?

D) What is the magnitude of the torque exerted by friction in N-m?

m = 0.750 kg

r= 0.125 m

wf = 23.04 rad/s (converting 220rpm)

t= 45.0 s

**2. Homework Equations**

Part A) Idisc= 0.5(mr^2) = 0.5(0.750kg)(0.125m^2) = 5.86*10^-2 kgm^2

**3. The Attempt at a Solution**

I was thinking along these lines... I could find part C first (angular acceleration - alpha) and use that to find part B (initial angular velocity - w0) by using this equation : wf = w0 + alpha(time). I could then find part D (torque - T) by using this equation : alpha = T/I

*My problem is finding angular acceleration, or alpha.*

This is what I did, please let me know if it's correct or not.

v(speed)= radius*wf

v = (0.125m)(23.04 rad/s) = 2.88

vf= v0 +a(linear)(time)

2.88 = 0 +a(45seconds)

a(linear) = 0.064 m/s

a(linear) = radius*a(angular)

0.064 = 0.125*a

__alpha = 0.512 rad/s^2__

If it's wrong, could you please advise on how to get the right answer?

Thank you in advance :)

Disclaimers: This is my first post, so please be gentle! Also, this is not homework, it is a practice test. Thanks again!

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