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Old Nov 22nd 2008, 08:24 PM   #1
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Another question! Angular acceleration! Torque! HELP!

I have a question that I'm not sure what to do with! HELP!

A wheel of radius 0.2 m is rotating around its axis with initial angular velocity of 30 radians/s clockwise. A braking force of 6.0 N is applied tangentially at the rim. 5.0 s later the angular velocity of the wheel is 10 radians/s clockwise.

(a) What is the magnitude and direction of the angular acceleration of the wheel while slowing down?
(b) What is the magnitude and direction of the torque acting on the wheel?
(c) What is the rotational inertia (moment of inertia) of the wheel?
(d) What is the initial angular momentum of the wheel (magnitude and direction)?
(e) What is the final Kinetic energy of the wheel?

This is a looong question and I'm not sure what to do or use for each part.
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Old Nov 24th 2008, 07:05 AM   #2
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For a) use the relation $\displaystyle \vec \alpha= \frac{\Delta \vec \omega}{\Delta t}$.
For b), you know that the torque is worth $\displaystyle \vec r \wedge \vec F$.
For c) check out your notes or wikipedia.
For d)$\displaystyle L_i=I_G \omega _i$.
For e) $\displaystyle E_f=\frac{I_G \omega _f^2}{2}$.
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