Conservation of Angular Momentum Homework Help

[hb94]

Homework Statement

"A student sits on a swivel seat. Initially the student is at rest, holding a spinning bicycle wheel that has an angular velocity of 12.6 rad/s (see figure below). The student turns the spinning wheel upside down, reversing the direction of its angular velocity. This causes the student to begin to rotate. Find the student's final angular velocity. The moments of inertia of the wheel and of the student are 0.340 kg · m2 and 2.25 kg · m2 respectively."

Homework Equations

How do you find the final angular velocity with the given information?

The Attempt at a Solution

Well since initial momentum is equal to final momentum, IWo = IWf +iw, where I is the moment of inertia of the wheel, Wo is the initial angular velocity of the wheel, Wf is the final angular velocity of the wheel, i is the moment of inertia of the girl, and w is the angular velocity of the girl. So, (I(Wo-Wf))/i = w. I guessed that Wf would equal -Wo, but I was mistaken, or I made a calculation error.
Thanks in advance for your help :D

 

[diazona]

Well since initial momentum is equal to final momentum, IWo = IWf +iw, where I is the moment of inertia of the wheel, Wo is the initial angular velocity of the wheel, Wf is the final angular velocity of the wheel, i is the moment of inertia of the girl, and w is the angular velocity of the girl. So, (I(Wo-Wf))/i = w. I guessed that Wf would equal -Wo, but I was mistaken, or I made a calculation error.

How do you know?

At any rate, your procedure seems to be correct. One additional thing you could think about is the fact that in the final situation, the wheel is not only spinning around its own axis, but also that axis is revolving around the girl's rotational axis. But in order for you to take that into account, I think they would have to give you more information, so I have my doubts that that is what's causing your problem.

 

[hb94]

Well, I(Wo-Wf)/i = 3.808, and if these are the only factors applied to the problem, then I got the wrong answer.

 

[diazona]

You forgot the units...

What's the right answer supposed to be?