Conservation of angular momentum

[bvan22]

Homework Statement

a professor steps onto a stationary turntable while holding a rotating bicycle wheel that is rotating with an angular velocity of 12 rad/s pointing upward. The wheels axis of rotation goes through the axis of the turntable. The rotational inertia of the wheel is 1.35 kg m^2 and the combined rotational inertia of the professor and the turntable about the turntable's axis is 9 kg m^2. The professor flips the bicycle wheel so that it is still rotating with the same angular speed, but pointing downwards. How fast will this cause the professor and the turntable to rotate?

Homework Equations

Iw(initial)=Iw(final)

The Attempt at a Solution

(1.35)(12)=(9)w(final)
w(final)=1.8 rad/s
I got it wrong on the test. Please help! Thanks

 

[kuruman]

Your expression for the angular momentum conservation is incorrect. The left side is OK because you have only the wheel spinning. After the wheel is flipped, you have two things spinning, a wheel and a professor plus turntable so you need two terms on the right side of the equation. Note that the wheel maintains the same angular speed, but its angular velocity is reversed (changes sign).

 

[bvan22]

(1.35)(12)=(1.35)(-12)+(9)w
2(16.2)/9=w
w=3.6rad/s
How does that look?

 

[kuruman]

Much better. I think you got it.