# Homework Help: Angular momentum problem

1. Apr 4, 2005

### Punchlinegirl

A bike wheel, of mass 1.36 kg is placed at the end of a rod 0.530 m in length, which can pivot freely about the other end.
The rod is of negligble mass. The wheel is turning rapidly such that it has an angular momentum of 10.8 kgm^2/s. At what angular speed does the wheel revolve horizontally about the pivot?

I used the equation
L= Iw, substituting .5MR^2 for I
L= .5MR^2 w
10.8=.5(1.36)(.530)^2 w
Solving for w gives 56.5 rad/s which isn't right

Can anyone tell me what I did wrong?

2. Apr 4, 2005

Your moment of inertia is incorrect. You need to use the parallel axis theorem.

Regards,

3. Apr 4, 2005

### Punchlinegirl

Ok I tried using
L=Iw, with I= MR^2 +MG^2
10.8= (1.36)(.265)^2+ (1.36)(.530)^2
solving for w gives 22.6 rad/s... which isn't right
I think I'm using the Parallel Axis Theorem wrong...

4. Apr 4, 2005

what is the moment of inertia or a hoop pivoted in the centre? use this and add it to the length between the centre to the pivot point squared, times mass. The moment of inertia for a hoop is not MR^2. Just look it up on google.

Regards,

5. Apr 6, 2005

### blz5k

Actually, a google search turns up I=MR^2 to be the moment of inertia of a hoop. But where does that play into this problem, seeing as you don't know the radius of the bicycle wheel? I'm having difficulty with a similar problem.

6. Apr 6, 2005

### Felix83

they are asking you for the precessional angular velocity which is mgr/L. where r is the length of the rod that the wheel is on. I got this straight out of a physics book, I haven't worked through it to understand how they derived it but it works. punchlinegirl, where do u go to school at, I had a very similar problem in my physics class

7. Apr 6, 2005

### blz5k

Thanks Felix. It looks like punchlinegirl actually goes to the same school as me (form of the question and date of posting), which is Penn State Behrend.

8. Apr 6, 2005

### Punchlinegirl

Thanks. Yes I do go to PSB

9. Apr 8, 2005

### Felix83

ha, me too. i guess alot of people have discovered this site