# Rotating Disk angular momentum

1. Nov 15, 2009

### Dark Visitor

I need some help with this problem. Any help would be greatly appreciated. It is due tonight.

http://session.masteringphysics.com/problemAsset/1035644/7/knight_Figure_13_47.jpg

Use the link above to solve the problem.

Part 1) What is the magnitude of the angular momentum of the 3.1 kg, 4.5-cm-diameter rotating disk in the figure ?

Part 2) What is it's direction?

For the first part, I think I need to use the equation L=Iw, but I got it wrong when I did, so I may have used the wrong numbers. If I am using the wrong equation, please tell me.

2. Nov 15, 2009

### cepheid

Staff Emeritus

3. Nov 15, 2009

### Dark Visitor

Well, as I said in this thread, I attempted it using the equation I posted and got it wrong. That's why I thought I needed to do for this problem, but I was wrong. Now I need help on finding where I went wrong and what to do next.

4. Nov 15, 2009

### cepheid

Staff Emeritus
Yeah, but you didn't post what formula you used for the moment of inertia, or any of the steps of your calculation, so I have no idea whether you got the wrong answer because you made a calcuation error, or whether it was because you used the wrong moment of inertia equation.

Angular momentum is always given by L = Iw, so your "big picture" conceptual approach to solving the problem is correct. You are aware of all of the physics you need to know. It's just the details that need sorting out. If I can't see any of those details, I can't help you.

5. Nov 15, 2009

### Dark Visitor

Nevermind. I realized I used the diameter in the Inertia equation instead of the radius. I will show you what I did anyways.

First I calculated that 600 RPM is equal to 62.83185 m/s. I then plugged that into the equation L=Iw, which gave me:

L = .5(M)(R)^2
= (.5(3.1 kg)(.0225 m)^2) * 62.83185 m/s
= .049 kg*m^2/s

And that was right. Now how would I get the direction of all that for Part 2?

6. Nov 15, 2009

### cepheid

Staff Emeritus
You mean 62.83 1/s, right? (or radians per second, if you prefer to call it that). Velocity and angular velocity don't have the same units, because they aren't the same thing

The answer hasn't changed since the last time you were asked this question. The relationship between the direction of the angular momentum vector and the rotation direction is still given by the right hand rule.

7. Nov 15, 2009

### Dark Visitor

Thanks. I figured it out shortly after you got off I think.