# Homework Help: Angular momentum of a two particle system

1. Nov 26, 2011

### majorbromly

1. The problem statement, all variables and given/known data
I will preface this with: I am in a mechanics class and the professor has made it his duty to explain every single concept in the most high level ways possible, often ignoring necessary basics. As a result, I suck at angular momentum (and plenty of other things)

My best attempt at a drawing from my textbook: http://imgur.com/qUvQE

A is the origin here.

What is are the translational, rotational, and total angular momenta of this system?

After a short time Δt, what is the linear momentum of the system, what is the rotational angular momentum of the system?

2. Relevant equations
L= r x p
mag(L)=mag(r(perp))*mag(P)
Lrot=Iω

3. The attempt at a solution
One problem I have is I'm unsure how to differentiate between translational and rotational L. I'm mainly confused as to when the general "r cross p" statement applies. I chalk this up to my professor being a string theorist that cannot teach all that well.

I've tried setting up a center of mass, and attempted to solve for L using a perpendicular r of h+1/2d, and a momentum using the center of mass velocity. So, L comes out to -((h+1/2d)*(mvcom))...I would assume this is a total angular momentum?

As you can see I'm a little lost, and I would appreciate any help greatly!

2. Nov 26, 2011

### grzz

Note that given a particle of mass m moving with velocity v at some distance from a point P, then 'angular momentum' of this particle about point P can also be interpreted as the 'moment of linear momentum' of the particle about point P.

3. Nov 26, 2011

### majorbromly

I'm sorry, but I'm not quite sure I understand what you mean, and my google searches haven't helped. Do you think you can try to clarify?

Also, one thing I forgot to mention is that the line between the two masses is a physical rod with length d.

4. Nov 26, 2011

### Staff: Mentor

One way to put it: The total angular momentum is the angular momentum of the system about the center of mass (the rotational angular momentum) plus the angular momentum of the center of mass (the translational angular momentum).

Start by finding the motion of the center of mass.