# Angular Momentum with Vectors

1. Apr 25, 2013

### mcnealymt

1. The problem statement, all variables and given/known data
http://session.masteringphysics.com/problemAsset/1070538/5/12.EX46.jpg

a)What is the magnitude of he angular momentum of the 200g particle relative to the origin.

b) What is the direction of the angular momentum relative to the origin of the 200g particle? Into the page or out of the page.

2. Relevant equations
L= r *p you can then substitute "mv" in place of "p"

3. The attempt at a solution

a) Okay I'm very confused as to how I'm supposed to get the velocity. I understand that radius is found using Pythagorean Theorem, but I believe the velocity needs to be broken up.

I saw that one person said v= 3(cos45-arctan(.5)) I understand the cos45, but I don't get how arctan is relevant.

b) Into the page because of right hand rule? Im still not sure about this rule, its still very confusing. I've read three different methods and they just don't make any sense.

Last edited: Apr 25, 2013
2. Apr 25, 2013

### jambaugh

Your "relevant equation" is a magnitude equation. for r a position vector and p a momentum vector the proper equation is $\vec{L} = \vec{r}\times \vec{p}=m\vec{r}\times\vec{v}$ where $\times$ is the cross product.

Pick a basis, (the standard i,j,k basis will do) express v as a vector and the position of the particle as a vector then take the cross product.

The velocity's magnitude and direction are given in your diagram. You just need to apply some trig.

3. Apr 25, 2013

### mcnealymt

After I take the cross product what do I do? I know that the equation L= *m*v*r*p and there's another one ABsin(beta)K-hat.

4. Apr 25, 2013