Register to reply

Vectors and angular momentum

by Tomsk
Tags: angular, momentum, vectors
Share this thread:
Sep15-06, 03:26 PM
P: 227
I hope this is the right forum, this is mostly about maths, I'm not looking for a physical interpretation of angular momentum... yet. It also involves *some* calc... anyway...

OK, firstly, I've come to the conclusion I don't get cross products. I understand the properties of them, and can use them OK, there's just something I came across that I don't get. Say you have [itex]\vec{a}\times\vec{b}=\vec{c}[/itex]. Apparently, the magnitude of c is given by the area of the parallelogram formed by a and b. I'm ok with the product axb having units of area, but when you then go and say c has a length that is an area.... I get a bit lost. How am I supposed to interpret that?

Actually, scrap the second part, I'm an idiot!

Oh, and my lecturer always seemed to swap between J and L, both apparently for angular momentum. They mean the same thing, right? Or have I completely not understood anything??

I'll be back. I hate angular momentum.
Phys.Org News Partner Science news on
'Office life' of bacteria may be their weak spot
Lunar explorers will walk at higher speeds than thought
Philips introduces BlueTouch, PulseRelief control for pain relief
Sep15-06, 03:36 PM
P: 227
This kinda looks pointless now, I should always think through a problem thoroughly before looking for help on here! I still don't get the first bit about cross products though.
Sep15-06, 03:53 PM
Astronuc's Avatar
P: 21,915
J is the 'total' angular momentum, which is a coupling of the orbital angular momentum and the spin angular momentum.

As for the cross product -

If vrectors a,b had dimensions of length, then in a x b = c, c would have magnitude of area, and vector would be parallel to the normal of the area.

When we do v x B for the Lorentz force, the resulting vector has units of T-m/s, which have to be equivalent to N/C, since F = q(v x B).

See also -

Sep15-06, 04:00 PM
HW Helper
radou's Avatar
P: 3,220
Vectors and angular momentum

Think about what quantities angular momentum contains, i.e., what information is the angular momentum vector composed of; the scalar quantity - mass, the vector quantities - velocity and position. Isn't it a intuitive need to know what mass a particle has, where it is located, and what it's velocity is?

Regarding the area/length affare - I wouldn't loose my head thinking about that too much if I were you. The vector c = a x b can have any physical meaning, so it's dimension can be length, velocity, acceleration, force, etc. It's absolute value always equals the area of the a x b paralelogram, but that doesn't mean the dimensions equal, too.
Sep16-06, 10:27 AM
P: 227
Thanks guys. I've not done any QM yet, maybe my lecturer was crossing between J and L subconciously. I think I see the connection, sort of! I can accept the thing about the magnitude of the cross product too, it kinda caught me off guard! Thanks again. :D

Register to reply

Related Discussions
Angular velocity and angular momentum vectors Advanced Physics Homework 0
Find angular velocity using angular momentum Introductory Physics Homework 3
Angular impulse and angular momentum questions Introductory Physics Homework 7
Angular momentum and orbital angular momentum problems Introductory Physics Homework 3
Angular momentum vectors Introductory Physics Homework 1