Vectors and angular momentum


by Tomsk
Tags: angular, momentum, vectors
Tomsk
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#1
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.
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Tomsk
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#2
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.
Astronuc
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#3
Sep15-06, 03:53 PM
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J is the 'total' angular momentum, which is a coupling of the orbital angular momentum and the spin angular momentum.

http://en.wikipedia.org/wiki/Angular...ng#LS_coupling

http://hyperphysics.phy-astr.gsu.edu...tum/qangm.html
http://hyperphysics.phy-astr.gsu.edu...um/vecmod.html

As for the cross product -

http://hyperphysics.phy-astr.gsu.edu/hbase/vvec.html

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 - http://en.wikipedia.org/wiki/Cross_p...metric_meaning

radou
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#4
Sep15-06, 04:00 PM
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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.
Tomsk
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#5
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


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