# Why is the symbol for angular momentum L?

1. Mar 9, 2009

### zhuangjieyi

Why is the symbol for angular momentum "L?"

So far, the best explanation I've come across is:

This comes from the "right hand rule" of vector cross products. The vector for the radius, r-> and the vector for the translational momentum p-> when made into a cross product :
r-> x p-> = L->
Since r and p are in the x-y plane, they are perpendicular to each other and form an "L" shape when made to be part of a cross product.

2. Mar 9, 2009

### confinement

Re: Why is the symbol for angular momentum "L?"

m is for mass.

Momentum can't be m, since mass is m, therefore momentum will be p. Now that we have accepted such things, magnetic field cannot be m, so it will be B. Then the vector potential from which B is derived should be known as A i.e. A precedes B, and the potential A is all important. Therefore we cannot have angular momentum be A or M (or P, but who would want that?). Therefore we may as well call it L, as in Leonhard Euler, "the master of us all" whose description of rigid body rotation is still used today.

3. Mar 10, 2009

### flatmaster

Re: Why is the symbol for angular momentum "L?"

I tell students that that the way to remember "p" stands for momentum is to imagine it spelled with a silent p.

pmomentum

4. Mar 10, 2009

### alxm

Re: Why is the symbol for angular momentum "L?"

Haha, no. As I mentioned in the other thread, I know this one. It's 'B' for 'Borealis' - North. That's where the field points.

At least in the hemisphere where all the cool folks live. ;)

5. Mar 10, 2009

### confinement

Re: Why is the symbol for angular momentum "L?"

Of course, the magnetic field of the earth always points to the north pole, even if you live in the southern hemisphere, since there is a magnetically south pole at the north pole!

6. Mar 10, 2009

### alxm

Re: Why is the symbol for angular momentum "L?"

No no, the magnetic field is defined as having the opposite direction south of the equator. And South is at the top of their maps.

See?

7. Mar 10, 2009

### Dr.D

Re: Why is the symbol for angular momentum "L?"

No reason at all other than an arbitrary decision someone made. It makes no difference what you call it, as long as you define your symbols.

8. Apr 5, 2010

### evoluciona2

Re: Why is the symbol for angular momentum "L?"

L from the sound of angulus which is angle in Latin?