Letters used in physics equations

[Baby Astronaut]

[math]r[/math] stands for...?

 

Is there a website anywhere that briefly explains all such letters, i.e. what each represents?

[ajb]

It will depend on the context. However, [math]r[/math] often stands for radius.

 

When one writes down an equation one should be careful to define all the symbols. In practice, some symbols have a specific meaning in different branches of physics. Note that the meaning may be very different depending on the subject.

[CaptainPanic]

Never blindly trust the meaning of a letter. Use your own brain, because different countries/books/websites have different letters.

 

Although I agree that "r" most likely is for radius.

 

Every good book or article will have a list of symbols. Unfortunately, there are many books and articles that aren't very good.

[D H]

It will depend on the context. However, [math]r[/math] often stands for radius.

Although I agree that "r" most likely is for radius.

 

Another common one: Pearson's correlation coefficient.

 

[math]\rho = \frac{\mathrm{cov}(x,y)}{\sigma_x \sigma_y}[/math]

 

Just as a lower case s is used in lieu of σ to represent the standard deviation as estimated from some population, a lower case r is typically used in lieu of ρ to represent the correlation coefficient as estimated from some population:

 

[math]r = \frac 1 n \sum_n \frac{x_i-\bar x}{s_y}\,\frac{x_i-\bar y}{s_y}[/math]

[CaptainPanic]

Ah, yes. Statistics. I try to ban it from my mind ever since I studied it at university, but it keeps coming back... perhaps because it's so useful?

 

Here's a link to D H's formula.

[ajb]

I have used [math]r \in \mathbb{R}[/math] to mean the order of degree of something, for example the order of a differential operator or the degree of a multivector field.

 

I have seen it used to denote an element of a ring.

 

So, without knowing the context, it is impossible to state what [math]r[/math] is.

[swansont]

Is there a website anywhere that briefly explains all such letters, i.e. what each represents?

I can't find anything that is comprehensive, but here are some collections

 

http://www.rwc.uc.edu/koehler/biophys/symb.html

http://en.wikipedia.org/wiki/Variables_commonly_used_in_physics

http://en.wikipedia.org/wiki/Greek_letters_used_in_mathematics

 

 

Also

http://en.wikipedia.org/wiki/Physical_constant

[Baby Astronaut]

Thanks swansont, everyone.

 

It will depend on the context. However, [math]r[/math] often stands for radius.

 

When one writes down an equation one should be careful to define all the symbols. In practice, some symbols have a specific meaning in different branches of physics. Note that the meaning may be very different depending on the subject.

 

Never blindly trust the meaning of a letter. Use your own brain, because different countries/books/websites have different letters.

Wow. I had thought it a far more organized and universal system.

 

If mathematics is the best (i.e. universal) language for contact with aliens, how's an extraterrestrial going to unravel what the letters represent in our equations? Hypothetically speaking

[iNow]

Through use of a crytographical key or primer. Geesh... Didn't anyone read Carl Sagan's Contact?

 

http://en.wikipedia.org/wiki/Key_%28cryptography%29

[StringJunky]

"If mathematics is the best (i.e. universal) language for contact with aliens, how's an extraterrestrial going to unravel what the letters represent in our equations? Hypothetically speaking "- Baby Astronaut

 

 

I thought it was common knowledge that aliens carried Babelfish...they hold one to their ear and the living fish translates anything for them.