Aligning multi-line equations

[HeinzAlex]

Hello everyone, I'l start by saying that this is my first time asking a question here and I just recently started using/learning rmarkdown so I hop eyou'll forgive me if I make/made some mistake(s).

My question is, I'd like to put two arrows under the divisors of two fractions in an equation formula, but the method I found for doing so is (I assume) abominable.
'''
$$P_{tot}=\frac{F_{tot}}{S}=\frac{m}{l}N\overline{{V_x}^2}\frac{1}{S}$$
$$\quad\quad\quad; \downarrow\ \quad\quad\quad\quad\quad\quad\quad\quad;:\downarrow$$
$$\quad\quad\quad;; lS=V_{box}$$
'''
Can anyone point me to a method to make this better/more elegant? I tried using the \begin{align} function but it won't allow me to use "&" in the divisor of a fraction.

Thanks in advance for your help!

[technocrat]

@HeinzAlex

abominable.

No, just LaTeX, which is as tricky as it is powerful.

Questions about intent.

1. is <em> supposed to start bolding?
2. are the ; and : intended?
3. formula block to be left aligned or centered aligned?

P_{tot}=\frac{F_{tot}}{S}=\frac{m}{l} *N* \overline{{V_x}^2} <em>\frac{1}{S}

\quad\quad\quad; \downarrow\ \quad\quad\quad\quad\quad\quad\quad\quad;:\downarrow

\quad\quad\quad;; l</em> S=V_{box}

[HeinzAlex]

Thanks for the reply! I didn't intend to put the thing there, it must have been a trascription issue. The colons and semicolons are probably to increase the spacing (: ;). The formula is center-aligned

[technocrat]

Then I think you want

P_{tot}=\frac{F_{tot}}{S}=\frac{m}{l} *N* \overline{{V_x}^2} \frac{1}{S}
\quad\quad\quad \downarrow \quad\quad\quad\quad\quad\quad\quad\quad\downarrow
\quad \quad \quad S=\quad\quad\quad \quad \quad \quad V_{box}

The code may render differently depending on the engine and output device, so you may need to adjust the number of quads

<center>
$P_{tot}=\frac{F_{tot}}{S}=\frac{m}{l} *N* \overline{{V_x}^2} \frac{1}{S}$<br>
$\quad\quad\quad \downarrow \quad\quad\quad\quad\quad\quad\downarrow$<br>
$\quad \quad \quad S=\quad\quad\quad \quad \ V_{box}$
</center>

[HeinzAlex]

Yeah, but I meant to ask if there was a way to do this in a way that wouldn't require me to use the 'ol "just put spaces until it looks good" and maybe would do it automatically