# How can I add left aligned text to an equation?

I have a document I am trying to copy to learn TeX. Here is what I have encountered: How can I have the "or" in the equation. This is what I have right now:

$f = ma;$
But $a$ is the change in velocity,

$f = m \frac{dv}{dt};$
$f = m \frac{d^2y}{dt^2};$


Edit: Trying out Stefan's answer Use the amsmath package and the commands \text{...} for text in the formula or \intertext{...} for text between the lines of multi-line formulas. For example:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
f &= ma;\\ \intertext{But $a$ is the change in velocity}
f &= m \frac{dv}{dt};\\ \intertext{or}
f &= m  \frac{d^2y}{dt^2};
\end{align*}
\end{document}


One advantage of align* to $...$ is that you can align the equations on relation symbols.

If you wish to put or in the same line, you could use \text and flalign* :

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{flalign*}
&& f &= ma;&\\ \text{or} && f &= m \frac{dv}{dt};&\\ \text{or}  &&f &= m  \frac{d^2y}{dt^2};
\end{flalign*}
\end{document} • & is used both for specifying the alignment position and as column separator, alternating. In columns of formulas, &= means that at = would be aligned, a following & would end the column, like in a table. && has been used to skip a column, i.e. to create an empty column. – Stefan Kottwitz Aug 13 '10 at 16:53