How can I add left aligned text to an equation?

Question

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

Answer 1

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}