# Newline within underbrace in math environment [duplicate]

Possible Duplicate:
How can I force a line break within \text{…}?

In the MWE below, is it possible to force a line break in the text under the Z so that the overall equation is more compact in the horizontal direction? I tried a normal linebreak, which I hoped would work since it's inside a \text, but that gave me errors.

Also, if there's a better way to make this sort of equation callout, please let me know. \underbrace and underset and variants thereon are the only methods I'm familiar with at present.

The Cauchy-Schwartz equation below from the Navier-Stokes Wiki article shows the kind of line breaks I was hoping to get in the underbrace.

MWE

\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
$\underbrace{z}_{\text{Initial Acceleration}} = x + y$
\end{document}


Underbrace linebreaks:

-