Is it possible to use \underbrace across align* environment?

Question

I have two sets of equations in an align* environment and they are all just how I want them, but I want to draw an underbrace from the left bottom equation to the right bottom equation, but it throws errors whenever I use & inside the underbrace argument. Is there a workaround?

---

MWE:

\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}
    \begin{align*}
    \text{Kin}&\text{ematics} &\ \ \ \ && \text{Rot}&\text{ation Kinematics}\\
    x &= x_ot+v_ot+\frac{1}{2}at^2 &&& \theta &= \theta_ot\omega_ot+\frac{1}{2}\alpha t^2\\
    v &= v_ot+at &&& \omega &= \omega_ot \alpha t\\
    % \underbrace{v^2 &= v_o^2+2a \Delta x &&& \omega^2 &= \omega_o^2 + 2 \alpha \Delta \theta}_{\text{Variables get replaced as per the table}}\\
    \end{align*}
\end{document}

The above code works perfectly until the last line is uncommented.

Answer 1

Some thing like this:

\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}
    \begin{align*}
    \underbrace{
    \begin{alignedat}{2}
    &\text{\rlap{Kinematics}} &  \text{\rlap{Rotation Kinematics}} &  \\
    x &= x_ot+v_ot+\frac{1}{2}at^2 & \qquad\qquad\qquad  \theta &= \theta_ot\omega_ot+\frac{1}{2}\alpha t^2\\
    v &= v_ot+at & \omega&= = \omega_ot \alpha t\\
    v^2 &= v_o^2+2a \Delta x &  \omega^2 &= \omega_o^2 + 2 \alpha \Delta \theta
    \end{alignedat}
    }_{\text{Variables get replaced as per the table}}
    \end{align*}
\end{document}

Answer 2

Here, I save the align* in a temporary \vbox and then stack under it. The [-23pt] controls the vertical separation of the underbrace; the to 270pt controls the length of the underbrace; and the {30pt} controls the horizontal placement of the underbrace.

\documentclass[12pt]{article}
\usepackage{amsmath,stackengine,lipsum}
\stackMath
\begin{document}
\lipsum[1]

\setbox0=\vbox{
    \begin{align*}
    \text{Kin}&\text{ematics} &\ \ \ \ && \text{Rot}&\text{ation Kinematics}\\
    x &= x_ot+v_ot+\frac{1}{2}at^2 &&& \theta &= \theta_ot\omega_ot+\frac{1}{2}\alpha t^2\\
    v &= v_ot+at &&& \omega &= \omega_ot \alpha t\\
    % \underbrace{v^2 &= v_o^2+2a \Delta x &&& \omega^2 &= \omega_o^2 + 2 \alpha \Delta \theta}_{\text{Variables get replaced as per the table}}\\
    \end{align*}
}
\noindent\hfil\stackunder[-23pt]{\box0}{%
  \stackunder{\hbox to 270pt{\upbracefill}}{\text{\scriptsize This is my caption}}%
  \hspace{30pt}}\hfill

\lipsum[2]
\end{document}