Raise/lower equation number

Question

Need to raise/lower equation number using tex macros given point value as variable.

Please find MWE file:

\documentclass{book}
\usepackage{amsmath}


\begin{document}
If the car in figure has a linear acceleration parallel to the ground, a point on the tyre's outer edge experiences a tangential acceleration relative to the axle. The same kind of reasoning as that used in the previous paragraph reveals that the magnitudes of these accelerations are the same and that they are related to the angular acceleration of the wheel relative to the axle.
\begin{align}%%\raisetag{30pt}
\begin{array}{ll}
\underset{\text{Linear speed}}{\underbrace{v}}=\underset{\text{Tangential speed}}{\underbrace{v_{\mathrm{T}}=r\omega }} & \left(\omega \text{~in~}\mathrm{rad}/\mathrm{s}\right)\end{array}%
\tag{8.12}%%\eqnoraise{30pt}{\tag{8.12}}
\end{align}

If the car in figure has a linear acceleration parallel to the ground, a point on the tyre's outer edge experiences a tangential acceleration relative to the axle. The same kind of reasoning as that used in the previous paragraph reveals that the magnitudes of these accelerations are the same and that they are related to the angular acceleration of the wheel relative to the axle.
\end{document}

Answer 1

You don't need array. I also suggest using a command for textual subscripts to an underbrace. In order to get the two aligned, a phantom in the first one is needed.

Also consider siunitx for getting uniform rendering of units.

\documentclass{book}
\usepackage{amsmath}
\usepackage{siunitx}

\newcommand{\textubrace}[2]{{\underbrace{#1}_{\text{#2}}}}

\begin{document}
If the car in figure has a linear acceleration parallel to the ground, 
a point on the tyre's outer edge experiences a tangential acceleration 
relative to the axle. The same kind of reasoning as that used in the 
previous paragraph reveals that the magnitudes of these accelerations 
are the same and that they are related to the angular acceleration of 
the wheel relative to the axle.
\begin{equation}
\textubrace{v_{\vphantom{T}}}{Linear speed}=
\textubrace{v_{\mathrm{T}}=r\omega}{Tangential speed}
\qquad
(\omega \text{~in~\si{\radian/\s}})
\end{equation}
If the car in figure has a linear acceleration parallel to the ground, 
a point on the tyre's outer edge experiences a tangential acceleration 
relative to the axle. The same kind of reasoning as that used in the 
previous paragraph reveals that the magnitudes of these accelerations 
are the same and that they are related to the angular acceleration of 
the wheel relative to the axle.

\end{document}

Answer 2

You can set the vertical alignment in your array with \begin{array}[t]{ll}