[Tex/LaTex] Putting a full stop in the end of an equation makes equation number appear one line below

Question

I'm having a problem. I am writing my thesis in physics, so I use a lot of equations. Since equations are to be treated as part of a sentence they include punctuation in the end. I have all the equations numbered, but punctuation makes the equation number appear on a new line. I use the following code to introduce equations into the paper:

\begin{equation}

\label{ix}

\centerline{$I_{aligned} = I - I_{rot}$.}

\end{equation}

This makes equations appear as follows:

But I want the equation number to be on the same line as the equation.

Answer 1

The presence of the full stop is not relevant at all. You get exactly the same effect with

\begin{equation}
\centerline{$a=b$}
\end{equation}

and

\begin{equation}
\centerline{$a=b$}
\end{equation}

The fact is that the input is wrong to begin with: \centerline is a Plain TeX macro and should not be used in LaTeX, unless very special effects are needed.

An equation environment already centers the math material without any further effort. So your input should be

\documentclass{article} % this class just for producing the example

\usepackage{amsmath} % for math typesetting

\begin{document}

\begin{equation}
\label{ix}
I_{\textup{aligned}} = I - I_{\textup{rot}}.
\end{equation}

\end{document}

Note that blank lines are not allowed inside equation and that the textual subscript should be in the upright font.