[Tex/LaTex] How to use intertext command at the end of align environment without adding an empty line

Question

I'm using the environment align of the package amsmath to display multiple lines of equations. I also use the \intertext command of the same environment to add text between the lines of equations. Now when I use the \intertext command in the last line of the align environment, it adds an empty line of equation. Do you know a simple way prevent this empty line to appear? If I use the \text command instead of \intertext, the indentation of the added text changes. So another way of solving the problem maybe to force the command text to print left aligned text in the same way the intertext command does.

Here is my code:

\begin{align*}
\frac{F_1}{m_1} \leq \frac{F_{12}}{m_{12}} \leq \frac{F_2}{m_2}\\
\intertext{Assuming additivity of mass and that joining the two bodies doesn't affect the force acting on each body (force is additive):}
\frac{F_1}{m_1} \leq \frac{F_1+F_2}{m_1+m_2} \leq \frac{F_2}{m_2}\\
\intertext{which is true iff:}
\frac{F_1}{F_2} \leq \frac{m_1}{m_2}\\
\intertext{which is equivalent to:}
a_1 \leq a_2\\
\intertext{which is always true by hypothesis (the same demonstration applies for $a_2 \leq a_1$)}
\end{align*}

Answer 1

The last line should not belong in the align.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
\frac{F_1}{m_1} \leq \frac{F_{12}}{m_{12}} \leq \frac{F_2}{m_2}\\
\intertext{Assuming additivity of mass and that joining the two 
  bodies doesn't affect the force acting on each body (force is additive):}
\frac{F_1}{m_1} \leq \frac{F_1+F_2}{m_1+m_2} \leq \frac{F_2}{m_2}\\
\intertext{which is true iff:}
\frac{F_1}{F_2} \leq \frac{m_1}{m_2}\\
\intertext{which is equivalent to:}
a_1 \leq a_2
\end{align*}
which is always true by hypothesis (the same demonstration applies for $a_2 \leq a_1$).

Some more text follows in order to show there's no added space.

\end{document}

On the other hand, I see no reason for forcing alignment of those equations and I'd typeset them as follows.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation*}
\frac{F_1}{m_1} \leq \frac{F_{12}}{m_{12}} \leq \frac{F_2}{m_2}
\end{equation*}
Assuming additivity of mass and that joining the two bodies doesn't
affect the force acting on each body (force is additive):
\begin{equation*}
\frac{F_1}{m_1} \leq \frac{F_1+F_2}{m_1+m_2} \leq \frac{F_2}{m_2}
\end{equation*}
which is true if and only if
\begin{equation*}
\frac{F_1}{F_2} \leq \frac{m_1}{m_2}\\
\end{equation*}
which is equivalent to
\begin{equation*}
a_1 \leq a_2
\end{equation*}
which is always true by hypothesis (the same demonstration applies for $a_2 \leq a_1$).

\end{document}