Spacing of consecutive equation versus a single gather

Question

I know the spacing produced by the following two approaches is slightly different. So my question is which one should I choose? Is it about personal preference?

With 3 consecutive equation environments

With a single gather environment

Code

\documentclass[preview,multi,border=12pt]{standalone}
\usepackage{amsmath}


\begin{document}

\begin{preview}
The following equations are too simple,
\begin{equation}
v_t=v_0+at
\end{equation}
\begin{equation}
s=v_0t+\tfrac{1}{2}at^2
\end{equation}
\begin{equation}
v_t^2=v_0^2+2as
\end{equation}
where $a$ is the acceleration.
\end{preview}


\begin{preview}
The following equations are too simple,
\begin{gather}
v_t=v_0+at\\
s=v_0t+\tfrac{1}{2}at^2\\
v_t^2=v_0^2+2as
\end{gather}
where $a$ is the acceleration.
\end{preview}

\end{document}

Answer 1

You definitely shouldn't do it with 3 consecutive equations. As you said yourself, the spacing is unbalanced. Here's why: The line before the first equation is "long", so TeX uses \abovedisplayskip above it, and \belowdisplayskip below. The other two equation don't have text before them, or you could say the text before the equations is "short". Thus, TeX uses \abovedisplayshortskip and \belowdisplayshortskip. As a result, you have

• \belowdisplayskip + \abovedisplayshortskip between the 1st and the 2nd equation, and
• \belowdisplayshortskip + \abovedisplayshortskip between the 2nd and the 3rd equation.

I'd use align (and not gather) to obtain horizontal alignment and balanced vertical spacing:

\begin{align}
  v_t &= v_0 + at \\
    s &= v_0t + \tfrac{1}{2} at^2 \\
v_t^2 &= v_0^2 + 2as
\end{align}

Answer 2

The line

s=v_0t+\tfrac{1}{2}at^2

produces a fraction which here does not give you a higher line. If you used a \frac{}{} instead, it might happen that one line will be higher than the others (if they are wo fractions), which is also unnatural. Then it helps to put \phantom{\frac{1}{2}} on the end of the other lines (after the dot or comma sign). At least then, even with consecutive equations, which I don't recommend, the spacing would be balanced.