[Tex/LaTex] typesetting a physical law

Question

How should one type set one of Newton's law?

\begin{definition} environment doesn't seem right since it should say the law's name in bold not definition (some number) and \begin{theorem} doesn't fit since we are dealing with a law.

Answer 1

Two options: the first one (possibly an overkill?), using thmtools as a front-end for amsthm with a dedicated, unnumbered structure, and the second one using mdframed:

\documentclass{article}
\usepackage{xcolor}
\usepackage{amsthm}
\usepackage{thmtools}
\declaretheoremstyle[bodyfont=\normalfont]{mystyle}
\declaretheorem[
  style=mystyle,
  numbered=no,
  name=Newton's First Law
]{newton}
\usepackage{mdframed}


\begin{document}

\begin{newton}
If there is no net force on an object, then its velocity is constant. The object is either at rest (if its velocity is equal to zero), or it moves with constant speed in a single direction.
\end{newton}

\begin{mdframed}[backgroundcolor=gray!30,frametitle=Newton's First Law]
If there is no net force on an object, then its velocity is constant. The object is either at rest (if its velocity is equal to zero), or it moves with constant speed in a single direction.
\end{mdframed}

\end{document}