Baby Collar, Dong people. China, Yunnan province, 20th century 37 x 18 cm. From the collection of Tan Tim Qing, Kunming. Photograph by D Dunlop. |

Newtonian particles are stable and dense. These are signature attributes, but then it immediately follows that they are also heavy and their Lorentz factor is always close to one

$\gamma \simeq 1$

So Newtonian particles are always at rest or in slow motion. Their mechanical energy $\, E \,$ and their mass $\, m \,$ are conserved under many common conditions. So if some isolated Newtonian particles $\mathbb{X}$, $\mathbb{Y}$ and $\mathbb{Z}$ interact like

$\mathbb{X} + \mathbb{Y} \leftrightarrow \mathbb{Z}$

then we can often assume that

$m ^{ \mathbb{X} } + m ^{ \mathbb{Y} } = m ^{ \mathbb{Z} }$and$E ^{ \mathbb{X} } + E ^{ \mathbb{Y} } = E ^{ \mathbb{Z} }$

These relationships are over-simplifications and approximations. But they are very useful, and applicable to a wide variety of everyday situations that we discuss in more detail over the next few articles.