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 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. Also, as discussed earlier, for Newtonian particles

$mc^{2} \simeq \left| \, H \, \vphantom{\sum^{2}} \right|$

where $m$ is the mass and $H$ is the enthalpy. But the energy $E$ of any material particle is approximately

$E \simeq \gamma m c^{2}$

Then substitution obtains

$E \simeq \left| \, H \, \right|$

The absolute-value signs can usually be ignored because ordinary particles are composed from electrons, neutrons and protons which all have an enthalpy greater than zero (see the WikiMechanics spreadsheet for details). So the enthalpy and the mechanical energy are almost interchangable for any macroscopic particle. That is, if we ignore collisions with anti-particles and processes like annihilation then

$H \simeq E \simeq m c^{2}$

But enthalpy is conserved for *all* particles and conditions, so energy and mass must be approximately conserved too. Finally recall that photons have no mass. So for interactions with photons, the mass of a Newtonian particle is usually a dependable constant. This is an important aspect of stability for Newtonian particles.