# What is meant by the rest energy of non-composite particle?

When talking about the rest energy of a composite particle such as a proton, part of the rest energy is accounted for by the internal kinetic energy of its constituent quarks. But what is physically meant by the rest energy of non-composite particles such as quarks?

• The energy obtained by turning the quark into pure energy (eg light &c). I guess you already know this, though.. I think rest energy is just another property you take for granted (no intuition behind it), like the spin of an electron.. Feb 16 '12 at 15:41
• Feb 16 '12 at 15:54
• Note that the valence quark masses make up several percent of the proton's mass. Feb 16 '12 at 16:11

the mass $m$ entering the relativistic equation where $p$ is the momentum and $E$ the total energy,
$E^2 - p^2c^2 = m^2c^4$
when $p=0$ is the rest energy.
For a non composite particle, as the electron, the energy when at rest, $p=0$, is its mass.