Rest Mass
Key Stage 5
Meaning
The rest mass of a particle is total mass of the particle when it is stationary relative to the observer.
About Rest Mass
- The SI Unit of rest mass is the kilogram (kg).
- Rest mass is a scalar quantity as it has magnitude but does not have a direction.
- The mass of subatomic particles depends up their relative motion. This becomes significant at speeds approaching the speed of light.
- The rest mass of a particle is equivalent to its total energy (when stationary relative to an observer) divided by the speed of light squared.
- The rest mass for a given subatomic particle is the same from all reference frames.
- The rest mass for a given particle is identical to the rest mass of its antimatter counterpart.
Equations
The rest mass is related to the rest energy by the mass-energy equivalence shown in the following equation
\(m = \frac{E}{c^2}\)
Where
\(m =\) The rest mass of a particle.
\(E =\) The rest energy of a particle.
\(c =\) The speed of light (\(3.00\times10^{8}ms^{-1}\) in a vacuum).
Examples
| Particle | Rest Mass / kg | Unified Atomic Mass Units | Rest Energy / MeV |
| Proton (\(p\)) | \(1.67\times10^{-27}\) | \(1.007\) | \(938\) |
| Neutron (\(n\)) | \(1.67\times10^{-27}\) | \(1.009\) | \(939\) |
| Electron (\(e\)) | \(9.11\times10^{-31}\) | \(0.05486\) | \(5.11\) |
| Muon (\(\mu\)) | \(1.88\times10^{-28}\) | \(0.1134\) | \(106\) |
| π-meson (\(\pi^±\)) | \(2.49\times10^{-28}\) | \(0.1500\) | \(140\) |
| π-meson (\(\pi^0\)) | \(2.41\times10^{-28}\) | \(0.1449\) | \(135\) |
| K-meson (\(K^±\)) | \(8.80\times10^{-28}\) | \(0.5300\) | \(494\) |
| K-meson (\(K^0\)) | \(8.87\times10^{-28}\) | \(0.5442\) | \(498\) |
| Photon (\(\gamma\)) | \(0\) | \(0\) | \(0\) |
| Neutrino (\(\nu\)) | \(0\) | \(0\) | \(0\) |