Mass and energy are equivalent: $E = mc^{2}$

$E$: energy The capacity to do work. Measured in joules (J). (J)
$m$: mass (kg)
$c$: speed of light in a vacuum (3.0 × 10⁸ m s⁻¹)

Nuclear Energy & Binding Energy — AQA A-Level Physics

Key Definition

Mass-energy equivalence — Einstein's principle that mass can be converted into energy and energy can be converted into mass, related by E = mc².

$$E = mc^2$$

A small amount of mass corresponds to a very large amount of energyThe capacity to do work. Measured in joules (J). because c² ≈ 9 × 10¹⁶.
Examples: hydrogen fusing into helium in stars, fission of uranium in nuclear reactors, particle-antiparticle annihilationThe process in which a particle and its corresponding antiparticle collide and convert their combined rest mass into energyThe capacity to do work. Measured in joules (J)., typically producing two gamma photons. in accelerators.
This equation applies to all nuclear reactions where the total mass of products differs from the total mass of reactants.