This section covers the equivalence of mass and energy and the relationship expressed by E = mc².
Mass as a form of energy
Special relativity establishes an equivalence between mass and energy. Mass can be converted into other forms of energy, while energy transferred to a system contributes to its mass. Removing energy from a system correspondingly reduces its mass.
E=mc2
where:
E is energy in joules;
m is mass in kilograms;
c is the speed of light in free space.
Because c2 is very large, a comparatively small change in mass corresponds to a large energy change.
Examples of mass-energy conversion
Process
Mass-energy change
Nuclear fusion
Some mass associated with the initial nuclei is released in other energy forms.
Nuclear fission
A mass difference corresponds to energy released by the reaction.
High-energy particle collisions
Energy supplied to a collision can contribute to the production of massive particles.
For an object at rest, its mass is its rest mass, m0, and its corresponding rest energy is
E0=m0c2.
Exam Tip: Keep mass and energy units consistent. In SI calculations use mass in kilograms, energy in joules and c=3.0×108m s−1.