E^2 = m^2 + p^2 where p is momentum and i’ve dropped unit adjustment factors like c

this allows light to have energy even if its massless

e=mc^2 is only correct for objects at rest. The full equation takes into account velocity, but for "low" speeds where v<<c, the term is close enough to zero than E=mc^2 is still a good approximation.

E^2 = (mc^2)^2 + (pc)^2,

where p is momentum. When an object is traveling at relativistic speeds, the momentum forms a more significant portion of its energy