Еквівалентність маси й енергії

Yes. Einstein’s equation expresses the relation between a system’s rest mass and rest energy: E₀ = mc², where c is the speed of light in vacuum. It means mass measures energy a system has in its own rest frame; changing a system’s internal energy changes its mass. This is one of the key consequences of special relativity. In the more general relation, the total energy of a particle with momentum p is E² = (pc)² + (mc²)²; therefore the short formula E=mc² specifically denotes rest energy, not the total energy of every moving body.