Mass–Energy Equivalence

Mass and energy are not separate substances in relativity; they are two ways of accounting for the same underlying quantity. The formula E = mc^2 gives the rest energy of an object: even when it is not moving, its mass corresponds to a huge amount of stored energy because the speed of light squared is extremely large. This relation comes from special relativity and shows that if a system loses mass, that mass can appear as released energy, and vice versa.

Use this when you want the energy equivalent of a given amount of mass, especially in nuclear reactions, particle physics, or high-energy astrophysics. It is also useful for estimating how much energy is released when a small mass is converted into radiation or kinetic energy.

• Using grams instead of kilograms without converting first
• Forgetting that c must be squared, not just multiplied by 3 × 10^8
• Thinking the formula applies to moving energy only, rather than rest mass energy