Quick questions on Mass-energy equivalence: WACE Year 12 Physics Unit 4

Nuclear masses are conveniently measured in unified atomic mass units, where $1\ \text{u}=1.661\times10^{-27}\ \text{kg}$. Converting a mass of $1\ \text{u}$ entirely to energy gives

At high speed the total energy of a particle is $E=\gamma mc^2$, which separates into the rest energy $mc^2$ (present even at rest) and the kinetic energy. At low speeds this reduces to the familiar $\tfrac{1}{2}mv^2$. The rest energy term is what becomes available when mass is destroyed in a reaction.

Because nuclear masses are tabulated in unified atomic mass units, the fastest route to energy is almost always to work in those units and use the conversion $1\ \text{u}\to931.5\ \text{MeV}$. Find the total mass of the reactants and the total mass of the products in atomic mass units, take the difference (the mass defect), and multiply by $931.5$ to get the energy released in MeV directly, with no powers of ten to juggle. Only convert to joules at the very end if the question demands SI units, using $1\ \text{MeV}=1.6\times10^{-13}\ \text{J}$. Keeping enough significant figures in the masses is essential, because the mass defect is a small difference between larger numbers, and rounding the input masses too early can change the final energy substantially.

For nuclear questions, working in atomic mass units and multiplying by $931.5\ \text{MeV per u}$ is fastest. If asked for joules, convert at the end. Always take the mass change as products minus reactants, and use its magnitude for the energy released.