alt Hacker NewsYes, there is no law of conservation for mass like there is for energy. Fusion is a good example for why it's not conserved. The teacher was right.
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tardedmeme • yesterday at 1:01 PM
There actually is a law of conservation of mass (it's the same law, because mass is energy) and it only appears violated if you forget about the particles that are zooming away at the speed of light. Of course the mass of a system changes if mass can flow in and out.
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He was right that it violates conservation of mass. He was completely wrong that it violated it by adding 2 atomic mass units when hydrogen fuses.
In reality heavier isotopes of hydrogen fuse, conserving the total number of nucleons, but the resulting hydrogen has a lower rest mass than the parent particles. The extra mass is released as energy and the total energy is conserved.
By his logic the system either violated energy conservation (by creating nucleons while releasing energy) or was endothermic (creating nucleons from the surrounding energy).