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Old 31st December 2018, 08:01 AM   #42
Roboramma
Penultimate Amazing
 
Join Date: Feb 2005
Location: Shanghai
Posts: 16,041
Originally Posted by kellyb View Post
How does that work?
I actually think it's wrong as he stated it, but something like it is true.

He said:
Quote:
If you were to disassemble a man into his constituent atoms and then weigh those atoms, they would be about the same weight as the man. But if you disassembled the atoms into protons, neutrons and electrons and weighed each of those, they would only weigh about 5% of the guy.
The mass of an atom is very close to the mass of the protons + neutrons + electrons. The binding energy isn't that high compared to the masses of those particles, so while it's a measurable portion of the mass, it's certainly not 95% of it. Actually, thinking about it the mass of, say, Helium is less than the mass of it's constituent particles, which is why you can get energy out of the fusion of hydrogen into helium. And each element up to Iron has less mass/particle than the ones before it (which is why you can keep getting energy out as you go up the periodic table up to Iron), so if anything the elements in your man will weigh more when disassembled into their component particles, not less. That extra mass is the energy that it took to disassemble them.

However, something like what he said is true, not of the atoms, but of the nucleons: if you were to disassemble the protons and neutrons (somehow?) into their constituent quarks, on the other hand, then you'd find that most of the mass was missing. Because the mass of a proton really is mostly in the form of the binding energy (the gluons I guess), and the quarks' mass makes up a small proportion of the total.
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