Food Mass

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Generic Goon
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Food Mass

Postby Generic Goon » Fri Sep 04, 2009 4:40 pm UTC

Maybe someone on these forums can help me out. I was wondering, when your body turns food into energy, is mass lost or does it stay conserved? I ask because I have heard that a significant amount of the mass was turned into energy, but I have also heard that even if you get energy from a chemical reaction, you only actually lose mass after nuclear reactions.

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thoughtfully
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Re: Food Mass

Postby thoughtfully » Fri Sep 04, 2009 5:08 pm UTC

There isn't a seperate conservation law for mass and energy, there is only one. Mass is energy. They are the same thing, expressed in different units. (the conversion formula: E=mc2) Unless its a truly humongous amount of energy, you won't notice the loss of mass, but it's still there.
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douglasm
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Re: Food Mass

Postby douglasm » Fri Sep 04, 2009 5:10 pm UTC

Mass is lost in the production of energy by the body, but it is such a tiny amount that even the best equipped labs on Earth probably wouldn't be able to measure it.

Essentially, any process that produces energy, no matter what its nature, consumes mass. No matter what its nature, the amount of mass consumed is governed by Einstein's famous equation, E = mc2. The only thing remarkable about nuclear reactions compared to chemical reactions in this respect is the sheer amount of mass converted and energy produced.

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thoughtfully
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Re: Food Mass

Postby thoughtfully » Fri Sep 04, 2009 5:20 pm UTC

Mass isn't "consumed", the entire system will weigh the same before and after whatever reaction being considered occurs. You have to count the escaping energy, just as you would escaping gasses.
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Re: Food Mass

Postby Tass » Fri Sep 04, 2009 8:40 pm UTC

But of course way more mass is lost the regular way in the form of breathing in O2 and breathing out CO2, (plus evaporation of water).

These will dwarf any mass given of in the form of energy.

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Re: Food Mass

Postby Yakk » Fri Sep 04, 2009 8:47 pm UTC

We can extract about 8 kiloCalories of energy from a gram of fat.

This comes to 3.72426227 × 10^-13 kg, or a fraction of about 1/3 * 10^-9 of the mass of the fat.

Now, our body doesn't store this mass as "raw energy" (there is no such thing), but rather in the chemical bonds of other chemicals in our body, or just by storing the fat itself.

So chemical reactions can change the mass of a chemical, but it is such a small amount that we cannot detect it practically -- we only know it happens because it would be a very inconsistent world if it didn't!

What our body does with food is it uses the chemical energy in it to build up local energy sources (and sometimes just breaks it down into raw materials, then imports those raw materials). We then store it in proteins, fats, sugars, and temporary energy storage molecules of various kinds. When we use energy, we break down those chemicals in order to fuel ourselves.
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Re: Food Mass

Postby thoughtfully » Fri Sep 04, 2009 8:57 pm UTC

Death, taxes, and entropy. The 2nd Law takes his cut, too. A lot of that energy becomes heat.
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