My friend and I became engaged in a debate when he made a joke about losing weight after a fart. My contention would be that if farts are composed mostly of methane, and methane is less dense than the air around us, we would become heavier after farting. Any insight would be greatly appreciated.
You’d have less mass.
But higher density.
Okay, maybe I’m missing something, but he didn’t answer the question at all.
He claimed you probably do weigh more after farting, since you lose the buoyant effect of the gas (mostly hydrogen and methane, both lighter than air).
He did claim uncertainty due to a question about intestinal pressure and a possible increase in gas density because of it. But methane’s specific gravity at STP is about 0.55, and hydrogen’s is 0.07 (both measured relative to air). Even supposing a pure methane fart, you’d have to just about double the absolute pressure in your intestine to get to a specific gravity of 1, at which point there is no net buoyant effect. That means a gauge pressure of ~14 psi in your intestine. No way that’s happening (nor is a fart ever pure methane); I don’t think your colon can take a gauge pressure of more than a few psi without causing considerable pain (enema height, measured in feet above colon, times 0.43, gives approximate pressure in psi).
Persuasive analysis. Farts probably are a little more dense than pure methane, due to larger organic molecules (especially sulfur-containing ones!), but can’t imagine it’s enough to make a difference.
But maybe we should go the obvious route and just ask Cecil to go ahead and ship the beans to M5 Industries?
Isn’t carbon dioxide a major component of farts, too? That’s heavier than air, so we’d need to know the relative proportions of carbon dioxide, methane, and hydrogen.
Wow. What a fascinating bit of formula. Is this something you knew or did you just pull it out of your ass?
Any engineer should know how to calculate the pressure at the bottom of a water column. In terms of engineering, this application may be unique, but the same principles apply.
What am I missing?
The molecules that make up a fart didn’t appear out of thin air. They were in you in one form or another and were formed by bacteria in the colon, yes? After the bacteria did their thing, you still have all the same molecules inside you, so you must still weight the same before and after the bacteria created the gas. Once you fart, you have expelled the molecules that are lighter than air and therefore you would weigh more.
That almost makes sense.
Until the last sentence.
I don’t think it makes any difference how buoyant the gas is. If you have a steel shell filled with gas under pressure, and you release some gas, the shell will get lighter, regardless of how buoyant the gas is. If 50% of the water in your body decomposed into H2 and 02, releasing the H2 is going to make you lighter, not heavier (although, I suppose it makes a difference if you ended up inflating like a balloon).
Joe Frickin Friday’s numbers show pretty clearly that intestinal gas isn’t pressurized nearly enough to make a difference.
So does this mean that your volume increases when you have gas?
You may not lose weight, but your friends will burn a few calories, running to clear the room.
This just begs the question of what solid/liquid/gas ratio a shart would have to be to be neutrally buoyant…
Ignoring the gas under pressure angle, as has been pointed out elsewhere, I’m not sure how expelling the lighter than air molecules won’t make you heavier. If I take a helium balloon floating up near the ceiling and let out all the helium, it will weigh more.
But the human body isn’t a rigid steel shell with a fixed volume. It’s flexible, and the volume is determined in part by what’s inside of it. So it’s more like letting gas out of a balloon, which can in fact increase the weight if the gas is less dense than air, because the volume decreases, giving less lift from air pressure. Think letting helium out of a balloon.
Yes, in very much the same way that your volume increases when you take a deep breath. Any SCUBA diver knows that breathing has a big impact on one’s buoyancy when underwater; anyone who has free-dived (even in a pool) knows it’s easier to stay on the bottom if your lungs are empty or only partly full.
Gas in your ass has the same basic effect, in terms of buoyancy, that gas in your lungs has. But whereas filling your lungs with air won’t change your net buoyancy while you are surrounded by air, having an assful of lighter-than-air gas will.
That’s the wrong way round. It isn’t breathing in that causes your volume to increase, it’s that increasing your volume will cause air to be sucked in.
I’m not sure if the same thing applies to your butt. Gas inside the intestines will cause them to swell like a balloon. But doesn’t it have room inside the body to expand? I’m not convinced that the body gets smaller when you fart.