Do you weigh more or less after you fart?

http://www.straightdope.com/classics/a2_380.html

I’m not at all convinced by Cecil’s answer. Hydrogen and methane may rise in air, but they still have weight. Losing gas still means you lose weight. Doesn’t it?
Undigested vegetable matter in your intestines weighs a certain amount. Bacteria acts upon it, turns part of it into methane and hydrogen. Surely the gas plus the remaining matter weighs the same as the original matter. Or am I wrong?

Cecil’s answer makes perfect sense. Weight and mass are two different things. A helium balloon has negative weight, but it still has mass. Let helium out, and the balloon has less mass, but weighs more.
A tank with helium under high pressure on the other hand, will lose both weight and mass if you open the valve.

Methane is lighter than ‘air’ so the question is what kind of pressure it is under in the intestine. Are humans like balloons or like air tanks?

I’m guessing air tanks, but that’s a truly WAG.

You are ignoring the bouyancy of the gas in the intestines. Even to a microscopic degree, if it is less dense than the air surrounding you, it lifts you up ever so slightly, reducing your weight. In the extreme, it makes a balloon float up and away. In your case, you simply feel lighter on your feet.

I see no reason why, inherently, gas in the intestines is under pressure. This might be true if your intestines were a closed system, but they are not. They have an opening at one end, out of which you release the gas naturally. They also have an opening back into the stomache, which admittedly doesn’t like the process of letting things back in, but it is doubtful the valve would withstand much pressure from gas. Of course, since this is guesswork, it might be wrong.

It also depends on what, precisely, one means by “weight”. One definition would be the amount of force gravity exerts on a body. In this case, both the helium balloon and the flautus assuredly have positive weight. Another definition would be the amount of normal force required by a surface to support the body, i.e., what a scale would read if you put the body upon it. By this standard, a helium balloon assuredly has a negative weight, and the flautus within the intestines possibly does.

I think it safe to say that the hydrogen and methane in the flautus are not sufficiently compressed to equal the density of air: Methane is only just over half the molecular mass of air, and hydrogen is much less than that, so the pressures needed would be over twice atmospheric pressures (neglecting the difference in temperature inside and outside the human body, which would make the situation even more extreme). A pressure difference of a single atmosphere within and without the body is quite sufficient to cause significant damage: This is the same pressure difference a person would feel if abruptly exposed to hard vacuum. However, methane and hydrogen are not the sole components of flautus, carbon dioxide (which is heavier than air) being another major component, as well as traces of aromatic organics, which are much heavier still. Without knowing the relative abundances of these various components, it is impossible to determine whether flautus is lighter or heavier than air.

Given the ambiguity of the term “weight” (as cogently explained by Cronos), we might prefer to say that after a fart, the body is slight less massive, but slightly more dense than it was before.

My weight is what shows on a scale.

What’s up with the typos? I must say that, given the subject matter, “arsies” is a pretty funny one.

I didn’t pay attention to my chemistry teacher, but… Does the fact that it’s rarely pure methane, but a mixture of methane, other gases and air that’s passed all the way through, have any effect. I know fancy terms like partial pressures and boyle’s law, but I don’t really know if they make any difference here. (Well, actually I know Boyle’s law isn’t of interest… but I needed two fancy terms.)

Chronos addressed the internal part of the equation…

One simple way to determine whether a given dose of flatus ( a flatii??) is lighter than air outside of the body would be to observe its effects. Noting that, for the most part, the gasses in question (hopefully they are gaseous) are emitted below nose level and further noting Fart Law (specifically: “the one that smelt it, dealt it”) it would seem that the gasses in question do immediately rise once released.

It does make me wonder, how do those gasses pass down into the lower intestines? Peristalsis?

Is it possible that, when I get into the mood and light my farts, I can convert the flame into BTU’s? Then I can submit the results to Mathochist for further analysis (don’t ask me what analysis) And I’ sure he will have certain comments. :wink:

Probably something to the effect of a nice burn cream for your cheeks… :smack:

At least some portion of the released gas does rise, but this could be due to simple diffusion, which would occur regardless of the density of the gasses involved. The Fart Law would then be a consequence of the fact that the closest nose to one’s own orifice is one’s own. For a reliable conclusion, one would need to compare the time needed for the odor to rise and to decend by an equal distance. But one does not often have noses below the level of one’s posterior, so less data are available for this case.

It always makes me feel lighter. :smiley: