Do you weigh more or less after you fart?

Cecil's Columns/Staff Reports
1

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

Weight is defined as (in this context at least) the vertical force exerted by a mass as a result of gravity which certainly decreases after you fart. The statement that you weigh less because the gasses expelled were lighter than air is misleading - your set of bathroom scales will certainly report that you weigh less (if they are that precise), but these do not exactly measure your weight, the simply measure the net force being exerted upon them after you factor in all the other forces affecting you. In this case, the force upon the scales is more because while your weight has decreased, the uplifting buoyant force of the air you are immersed in has decreased by more.

This may be a minor technicality, but it implies that a lighter than air object would have a negative weight, and be repelled by gravity, which is of course rediculous.

2

Misread as: Do you weigh more or less after the fact?
I think this is the first time I’ve misread a header as LESS absurd than it really is. Although I did get a giggle or two thinking what “the fact” might be.

3

Ummm… Actually, a lighter-than-air object does have negative weight. You’re confusing weight with mass.

4

It depends, did the fart have lumps?

5

[QUOTE=John W. Kennedy]
Ummm… Actually, a lighter-than-air object does have negative weight. You’re confusing weight with mass.
[/QUOTE]

No, you don’t know what you’re talking about. You’re confusing weight with net vertical force. Weight is a product of the objects mass and the acceleration acting upon it (in this case, earths gravity). Hence if the mass of an object decreases, it’s weight also decreases. Weight is a force, which is derived from mass and gravity. Mass is a quantity of the amount of matter and energy an object has.

6

[QUOTE=John W. Kennedy]
Ummm… Actually, a lighter-than-air object does have negative weight. You’re confusing weight with mass.
[/QUOTE]
Ummm…No, actually, he’s not. He’s pretty clearly talking about the difference between the technical definition of weight and the nontechnical usage (let’s call it apparent weight) that includes the buoyant force.

7

zut… the fact that random5 is talking about the technical definition was made clear by the OP. John W. Kennedy was confused about something, maybe not mass.

random5, you have a quality nitpick.

8

[QUOTE=Fir na tine]
It depends, did the fart have lumps?
[/QUOTE]
If it does, the lumps aren’t going anywhere.

9

Yes, you weigh more or less the same.

10

Now, wait a minute Random 5. Do you know the density of fart gases?

If the gases that comprise the fart were lighter than air, when they were expelled, you would have more unit mass and would apparently weigh more. Think of an analogy of holding a helium balloon, weighing yourself, letting go of the balloon and then re-weighing yourself.

What if they’re heavier than air? Expelling a more dense gas would make you have less unit mass and then you’d weigh less. Visualize the analogy above except use a brick instead of a balloon.

11

Well, I submit farts must be lighter than air, because if one smells them, they must rise, to some extent.

Would compression in the bowel significantly affect this lifting property?

I think fart gasses have little enough either mass or lifting property that your weight is probably more affected by whether your breathing in or out, than if you’ve just let one rip.

My gut (ha ha) tells me you’d weigh slightly less after a fart, that the gasses in the bowel don’t extert any lifting property.

12

Two things to think about:

First, think of a tank of helium, with rigid sides. If you release the helium, the tank weighs less. It has less mass in the same volume.

Second, think of a helium balloon. Release the gas, the balloon weighs more. This is because volume decreases.

So, does a human body act like a ballon or a tank? I’m not certain, but I tend to think tank. Fart gasses might make the gut inflate inside the body, but does the body itself swell up?

13

[QUOTE=Peter Morris]
Fart gasses might make the gut inflate inside the body, but does the body itself swell up?
[/QUOTE]
Yes. In the Pop Rocks & Soda/Exploding stomach myth, the Mythbuster guys proved the pressure of gases in the body did not increase appreciably, the stomach just expanded. So the gas doesn’t not compress much in the gut.

14

[QUOTE=Science Geek]
Now, wait a minute Random 5. Do you know the density of fart gases?

If the gases that comprise the fart were lighter than air, when they were expelled, you would have more unit mass and would apparently weigh more. Think of an analogy of holding a helium balloon, weighing yourself, letting go of the balloon and then re-weighing yourself.

What if they’re heavier than air? Expelling a more dense gas would make you have less unit mass and then you’d weigh less. Visualize the analogy above except use a brick instead of a balloon.
[/QUOTE]

Would you read the first post, not just the first sentence after the link? Or if you did perhaps you should turn in the physics section of your ‘science geek’ title there.

In any case, I’ll try to explain it from a different angle.

Releasing a lighter than air object only makes you heavier because you’re completely surrounded by air. Sitting in the middle of a pool, and you are holding say a basketball, which is filled with air and lighter than water. You are also holding a weight heavy enough to counter the uplifting effect of the basketball (for the point of the argument you are holding both underwater). If you release the basketball, you become ‘heavier’ and sink to the bottom. If you release the weights you float up a bit higher. But on land, standing on your scales if you release either you get lighter.

But in truth, even in the water you got lighter. Why did you sink like you were heavier then? Because any fluid (liquid or gas) exerts a force called bouyancy upon anything immersed (or floating on the surface of it). This force is equal to the weight of the volume of the fluid your volume displaces, in an upwards direction. Your weight fights against this, in a downwards direction.

Lets take the case of a lighter than air object, one cubic meter in volume sitting in a whole bunch of air. Say 1m^3 of water air 2kg, while 1m^3 of helium weighs 1kg. So bouyancy on the lighter than air object pushes it upwards for 2kg (i’m going to leave it in kgs to keep it simple for everyone) while gravity pushes it downwards for 1kg. Net force: Upwards, 1kg. But this doesn’t mean the object weighs -1kg, though thats what your bathroom scales would say.

15

Oops,

should say

16

i think i just sh1t my pants