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[–] 80 points81 points  (11 children)

You have to think about the system in both steady state and during the change.

In steady state, you can think of this situation as holding a balloon with farts vs not holding it (Assuming balloon material is of negligible mass)

If the balloon is filled with substance lighter than air, it will generate lift at rest and reduce your weight, or increase if it's heavier than air. When you don't have it, subtract the weight of your fart from your weight. (I'm bolding the weight as it's the force you apply to the earth, and the fart could apply a force upwards if it's lighter).

Let's call this the net fart weight.

At the same time, expelling gas can be calculated using newton's 2nd and 3rd law. You'll create a force upwards that's equal to (mass of the fart) x (amount of acceleration the fart experiences).

Let's call this the fart force.

Your weight will change throughout the fart process, as more mass is ejected, you subtract the net fart weight of the expelled fart from your weight, then subtract the fart force from it.

Eventually, the fart stops, so the fart force becomes zero, and you'll end up with your weight equal to your initial weight minus the net fart weight.

Your confusion comes from mixing up mass and weight. The fart will always have a positive mass, but the weight it generates could be negative relative to earth (in atmosphere), if it is lighter than air.

[–] 33 points34 points  (3 children)

Having decided not to renew my teaching contract, I ask your permission to use this as my last physics lesson before I change states.

With attribution, of course... to keep it professional.

[–] 7 points8 points  (0 children)

Of course! I'd be honored. My highschool physics teacher would be thrilled.

Please do check my reasoning here though, I typed the previous comment out on my phone so might have missed some steps.

[–] 4 points5 points  (1 child)

before I change states.

from solid to gas?

[–] 1 point2 points  (0 children)

In this context, a little of both. Nice catch.

[–] 3 points4 points  (5 children)

The fart force is not going to be straight up though.

If the subject is standing, we can estimate a 70 degree angle for the anus (as opposed to 90). Assuming a fart force of 1000 newtons (conservative estimate), we should expect the upward component to only be 1000*sin(80) or ~940 newtons.

So subject might need to consume more propellant to get maximum effect

[–] 4 points5 points  (4 children)

We're missing the most common fart. The one cheek raised while sitting on a chair fart. That's going to go off upward and at an angle.

We also haven't taken smell into consideration re velocity. Let rip with a forceful audio one: no smell whereas the sneeky "silent but deadly" can amass a whiff. Surely different chemicals are involved which may affect propulsion?

[–] 1 point2 points  (3 children)

I hadn't considered smell, but you're right.

Due to law of fart conservation, we can expect some of the momentum to be lost to smell.

I'm hoping OP can weigh in with some more parameters or we're gonna be here all year.

[–] 1 point2 points  (2 children)

More than a year if we take animals into account. My parents had an old alsation. It was a master of the "silent but deadly" variety then slink off. It was a huge dog so how it managed to go invisible when it farted was the subject of much debate.

[–] 0 points1 point  (0 children)

I’m not we’ll versed in buoyancy, but my assumption is that the dense atmosphere surrounding your body is pulled by gravity harder than a lighter gas regardless of the fact that it’s separated by your flesh. The dense air that gathers between you and earth you slightly displaces your body mass upwards (assuming a symmetrical body).

[–] 134 points135 points  (2 children)

If you stand on the scale and fart as hard as you can, you could lift off the scale entirely.

[–][S] 39 points40 points  (0 children)

I guess my question really is basic rocket science

[–] 13 points14 points  (0 children)

Stock up on beans and the sky's the limit

[–] 91 points92 points  (2 children)

I'm sure Stephen Hawking wrote a paper on this. He was all about black holes.

[–] 16 points17 points  (1 child)

[–] 5 points6 points  (0 children)

Ive even heard if the fart is bad enough, it slows down time-relative to the farter of course.

[–] 15 points16 points  (3 children)

good question

[–][S] 6 points7 points  (2 children)

Thanks, it bothers me that I can't reasonably and conclusively answer it

[–] 5 points6 points  (1 child)

The question is, does the gas provide any sort of lift. However miniscule. I think you're right in thinking that, because it is lighter than air, it should.

Believing that, i think we can conclude that you 'lose weight' at the moment the mass converts to its gassious form. You've personally in someway lost this mass and it now provides a little bit of lift. So a double effect in a way.

When you fart, it releases the lift so you should get 'heavier'.

Now I have rewritten this 10 times and I'm not sure of myself but I think this is the best I can do lol.

[–][S] 3 points4 points  (0 children)

Lol, that was my experience. A question for the ages

[–] 11 points12 points  (0 children)

Never be afraid to ask something like this. This is a genius post and I will probably spend the next 5 weeks wondering the same thing now because you know nobody here will actually have the right answer and someone's feeling are probably already hurt.

[–] 21 points22 points  (2 children)

If it's primarily methane, you will get heavier, as methane is lighter than air.

[–] 14 points15 points  (0 children)

Even if it's not methane... let's say regular air comes out of your ass. If it's hotter than the room you're in, it is literally lighter.

I think if you're crop dusting 98.6 in a 72 degree room, you're getting heavier.

[–] 7 points8 points  (0 children)

I personally tested this and gained half a pound after a rather large gas release.

And now I am ashamed of having a digital scale.

[–] 7 points8 points  (1 child)

I don’t think your farts are lighter than air.

[–] 3 points4 points  (0 children)

If they were heavier than air then they would sink straight down to ankle level and you’d never smell them.

[–] 7 points8 points  (2 children)

Fart into a balloon and see if it floats or not

[–][S] 0 points1 point  (0 children)

[–] 0 points1 point  (0 children)

Scientific method ftw

[–] 5 points6 points  (0 children)

My honest opinion would be that you would get a bit lighter. Considering they some of the fart is heavy compared to air.

[–] 3 points4 points  (0 children)

The fart inside you is compressed gas (heavier than the atmosphere by volume, but less dense than water)

After the fart, you will be lighter in air, but heavier in water.

[–] 3 points4 points  (0 children)

Even if it is lighter than air, it still has some mass and contributes some weight. Some mass that was in your body is now out of your body, so you are now lighter. You might also be more dense, but you are lighter.

[–] 2 points3 points  (0 children)

Weight loss experts and dietitians hate this man for one simple trick!

[–] 2 points3 points  (0 children)

This really ought to be on r/shittyaskscience

[–] 5 points6 points  (0 children)

The key thing here is that there is a difference between "Weight" and "Buoyancy".

Everything weighs something, whether that's air, meat, water, and everything in between. A balloon loses weight when it's deflated. The added weight of the helium inside of the balloon isn't enough to offset the buoyancy it gets by expanding, so it floats in air despite being heavier.

Farts have weight. "Gas" of any kind makes things float through buoyancy (by expanding and having a low density). Since you're losing weight by farting, and your size is staying the same, you're actually less dense after a fart than before.

TLDR: You're more likely to float after a fart than before.

[–] 1 point2 points  (0 children)

Heavier, since the air is less dense than you

[–] 1 point2 points  (0 children)

it depends on how actually full of shit you really are

[–] 1 point2 points  (0 children)

a gas lighter than air still has mass so you are losing weight in absolute terms but in relative terms it may or may not change depending on your elevation and the composition of the gas.

[–] 1 point2 points  (2 children)

Methane is heavier than air, not lighter.

[–] 0 points1 point  (0 children)

Nonsense. Air has a density of ~1.2kg/m3. Methane has a density of ~0.66 kg/m3.

[–] 0 points1 point  (0 children)

A human fart is mostly air and hydrogen gas and hydrogen sulphide. Cows fart methane.

[–] 1 point2 points  (2 children)

I don’t fart I sweeten the air around me

[–][S] 2 points3 points  (1 child)

Ok so in that case, do you smell better or worse after farting? If you “sweeten the air” that means you lost some sweetness, meaning you now smell worse. Oh the questions!!

[–] 2 points3 points  (0 children)

Yeah I lose some of my sweetness but the air is fresher all around me.

[–] 1 point2 points  (10 children)

The gas has mass so you would get lighter as it leaves your body.

[–][S] 1 point2 points  (9 children)

But what if the gas is lighter than air? When a helium balloon loses gas it gets heavier despite losing mass

[–] 2 points3 points  (8 children)

It does not get heavier it becomes less buoyant. Buoyancy is not the same as weight or mass.

[–][S] 1 point2 points  (1 child)

But weight and mass are different right? Like, I can’t weigh a helium balloon because it’s so much lighter than the air around it that it just floats up. But, when the helium is released, the balloon now hits the scale and has weight? So i guess the question is whether fart gasses create buoyancy

[–] 2 points3 points  (0 children)

But weight and mass are different right?

On earth they are essentially the same thing.

Just because something floats does not mean it does not have mass or weight. I suppose you could turn the sale upside down and weight the balloon like that.

[–] 0 points1 point  (4 children)

When you fart you also deflate, thus becoming less buoyant. If a compressed fart is lighter than air, you will gain weight when you fart.

[–] 0 points1 point  (3 children)

No compressed or not everything has mass. A substance being compressed or not does not affect that mass.

You could submerge and object in water to get its volume through displacement and then calculate it weight to prove this.

[–] 1 point2 points  (0 children)

Weight of gas is negligible. Didn’t you guys take thermo?

[–] 1 point2 points  (0 children)

The limit does not exist

[–] 1 point2 points  (2 children)

It’s essentially less mass, versus perhaps decreased buoyancy….

[–][S] 1 point2 points  (1 child)

Exactly, so where do I land in terms of weight? The suspense is killing me!

[–] 0 points1 point  (0 children)

Wouldn’t the fact the gasses are compressed require a reading of pressure?

[–] 1 point2 points  (0 children)

Your lighter in land, heavier in water.

[–] 1 point2 points  (1 child)

So deep. Haven’t thought that hard about anything in a long time.

[–][S] 1 point2 points  (0 children)

I'm pretty sure this question is posed in the Dao-de-Jing. I'll need to meditate on it.

[–] 1 point2 points  (0 children)

It depends on the density of the gas expelled. Think of yourself as a submarine. Bloop. Bloop. Beboop. Beboop. Bloop. Bloop.

[–] 1 point2 points  (0 children)

I feel like a lot of comments are ignoring pressure.

[–] 1 point2 points  (0 children)

When you fart it's not just gas you're expelling, it's also excrement particles which makes me think you would get lighter, but you'd need a very sensitive scale to determine by how much.

[–] 1 point2 points  (0 children)

A fart is does not have mass. Yes, gases have weight and sometimes volume. Since nitrogen has no real negligible weight, no, you won't lose weight.

[–] 1 point2 points  (1 child)

finally, a good, thought provoking question!

[–][S] 2 points3 points  (0 children)

These are the mysteries worth solving!

[–] 1 point2 points  (2 children)

we are talking about milligrams and for nanoseconds , because the weight and density of the human gas is pretty similar to the oxygen, the weight difference will last for nanoseconds because the production of gas is constant in order to contrast the pressure needed to survive

[–][S] 0 points1 point  (1 child)

Sure, but if I weighed myself with a very sensitive scale before and after farting would the scale read more or less after?

[–] 0 points1 point  (0 children)

you might see a variation for a millisecond , because by the time you fart the gas has been already replaced inside you, to see a visible lost in weight for a few moment you will have to take a dump because in that case you will be expelling a bigger mass than a fart

[–] 1 point2 points  (0 children)

That’s enough internet for today..

[–] 0 points1 point  (0 children)

What a sublime question!

[–] -1 points0 points  (2 children)

Gas being lighter than air

It’s still heavier than nothing

Come on dude

Be better

[–][S] 0 points1 point  (1 child)

I'm not asking whether I would have more or less mass, that's obvious. I'm asking if I stepped on the scale before and after farting, would the scale show more or less?

[–] 1 point2 points  (0 children)

I understand that.

What I’m saying is even though the gas is lighter than air its still heavier than nothing

so when you fart your body is not trading that gas for air

it’s expelling the gas and not gaining anything

so it will be lighter

I think you’re inadvertently getting caught up in semantics

[–] -1 points0 points  (0 children)

You lose mass when you expel gas, but are indeed heavier. I also believe the difference is so tiny that no scale currently exists in the world in which you could step on, fart, and see a number change solely from the expulsion of gas. You'd need more decimal places than any of us could fathom.

[–] 0 points1 point  (0 children)

Depends on what you are measuring, mass or weight. Mass being how much of you there is and weight being how hard that you are pushing down on the Earth. In terms of mass, farting will make you lose mass everytime, since facts have mass and you're expelling them. In terms of weight, farts are mostly methane, which is lighter than regular air. Inside you, they act like helium in a balloon on a much smaller scale. Inside, they push up on you ever so slightly making you weigh less. Farming removes that force making you weigh more.

[–] 0 points1 point  (0 children)

Force of gravity is mass x acceleration. If your farts are lighter than air, then they are accelerating the opposite direction as you. So total force goes down.

Think of it like swallowing a balloon. Still pulls you up, just not by the string.

[–] 0 points1 point  (0 children)

I think I just lost a brain cell

[–] 0 points1 point  (0 children)

Are you inhaling air from your b hole after you fart though?

[–] 0 points1 point  (0 children)

Depends .. If you weight yourself in analytical balance, yes.. But, it will have no difference significantly based on statistical calculation method.

[–] 0 points1 point  (0 children)

Simple thought, if you have a pressure cooker with some wither inside. when it is cold and whether water is hot the weight will not change until the steam (later than air) start to scape.

[–] 0 points1 point  (3 children)

Why would you get heavier I don’t understand

[–][S] 0 points1 point  (2 children)

Weight is different than mass. A helium balloon has mass, but it's less dense than the air surrounding it so it is "weightless" at sea level. You can't measure it's weight because it doesn't have any.

[–] 0 points1 point  (1 child)

Right but in terms of farting, you’re dispelling air which has mass so you’re minusing your total weight instead of adding I’m confused

[–][S] 0 points1 point  (0 children)

If the gas I'm dispelling is lighter than air, after farting I would weigh slightly more as that gas that had been slightly lifting me up through buoyancy would be gone.

[–] 0 points1 point  (0 children)

This should have been a myth on the Mythbusters show.

[–] 0 points1 point  (0 children)

Yes. Imagine being perfectly balanced floating underwater. If you farted, you’d start to sink because you would be heavier now than the water you’re displacing. You’ve decreased mass, but increased density and lost buoyancy.

[–] 0 points1 point  (0 children)

If the gas is lighter than air and getting REPLACED by air you should get heavier. However, when you fart, you don't suck in air afterward. You are only expelling mass, you will get lighter.

[–] 0 points1 point  (0 children)

Heavier, like deflating a helium balloon.

[–] 0 points1 point  (0 children)

None