Will you weigh less on a scale if you exhale?

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Will you weigh less on a scale if you exhale?

Postby pernero » Tue Jan 05, 2010 10:27 pm UTC

No........

...right?
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Re: Will you weigh less on a scale if you exhale?

Postby Yakk » Tue Jan 05, 2010 10:34 pm UTC

PV = nRT

If your lungs are pressurised, they will be smaller in volume while having the same mass. The rest of you is mostly incompressible, so your volume drops without a mass change, which increases your weigh on a scale because you displace less air.

Of course, this will happen at nigh-undetectable amounts.

While exhaling, you are both emitting a jet of air, and have increased pressure in your lungs. The effects will be complex, and might depend on the shape of your mouth/nose and what direction they are pointing.

Why do you ask?
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Re: Will you weigh less on a scale if you exhale?

Postby pernero » Tue Jan 05, 2010 10:47 pm UTC

Lets say someone needs to weight less than 175.000 lbs for a competition. Will his chances of reaching this target increase if he exhales before stepping on the scale?
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Re: Will you weigh less on a scale if you exhale?

Postby GeorgeH » Tue Jan 05, 2010 11:25 pm UTC

Will there be a weight difference -

In the real world? No.

With spherical lungs in theory land? Sure.

Reasoning:
When you're full of air, you're more buoyant and will weigh less. The reason is that you're filled with air that is warmer than your surroundings (presuming you live in a civilized climate, anyway.) Therefore in the reduction "human"="hot air balloon" you will weigh less after inhaling.

EDIT - Completely failing at reading comprehension, I just realized you wanted to weigh less by exhaling, not inhaling. Even in theory land, the only way to do that would be to completely exhale, close your mouth, and then expand your lungs to contain a vacuum.
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Re: Will you weigh less on a scale if you exhale?

Postby lulzfish » Wed Jan 06, 2010 1:36 am UTC

Even if you were breathing helium, you could not possibly change your weight by more than a pound.
It's completely irrelevant. You'd do better by far to take laxatives or something.
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Re: Will you weigh less on a scale if you exhale?

Postby Kobayashi_Maru » Wed Jan 06, 2010 2:28 am UTC

In a vacuum? Absolutely!
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Re: Will you weigh less on a scale if you exhale?

Postby Talith » Wed Jan 06, 2010 2:51 am UTC

weight = mass x specific gravity. Do we include the air inside your lungs as you? If yes then exhaling will reduce your weight, if no then your weight doesn't change.

[EDIT]Just saw the 'on scales' bit. Vote to change the wording of the problem please :P[/EDIT]
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Re: Will you weigh less on a scale if you exhale?

Postby Kobayashi_Maru » Wed Jan 06, 2010 3:17 am UTC

Archimedes wrote:Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

When I exhale a deep breath, I release about 5L (50 cm^3) of air. So the real question is, when I exhale that much, do I get 5L smaller? More than 5L smaller? Less?

If I get exactly 5L smaller: my weight doesn't change.
If I get 6L smaller: I become 6L "heavier," because I am displacing less air and the upward buoyant force is less, but I am 5L "lighter," because I've released that much air and the downward gravitational force is also less. So overall I am 1L "heavier."
If I get 4L smaller: I become 4L "heavier," due to reduced buoyancy, and 5L "lighter," due to reduced weight. So I am 1L "lighter."

My first instinct is to say that when I take 5L of air from the atmosphere my volume increases by less than 5L, because I can sustain an internal lung pressure greater than atmospheric pressure. Considering only this, the scale would say I weigh less when I exhale. But the real answer depends on the specific structure of the lungs/chest muscles/torso, and the detailed way they all work together to expand and contract as I breathe. So I'm not sure. One way to measure this might be to submerge someone in a full bathtub, have them breath out, and measure how much the water line drops.

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Re: Will you weigh less on a scale if you exhale?

Postby Jakell » Wed Jan 06, 2010 3:50 am UTC

You can sustain a pressure higher or lower then the ambient pressure, so if you take a big breath and hold it, compressing it, you should increase your effective weight. If you take a small breath, seal your throat and try to inhale more (decreasing the pressure) you should be able to decrease your weight.
Unfortunately, (rough estimates using a tube and water here at home) you can only change the pressure a few percent, and since the lungs are only 6 out of 80ish liters (me-sized), and their full weight has only changed by a few percent, I would guess you could change your weight from 785N down to about 784.999 N, or about .0002 pounds. (or up to 785.001 N).
The physicist in me says yes, your chances would increase, but not appreciably. you would do far better cutting your hair/nails.
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Re: Will you weigh less on a scale if you exhale?

Postby gmalivuk » Wed Jan 06, 2010 5:00 pm UTC

It's actually possible to check this in a pool: if you have a lungful of air, and tense your abdomen to compress that air, you can make yourself noticeably less buoyant in the water.

However, if your throat is open and air is free to move in and out of your lungs, it'll be the same pressure (and thus approximately the same density) in your lungs as out. Which means your weight won't change.

Of course, this is only approximate, since the air in your lungs is warmer and more humid than the air around you, so it could change your weight a tiny bit.
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Re: Will you weigh less on a scale if you exhale?

Postby Krikkit_Robot » Wed Jan 06, 2010 5:22 pm UTC

You do not become more buoyant by having a lung full of air.

Yes your volume increases but so does your overall weight.

If the air does not compress the extra volume of air displaced will weigh as mush as the air you just inhaled.

But air in lungs does compress meaning that the air you intake weighs more than the air you displaced by increasing your volume. So you do become slightly heavier.

I just think of a balloon under water. An empty balloon (with no inside volume) will be as buoyant as as a balloon filled to capacity with water. But if you put more water in the balloon to the point where the rubber starts to stretch the water inside is slightly compressed and becomes slightly (very slightly) more dense and less botany.

But once again there are way too many variables in RL to make any difference.
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Re: Will you weigh less on a scale if you exhale?

Postby gmalivuk » Wed Jan 06, 2010 6:02 pm UTC

Krikkit_Robot wrote:You do not become more buoyant by having a lung full of air.

Yes your volume increases but so does your overall weight.

Yes, so the *net* force doesn't change. But that's because your mass and your volume go up in parallel, so the increased downward force from your mass balances the increased buoyancy from your volume.

I just think of a balloon under water. An empty balloon (with no inside volume) will be as buoyant as as a balloon filled to capacity with water. But if you put more water in the balloon to the point where the rubber starts to stretch the water inside is slightly compressed and becomes slightly (very slightly) more dense and less botany.

Bad example, because it's so incredibly hard to compress water at such low pressures. Air is actually a better example, if you've got a balance handy. (You need the balance because people's intuition sucks, so you need to actually prove experimentally that the air-filled balloon weighs more than an empty one.)
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Re: Will you weigh less on a scale if you exhale?

Postby Yakk » Wed Jan 06, 2010 6:36 pm UTC

A neat trick with a lung full of air and a swimming pool that is reasonably deep is to inhale, then swim to the bottom.

Your buoyancy goes down a lot near the bottom, as the water pressure compresses your lungs, reducing your volume.
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Re: Will you weigh less on a scale if you exhale?

Postby Izawwlgood » Wed Jan 06, 2010 7:11 pm UTC

Assuming the surrounding air is less humid then the air in your lungs, you will lose water. A scale sensitive to pick that up will likely be bouncing around as you shift your weight however.
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Re: Will you weigh less on a scale if you exhale?

Postby Halebit » Thu Jan 07, 2010 9:46 pm UTC

One question I've had since I took chemistry was, Does breahting cause you to lose weight? Considering your exchanging 02 for C02 (I'm assuming on a 1:1 ratio), wouldn't you be losing weight? Albeit an increadibly insignificant amount.
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Re: Will you weigh less on a scale if you exhale?

Postby Izawwlgood » Thu Jan 07, 2010 10:59 pm UTC

That's an interesting side of the question, considering the body uses CO2 to maintain blood pH.
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Re: Will you weigh less on a scale if you exhale?

Postby Kobayashi_Maru » Fri Jan 08, 2010 2:22 am UTC

Interesting point! According to Wikipedia: the atmosphere is about 21% oxygen, about 5% of the air we breathe out is CO2 (which was O2 when we breathed it in), we breathe in about 500mL in a normal breathe, and an average adult breathes 12-20 times per minute.

Now, applying the Ideal Gas Law at STP gives us: PV/RT = n = 0.0208 moles of air per breath, or 0.0044 moles of O2 (in) and C02 (out) per breath. So we lose ~.052g of carbon per breath, which amounts to 38-63 grams of carbon per hour, or 0.9-1.5 kg of carbon per day.

That seems high by at least an order of magnitude. There's no way we breath out that much carbon a day. Have I made a mistake in my figures, or are the input values from Wikipedia just BS? Can I get some corroboration?
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Re: Will you weigh less on a scale if you exhale?

Postby gmalivuk » Fri Jan 08, 2010 2:32 am UTC

Why do you think that seems an order of magnitude too high?
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Re: Will you weigh less on a scale if you exhale?

Postby jareds » Fri Jan 08, 2010 8:42 am UTC

Kobayashi_Maru wrote:Interesting point! According to Wikipedia: the atmosphere is about 21% oxygen, about 5% of the air we breathe out is CO2 (which was O2 when we breathed it in), we breathe in about 500mL in a normal breathe, and an average adult breathes 12-20 times per minute.

Now, applying the Ideal Gas Law at STP gives us: PV/RT = n = 0.0208 moles of air per breath, or 0.0044 moles of O2 (in) and C02 (out) per breath. So we lose ~.052g of carbon per breath, which amounts to 38-63 grams of carbon per hour, or 0.9-1.5 kg of carbon per day.

That seems high by at least an order of magnitude. There's no way we breath out that much carbon a day. Have I made a mistake in my figures, or are the input values from Wikipedia just BS? Can I get some corroboration?


You use the atmospheric oxygen content, 21%, to go from 0.0208 moles to 0.0044 moles, although you earlier state we exhale 5% carbon dioxide. The oxygen content is irrelevant since we don't metabolize 100% of what we inhale. So, if we use your input values that actual result will be about 1/4 of the one you gave. I have no particular expertise to know whether the input values are correct.
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Re: Will you weigh less on a scale if you exhale?

Postby Kobayashi_Maru » Fri Jan 08, 2010 12:53 pm UTC

jareds wrote:You use the atmospheric oxygen content, 21%, to go from 0.0208 moles to 0.0044 moles, although you earlier state we exhale 5% carbon dioxide. The oxygen content is irrelevant since we don't metabolize 100% of what we inhale. So, if we use your input values that actual result will be about 1/4 of the one you gave. I have no particular expertise to know whether the input values are correct.


Yes, of course. Thanks for catching that. So we lose about 216-359g per day, which seems more reasonable.

gmalivuk wrote:Why do you think that seems an order of magnitude too high?

I have no rigorous reason to think that, it just seems contrary to personal experience. That being said, I've never done the experiment myself.
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Re: Will you weigh less on a scale if you exhale?

Postby gmalivuk » Fri Jan 08, 2010 2:48 pm UTC

Do you weigh your poop? The reason I was skeptical was because I really don't know what my personal experience would tell me about how much weight I exhale daily, compared to how much I excrete in liquid or solid form and compared to how much I eat and drink.
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Re: Will you weigh less on a scale if you exhale?

Postby Yakk » Fri Jan 08, 2010 4:12 pm UTC

I suspect sweat will be harder to control for...
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Re: Will you weigh less on a scale if you exhale?

Postby Kobayashi_Maru » Fri Jan 08, 2010 4:49 pm UTC

gmalivuk wrote:Do you weigh your poop? The reason I was skeptical was because I really don't know what my personal experience would tell me about how much weight I exhale daily, compared to how much I excrete in liquid or solid form and compared to how much I eat and drink.

I reasoned that I probably only ingest about 4kg of food/drink per day. The idea that breathing out carbon counted for somewhere between 23 and 38 percent of my excretory functions seemed unreasonable.

No, I don't weigh my poop. I cited 'personal experience' above because I didn't run through these numbers until just now. Before it was simply an intuitive (read: non-quantitative) idea of about how much I eat and drink (and poop and pee) the caused me to be doubt my original result. Like I said, there was no rigorous reason.
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Re: Will you weigh less on a scale if you exhale?

Postby yusri106 » Sun Jan 17, 2010 4:04 pm UTC

yea,

The mass of air you inhaled or exhaled is actually really small and can be neglected, so the mass changes is negligible

At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m3.

amount of air can be stored in lung is about 6L = 0.006 m3, therefore the mass chnage during inhaled/exhaled is around 0.0072246 kg ! which is a very small number.

it is like holding a slice of an apple which been cut into 14 equal size...

Therefore, it can be concluded that, there is no difference occurs when you are weighting while inhale or exhale air...

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Re: Will you weigh less on a scale if you exhale?

Postby gmalivuk » Mon Jan 18, 2010 3:11 pm UTC

yusri106 wrote:The mass of air you inhaled or exhaled is actually really small and can be neglected, so the mass changes is negligible

Actually, no, it's not. Not when the question is specifically about that mass change. If the question were about making it into one fighting weight class or another based on whether you've got air in your lungs, then sure, it's worth pointing out that the air can't make enough of a difference to pay attention to. But that's not what's being asked here.
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Re: Will you weigh less on a scale if you exhale?

Postby Fume Troll » Mon Jan 18, 2010 4:13 pm UTC

Am I the only one that did this in school?

http://www.ehow.com/way_5294842_experim ... -mass.html
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Re: Will you weigh less on a scale if you exhale?

Postby gmalivuk » Mon Jan 18, 2010 10:39 pm UTC

Nope. I mentioned the balloons thing 12 days ago, if you read up the page a bit...
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Re: Will you weigh less on a scale if you exhale?

Postby Fume Troll » Tue Jan 19, 2010 12:23 pm UTC

Ah yes, begging your pardon.
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Re: Will you weigh less on a scale if you exhale?

Postby vibes » Wed Jan 27, 2010 5:11 pm UTC

Although not completely relevant given that what you're really playing with here is bouyancy rather than weight, here's another pool experiment, straight out of scuba diving and teaching neutral bouyancy:

Step 1: In the pool, take a deep breath and float vertically. You should float with your eyes just above the water. (Disclaimer - at this point, some people sink like a stone. But at least they took a deep breath just before :P ).

Step 2: Breathe out.

Step 3: Sink. Personally I don't completely sink, the top of my head is just about above water. Having just exhaled, staying under water to establish whether or not I completely sink is something of a time sensitive issue.

Anyone who's done a PADI course will also have done the flipper pivot - basically, you lie flat on the bottom and deflate your BCD (Bouyancy Control Device - the vest you wear when diving that you can fill with air on your tank to increase bouyancy or evacuate to decrease it). Having achieved negative bouyancy, you then slowly inflate your BCD until, when you breathe in, you start to lift from the bottom. Then, as you exhale, you start to sink down again. Breathe in, up, breathe out, down, etc. In effect, you're using your lungs as a secondary BCD. It's fun, honest!
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Re: Will you weigh less on a scale if you exhale?

Postby pernero » Tue Oct 23, 2012 1:54 am UTC

I asked this question (what feels like) ages ago and I seem to have gotten contradictory answers with no concensus.

The following answer makes the most sense to me:

"Neither.

To calculate the weigh of a body on earth, we have to consider two forces: Gravitational force and the buoyant force.

F_graity=m_body*g

F_buoyant=m_fluid*g

That means, the buoyant force is equal to the weigh of the displaced fluid.

Thus; the net force is proportional to the Volume to Mass ratio of a body.

If you breath out, you emit mass, but your volume decreases. Which means both forces decrease at the same rate. Thus the weigh should not change."

Is this a solid solution?
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Re: Will you weigh less on a scale if you exhale?

Postby Meteoric » Tue Oct 23, 2012 3:12 am UTC

Only if the density of the air in your lungs is the same as the density of the air around you, which is not necessarily a good assumption since it can have a different temperature, humidity, CO2 content, etc.
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Re: Will you weigh less on a scale if you exhale?

Postby Copper Bezel » Wed Oct 24, 2012 1:54 am UTC

The humidity is from you, though. You lose water and CO2 when you exhale, so there's an actual loss of mass there (taking it out of the scope of the buoyancy question on inhaling.) The pressure has to be the same as long as you hold your diaphragm (breathing manually) and the heat really could be an immediate factor, so it sounds like you'd gain just a little extra buoyancy.
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Re: Will you weigh less on a scale if you exhale?

Postby gmalivuk » Wed Oct 24, 2012 3:01 pm UTC

You've gotten contradictory answers because people have made contradictory assumptions. If you specify the temperature and humidity and how efficient this particular person's respiratory system is at replacing oxygen with carbon dioxide in the lungs, we might be able to answer the question better. As it stands now, the best answer you'll get is "it depends".
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Re: Will you weigh less on a scale if you exhale?

Postby macgyver5 » Thu Dec 06, 2012 5:51 pm UTC

Related to the balloon on a scale question:

The differences in mass of a blown up balloon vs a deflated balloon only occur because of the increase in pressure inside the inflated balloon, correct? The increase in density is due to more mass in the same place vs the outside air being more spread out. So if you took a ziplock type bag and filled it with air without allowing it to stretch then it should weigh the same inflated as empty, right? Otherwise where would the difference come from? Whether the gas is inside the bag or not its density would be the same. It wouldn't matter if the air was inside or outside since it would be applying force everywhere equally (walls of the bag, the scale itself, the ceiling, floor, etc).

All of this neglects any added humidity from inflating the bag.
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