# Does an Airplane get lighter after dinner is served? **URL:** \ **Category:** Factual Questions\ **Created:** [December 2, 2011, 4:03pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849 "2011-12-02T16:03:58Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [December 2, 2011, 7:51pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/21 "2011-12-02T19:51:45Z")
What kind of power does the in-flight cooker use anyway? I’m guessing it’s all electric, from power stored in a battery. That doesn’t get any lighter after using the power … does it? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 2, 2011, 8:13pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/22 "2011-12-02T20:13:02Z")
Technically, when a hot object cools off, its mass decreases in the process. The mass decrease is negligibly small, though, well below anything we could measure. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [December 2, 2011, 8:13pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/23 "2011-12-02T20:13:54Z")
Yes, it gets a teensy bit lighter. A battery works because chemical reactions cause an electrical current. And the reacting chemicals end up a whisper lighter at the end of the reaction because e=mc^2. So there’s a tiny difference in the mass of the battery in its charged state and in its discharged state. However, the difference would be so slight you’d need specialized ultra-precise laboratory testing equipment to measure it. A scale on a real moving airplane, even a precise one, would be subject to so many random jolts and vibrations you’d never be able to extract the signal from the noise. Or perhaps not even real-life lab equipment would be able to detect the difference. We could calculate the theoretical difference, but we might not have instruments precise enough to actually measure an amount that small. ---
**Author:** ![mnemosyne](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@mnemosyne](https://boards.straightdope.com/u/mnemosyne)\ **Post date:** [December 2, 2011, 8:47pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/24 "2011-12-02T20:47:13Z")
> [@Peter\_Morris](#): > > What kind of power does the in-flight cooker use anyway? I’m guessing it’s all electric, from power stored in a battery. That doesn’t get any lighter after using the power … does it? Basically, fuel powers the engines, which powers the generators, which power the on-board electronics and electrical devices. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [December 2, 2011, 11:43pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/25 "2011-12-02T23:43:41Z")
So, it uses a slightly larger amount of fuel when cooking? ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [December 2, 2011, 11:56pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/26 "2011-12-02T23:56:24Z")
I’m talking about a simple battery, not charged by any generator, that powers a microwave. Simply discharging the battery makes the battery every-so-slightly lighter. This is a consequence of Einstein’s equation e=mc^2. If a system loses energy, it loses mass. Mass is converted to energy. This happens even in normal chemical reactions, not just nuclear reactions. It’s just that the amount of maass converted to energy is so infintesimal that it’s nearly impossible to measure. > **[Mass–energy equivalence](https://en.wikipedia.org/wiki/E%3Dmc%C2%B2#Binding_energy_and_the_.22mass_defect.22)** > > In physics, mass–energy equivalence is the relationship between mass and energy in a system's rest frame, where the two quantities differ only by a multiplicative constant and the units of measurement. The principle is described by the physicist Albert Einstein's formula:  > > > E > = > m > > c > > 2 > > > > E=mc^{2} > > . In a reference frame where the system is moving, its relativistic energy and relativistic mass (instead of re The form... ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [December 3, 2011, 12:24am UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/27 "2011-12-03T00:24:45Z")
> [@Mijin](#): > > It’s important to appreciate that chemical reactions do not result in any mass being lost. > > If you burn a piece of wood, then somehow collect all the ashes, smoke particles and gasses, you’d find they have exactly the same mass as the original piece of wood. As **Lemur866** notes, not quite. Very, very close to zero, but not exactly zero. Even chemical reactions have a (tiny!) relativistic effect. Some of the energy in chemical bonds is in the equivalent form of mass. (It’s a little like noting that when I go for a walk, I weigh more than when I’m sitting still, due to Lorentz mass expansion. Yeah, it’s true…but the difference is so close to zero as to make no difference at all, except to us nitpickers and pedants!) ---
**Author:** ![Rusalka](https://avatars.discourse-cdn.com/v4/letter/r/ecd19e/32.png) [@Rusalka](https://boards.straightdope.com/u/Rusalka)\ **Post date:** [December 3, 2011, 1:46am UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/28 "2011-12-03T01:46:11Z")
> [@Lemur866](#): > > The act of serving dinner has no effect on the weight of the plane. Yes, the passengers convert glucose to CO2 and H2O, and those gasses are vented from the plane. However, they don’t exhale more CO2 and H2O just because they’ve been served dinner. They do it constantly, because if they didn’t they’d be dead. The passengers get a tiny bit lighter every time they breathe. And pressurized fresh air is constantly added to the plane, so the net weight of the air doesn’t change. Don’t mean to nit-pick, but don’t the acts of serving and eating dinner require more energy than simply sitting still? More energy means more CO2, etc., expelled, and a plane that weighs less. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [December 3, 2011, 2:48am UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/29 "2011-12-03T02:48:27Z")
Does anyone have a rough estimate of how much weight is lost by respiration? I’m sure it varies from person to person and probably changes with the environment but now I’m curious. I’m in a position to weigh myself on a balance that is accurate to .1 gram (100mg). Could I see a difference over the course of, say, 6 hours if I don’t take or leave any extra mass? I’ll consider an experiment if I know I can measure the results. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [December 3, 2011, 3:17am UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/30 "2011-12-03T03:17:46Z")
What if the food were cooked by re-routed energy from the engines themselves, or their exhaust heat? And this would open the system. Cooking on the engine has its adherents (see, e.g., _[Manifold Destiny](http://www.amazon.com/Manifold-Destiny-Guide-Cooking-Engine/dp/0375751408)_), where cooking times are given in mph. I have no idea how to do the conversions – including the amount and type of food to be cooked – for how to apply this method with commercial jetliners. It would be fun to figure it out though. Takers? ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [December 3, 2011, 12:19pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/31 "2011-12-03T12:19:42Z")
A pedantic person could say that extra calories are used by the chewing motion of the mouth, which wouldn’t have occurred had a meal not been served. So there’s a miniscule weight loss. And if we’re talking trans-oceanic flight, there will be a lot more than 150 meals. ---
**Author:** ![Alley\_Dweller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alley_dweller/32/430_2.png) [@Alley\_Dweller](https://boards.straightdope.com/u/Alley_Dweller)\ **Post date:** [December 3, 2011, 12:55pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/32 "2011-12-03T12:55:54Z")
The act of heating the food causes water and other volatile compounds to evaporate from the food. If you can smell the food, that means little bits of chemicals that were formerly in the food are now flying around in the air. This water vapor and these volatile compounds can be sucked out of the plane by the air circulation system. Water loss from cooking can be significant. Also, some types of foods can cause certain people to sweat more. Think spicy foods or foods to which an individual might be allergic. The additional perspiration can evaporate and be exhausted from the plane. ---
**Author:** ![Alley\_Dweller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alley_dweller/32/430_2.png) [@Alley\_Dweller](https://boards.straightdope.com/u/Alley_Dweller)\ **Post date:** [December 3, 2011, 1:02pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/33 "2011-12-03T13:02:38Z")
Eating can also lead to flatulence which is expelled into the air and sucked out of the plane. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [December 3, 2011, 1:07pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/34 "2011-12-03T13:07:48Z")
But flatulence is a lighter than air gas. If it is sucked out, it makes the plane heavier. ---
**Author:** ![Alley\_Dweller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alley_dweller/32/430_2.png) [@Alley\_Dweller](https://boards.straightdope.com/u/Alley_Dweller)\ **Post date:** [December 3, 2011, 1:47pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/35 "2011-12-03T13:47:33Z")
> [@Peter\_Morris](#): > > But flatulence is a lighter than air gas. If it is sucked out, it makes the plane heavier. Before the gas is expelled from the human body, there is a certain amount of air in the cabin. The body then begins turning the food and other chemicals that were previously stored in the body into gas. It then pumps the gas into the cabin, thereby increasing the amount of total gas (air plus flatulence) in the cabin, thereby increasing the pressure. If the pressure control system is working properly, it will expel more gas to the outside than it takes back, in order to reduce the pressure to the desired level. In other words, you start out with a mass of air in the cabin, the human digestive system pumps some more gas into the air, raising the air pressure. The pressurization system will eventually expel enough gas from the cabin to bring it back to the original pressure. As the air is circulated out, you should eventually get back to the same mixture of cabin air as you had before the flatulence was pumped out of the body. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [December 3, 2011, 3:55pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/36 "2011-12-03T15:55:44Z")
> [@Trinopus](#): > > As **Lemur866** notes, not quite. Very, very close to zero, but not exactly zero. Even chemical reactions have a (tiny!) relativistic effect. Some of the energy in chemical bonds is in the equivalent form of mass. Doh! I shouldn’t have used the word “exactly”; I’m aware of the whole binding energy and difference in mass thing. I was just trying to counter the misconception some had that energy-holding materials are “consumed” for energy. ---
**Author:** ![aceplace57](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aceplace57/32/3500_2.png) [@aceplace57](https://boards.straightdope.com/u/aceplace57)\ **Post date:** [December 3, 2011, 4:10pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/37 "2011-12-03T16:10:33Z")
I don’t doubt the physics and science. This still would be a great Mythbusters experiment. Get a plane in a hanger and on a scale. Fill with at least 150 people and stock the galley. Serve dinner and watch movies for a few hours. A few people will visit the planes restroom. See what the scale shows. It’s fun to see science put to a practical test. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [December 5, 2011, 1:07pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/38 "2011-12-05T13:07:19Z")
> [@jnglmassiv](#): > > Does anyone have a rough estimate of how much weight is lost by respiration? > I’m sure it varies from person to person and probably changes with the environment but now I’m curious. I’m in a position to weigh myself on a balance that is accurate to .1 gram (100mg). Could I see a difference over the course of, say, 6 hours if I don’t take or leave any extra mass? I’ll consider an experiment if I know I can measure the results. [The Wikipedia entry for cellular respiration](http://en.wikipedia.org/wiki/Cellular_respiration) lists the chemical equation along with the amount of energy released per mol of ATP. An at-rest human being needs [about 61 watts of power.](http://en.wikipedia.org/wiki/Basal_metabolic_rate) Between that and the cellular respiration equation, you should be able to calculate how much ATP is used per hour (and therefore how much CO2 and H2O is produced per hour). I’ll leave the math up to you, but I’ll also wager that the mass rate of your CO2 and H2O output due to cellular respiration is small compared to the amount of water your respiratory system uses to humidify the air coming into your lungs - especially on an airplane, where the air you inhale has single-digit relative humidity. Your respiratory system brings that up to 100% relative humidity, and then you exhale all that moist air. ---
**Author:** ![drastic\_quench](https://avatars.discourse-cdn.com/v4/letter/d/e36b37/32.png) [@drastic\_quench](https://boards.straightdope.com/u/drastic_quench)\ **Post date:** [December 5, 2011, 1:33pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/39 "2011-12-05T13:33:32Z")
Yes, but what if it is also on a treadmill which is in turn balanced on the head of a pin, and the guy in a bathroom flushes at the precise latitude to result in the Coriolis effect? ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [December 6, 2011, 7:10pm UTC](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849/40 "2011-12-06T19:10:18Z")
> [@aceplace57](#): > > I don’t doubt the physics and science. > > This still would be a great Mythbusters experiment. Get a plane in a hanger and on a scale. Fill with at least 150 people and stock the galley. Serve dinner and watch movies for a few hours. A few people will visit the planes restroom. See what the scale shows. > > It’s fun to see science put to a practical test. You don’t need a full airplane, for crying out loud. You can do the same experiment at home that I outlined earlier. Stand on a scale. Note your weight. Grab a sandwich. Note the combined weight of you and the sandwich. Put the sandwich in your mouth. Note the combined weight of you with the sandwich in your mouth. Chew the sandwich. Note the combined weight of you and the chewed sandwich. Swallow the sandwich. Note the weight of you with a sandwich in your stomach. The act of chewing and swallowing the sandwich does not make the sandwich disappear. It is still there inside your tummy, even though you can’t see it. But if you want to get complicated can also measure the effect of losing water from your body through exhalation and insensible perspiration. This process, however, is continual, and doesn’t change just because you ate a sandwich. You can also measure the effect of metabolism. Your body converts fats and proteins and starches into sugar, then metabolizes the sugar with oxygen, creating water and carbon dioxide. The water remains in your body to be lost through the normal processes above. The CO2 is exhaled. However, you’re also inhaling oxygen, so we’d have to measure that. Then you’re excreting urine, which is a mix of water and urea and salts and such. And you’re excreting solid waste. And spitting. And crying. And sweating. And ejaculating. And vomiting. And blowing your nose. And farting. And cleaning out earwax. And clipping your toenails and cutting your hair. So we can account for all this by putting you in a sealed container on a sensitive scale and measuring all the gasses and liquids and solids that come in and out. People have done such experiments. But an airplane is not a closed system, air is constantly coming in and going out, carrying in oxygen and carrying out CO2 and H2O and methane and whatever. But all this is dwarfed by the change in mass of the burning jet fuel. After a 6 hour flight, the weight of the passengers + food + drinks is pretty much exactly the same from the start of the flight to the end of the flight. With sophisticated controls you could measure the differences and account for everything, but in real life you can’t, and in any case the difference would be ounces. All that said, the bottom line is clear. I’ll make it all caps to make it stand out: EATING A SANDWICH DOES NOT MAKE THE SANDWICH DISAPPEAR. THE WEIGHT OF YOU AND THE SANDWICH TOGETHER IS THE SAME, WHETHER THE SANDWICH IS IN YOUR HAND OR IN YOUR TUMMY. [Previous page](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849.md?page=1) [Next page](https://boards.straightdope.com/t/does-an-airplane-get-lighter-after-dinner-is-served/604849.md?page=3)