Does an Airplane get lighter after dinner is served?

You’re on a long transcontinental flight. The galley is filled with trays of food that will soon be heated and served.

Will the plane weigh less after serving 150 people dinner?

Some of the food we eat dissolves in our mouth. Food in the stomach is digested. Some turns into stinky waste.

Lets assume the flight is long enough that digestion has time to occur.

How much of the foods initial weight is lost after that many people eat? 10%, 20%? Zero?

None, it’s a closed system.

Sure, because it takes some energy to heat the meals. That energy comes from liquid fuel that is burned and exhausted from the plane.

I’ll leave the food issues (noshing, digesting) to others.

If you eat sugar, and it’s converted to energy before you end your flight, then it can escape the closed system.

Zero. The mass of the food is still on the airplane. Food that’s digested, dissolved, or turned into waste just changes form. Its mass doesn’t disappear.

*Not counting the fuel burned to heat the food, or the very small amount of food mass converted to energy during the flight.

Yes. The plane will be lighter by the amount of fuel that it burns during dinner.

But none of the food’s weight is lost untill people deplane and the plane is serviced.

Or unless a chunk of the blue ice from a leak in the lavatory system breaks off the plane.

The plane is not a closed system. Dry air is constantly being brought in and warmed while moist air is being expelled. In addition to that CO2 from metabolism is also being expelled.

Um the whole point of digestion (from the point of the human body) is to produce energy to use, some of it for basal energy.

We know that an average human uses x kaloric joules per hour in rest mode which are equal to y joules which can be converted into z grams of food (depending on kind of food).

We also know that waste products of digestion are CO2 and water vapour, some of which might get expelled during the AC system (an airplane is not hermetically sealed AFAIK).

So we could distract w% of waste during digestion and the basal amount of calories times the hours people are on board, and see if this is big enough to make an effect.

On a transcontinental flight (or any flight) in an airplane, the airplane is constantly decreasing in weight as fuel is burned. This occurs whether dinner is being served or not.

The waste products of cellular respiration (not digestion) are CO2 and water. This is happening whether you eat dinner or not. You could fly from LAX to Narita without eating or drinking a thing, and you’d be exhaling CO2 and water vapor the whole time.

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.

The Master speaks on this exact topic.

“beanbrain,” “Put a sock in it,” “you slime”… is it any wonder that the tone that’s set around here sometimes partakes of insulting or abusive language? I should probably take this to ATMB, and I will.

No, it converts energy. It doesn’t produce it. The sun does that. Photosynthesis traps that energy. From there on out, it’s just changing form.

Anyway, big jets burn about 400 lbs of gas per minute while in cruise. There’s no way the passengers are shedding anywhere near that much weight through heat loss or (pulmonary) respiration. If they were, you’d lose 2,000 fat people by the time the in-flight move was over.

Doesn’t waste from the toilets get ejected in-flight any more? Or was that just something I believed as a kid?

they just let people hang their butt out the window. the high velocity air stream cleans them off too.

leaks of the system has had blue ice form and fall.

that happening and popular culture claiming it was like trains dumping their load probably generated the idea.

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.

Think of it this way. Stand on a scale, holding a sandwich. Look at your weight. Stuff the sandwich in your mouth. Look at your weight. Chew the sandwich. Look at your weight. Swallow the sandwich. Look at your weight. Every reading will be the same. If you had a sensitive enough scale you could detect yourself getting slightly lighter every time you exhaled, because the CO2 and H2O you exhale mass more than the O2 you inhale. Plus you’d see weight loss through insensible persperation as you evaporate water from your skin and mucous membranes. But those tiny changes aren’t affected by the act of chewing and swallowing a sandwich, they go on whether you chew and eat or not. The only time you’d see a change is if you shit or pissed or spit or vomited or expelled any other random bodily fluid off the side of the scale.

If you put someone in a sealed can with some food and put the can on a scale, you’d see no change in mass even from breathing and persperation, because the gasses wouldn’t be able to escape the can. The can would weigh the same as the person ate, drank, exhaled, pissed, shit, sweated, jacked off, suffocated, died, and decomposed.

I said “energy to use” - the energy stored in a gram of fat is not-usable by the body, only the energy stored in the bonds of the ATP-molecule is usable.

I’ve heard that it used to be, but is no longer. The waste is stored and sucked from the tank at the airport, to be properly dealt with in a real sewage plant.

The problem was not the danger of occasional ice comets striking people, but more that dumping untreated shit and urine regularly into the air is kind of bad for the enviorment, mkay?, and major airports being near major cities have the equipment for treating shit properly.