/sci/ - Science & Math

a little more than 1.5kg, since there's also air in the jar

>a fly weighs 100 grams
unphysical situation. don't care

>>15123250
Diagram not drawn to scale. There could be 30K flies in the jar, and the nature of the problem doesn't change.

>>15123232
It's 0.5kg because flies exert 0.5kg of negative pressure on the lid due to flying

>>15123232
>weighs
>grams

Oooof

Flies displace air to fly.
The scale measures weight and not mass.
Therefore 1.5kg approximately. If the flies were suspended by strings the reading would be the same.

>>15123232
wtf do you need 500g of flies for

>>15123232
the problem is not gauge invariant

>>15123232
1.5kg

Here's a less retarded question: if i stand directly under a flying helicopter will i be crushed by the air weight?

>>15123617
Depends how close the helicopter is to your head. I.e. the helicopter propeller exerts a force that is at least as large as the weight of the helicopter. Luckily all the surrounding air 'mitigates' this force so it is not applied directly to you.

>>15123232
Should read 1.5, at least if the flies are hovering in place. If they let themselves fall it’ll dip to1, until they hit the floor of the jar, then it’ll be 1.5 again

>>15123617
>if i stand directly under a flying helicopter will i be crushed by the air weight?
The answer is obvious and you solved OP's problem, contrary to every other retard ITT. Good job.

>>15123617
The jar is a closed system. The impulse of all air must be 0.

>>15123712
So?

>>15123232
The scale will read 1.5 kg. because the flies will generate an airflow counteracting the weight.

>>15123724
>the flies will generate an airflow counteracting the weight.
So?

>>15123239
it’ll be a little less than 1.5kg; the buoyant force on the jar (assuming the jar is in an atmosphere with the same composition as the air inside the jar) is equal to the weight of the air that the jar displaces, which is the weight of the air inside the jar, plus a little extra, since the jar displaces slightly more air than it contains (since the walls of the jar presumably have some thickness). this is of course assuming that the buoyant force on the jar isnt included in the 1kg figure given.

>>15123730
Take a jar. Put it on a scale, set to zero, bring out your hair dryer, and blow into the jar, see what happens.

>>15123739
So? Are you actually retarded?

>>15123232
|| ||||

>>15123617
Helicopters mainly fly through Bernouilli's principle, not the physical thrust of air

>>15123717
This means (in my understanding) the weight will not change based on the air flow in the jar but only based on the displaced air by the flies and by the jar. This means the analogy of a helicopter changing the weight (force) measurement would only be right, would you allow a counteracting force of air against the atmosphere. The jar must be open to "weigh" air flow.

>>15123739
This misconception is basically what I was trying to mention.

>>15123766
A force equivalent to the weight of the flies is distributed across the jar as pressure forces. Why would you assume the pressure forces on the bottom should cancel out with the ones on the top? It very obviously won't be 1.5kg, as the other anon's helicopter example clearly illustrates, but it will be more than 1kg.

>>15123766
If the impulse must be 0, how can the flies even get off the ground? The flies try to push air downward, creating a pressure difference, which pushes them downward again

>>15123807
Air travels down while the force exerted by the air on the wing is what makes the fly go up.

>>15123816
Yes, but in his model, the force of the air ripples through the rest of the container and pushes up on the lid with equal force. It should then follow that it also pushes down on the fly with the same force.

The counterargument is that the fly creates a permanent pressure differential throughout the container, with a net downward force on the jar. But at the moment, I'm not quite sure what the correct answer is, so I just want to see a better defense of the 1kg argument.

>>15123766
As another thought experiment, imagine the container is a vacuum, but each fly has a ball. The flies throw the balls down with enough force to keep themselves propelled off the ground. The balls bounce off the ground and then return to the flies, who catch the balls and then repeat the process to continue floating.

What does the scale read? What is the impulse on the jar? How do we get from that case to the "continuous" air case?

>You WILL eat the bugs

>>15123848
The air should lose momentum due to friction between the container and the air and friction between the air and itself.

>>15123848
You are correct. My mistake was interchanging momentum and impulse (i am german).

I was talking about air momentum being the same (which is irrelevant) while you were misunderstanding me to claim that the air forces acting on the jar would stay the same.

The air force does not stay the same as the flies act with a force on the air which transfers this to the jar floor. Before starting the flies were directly pushing on the floor.


I think that was my error (do you agree)?

>>15123232
the scale will read 1 kg. its based on the normal force exerted on the scale, which is only dictated by the jar. the flies don't exert any force on the jar therefore none on the scale. (technically speaking they do exert a certain force - pressure - but it's so weak that it becomes negligible).

>>15123922
See thought experiment
>>15123859

>>15123232
1.5kg from the downward thrust of the fly

>>15123232
Interesting question, have a bump

>>15123617
If all of the force generated by the propellers was concentrated onto an area equivalent to the top of your head, probably.

>>15124320
Wrong.

Are flies dense enough to fit 500g worth in a jar weighing only 1kg? If memory serves me correctly, a 16 oz mason jar without the lid is 271g, so half gallon is roughly the jar size and the weight of a housefly is about 15mg, so its gonna be 35k or so flies in the jar, but i don't know if they're dense enough to fit allowing for room for each of them to work their wings.

>>15123232
https://www.youtube.com/watch?v=N0IGrSjcBZs

>>15123617
Lets take a Huey as an axample of a helicopter.
Mass 4000 kg
Rotor diameter 14.5 m
That makes 24 kg/m^2, so no, you will not be crushed.

>>15123232
Someone should try this experiment. The hard part is finding 0.5 kg of flies, of course.

>>15125919
>The hard part is finding 0.5 kg of flies, of course.
The hard part is stuffing 12,000 flies into a single jar.

Replace the flies with a single small drone, keep it flying in the jar and see how it goes.

>>15125934
The aerodynamics of 12,000 flies is different.

>>15123232
I read most of the answers and they're all wrong.

That many flies flying probably wouldn't have significant impulse variances. 1.5kg all the time whether all the flies are landed or flying and also regardless of whether the jar is enclosed or not.

>>15123250
There are actually scales precise enough to measure difference of a fruit fly.

Despite all my rage I'm still just a fly in a jar

>>15125865
Gallon is about 3.8 million cubic mm. Approximate fly as a sphere of radius 1 mm with a volume of about 4 cubic mm. You can fit 35k flies in there for sure. Kind of a mind fuck, and I feel like I just NDT'd myself.

>>15123232
It's, on average, 1.5 kg (depending on the flight of the flies, the value might fluctuate a bit, but it will average 1.5 kg, as the system can be treated as a closed one, with a zero-sum force balance) .
If you remove the lid, the further away the flies go from the bottom of the jar, the closer the value read by the scale goes to 1 kg.

>>15125865
>Approximate fly as a sphere of radius 1 mm
they're already larger than that with their wings folded, whats their wingspan? use wingspan as the diameter of the sphere.
NDT can't do math and he doesn't have enough meta-cognitive thought to ever second guess himself and correct his own mistakes before they go embarrassingly public, you're already way above his level because you did some math and double checked yourself before opening your mouth.

>>15126840
NDT as in black science man? If so, what are some of his mistakes?

1kg
>the flies wont exert their gravitational force on the scale until they attach their mass to the jar. While flying they emit no gravitational force

Assuming that the flies' elevations are not changing, the downward force of the flies wings acting against gravity should be transmitted through the air and balanced by a normal force of 0.5kg at the bottom. Therefore, in the way that a compressed air nozzle can be used to make the scale show weight even when nothing is resting on the scale, I expect that the scale will show 1.5kg.

>>15128316
> the downward force of the flies wings acting against gravity should be transmitted through the air and balanced by a normal force of 0.5kg at the bottom
I don't understand this logic. It's not like there would be a column of air going straight down. Some it will inevitably end up spreading sideways more and more, pressing against the sides of the jar instead of the bottom.

>>15123232
1.5 kg

the beat of the flies wings will make the air heavier

>>15123736
thank you for your service

>>15126399
top kek

>>15123232
>Physics thread
>anons don't know the difference between mass/weight and what units they have

>>15129200
As for the mass, it'll be 1kg,because the flies don't exert a force on the jar.