(Bell?) 212man

Your analogy again has a subtle flaw - a helium balloon is an aerostat, not an aerodyne. There is no action or reaction on the air from an aerostat (static), whereas there is from an aerodyne (dynamic).

No action or reaction on the air? that's novel, so what's supporting the balloon? When you lie in a swimming pool doing starfish impressions, is there no hydrostatic force acting on your lower body to counteract the weight?

Dragchute, if my analogy ignores Archimedes, so does the asertion that the force from the geeses wings will apply itself to the a/c floor. Any rise in air pressure would vent through the open window.

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Another day in paradise

212man,

Re-read my reply - my comment had referred to your first analogy. If you have a sealed container filled with various compounds such as helium, lead, air or what ever, the mass of those compounds remains constant regardless of what actions or reaction occur within.

I made no comment regarding your second analogy.

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dragchute
email: [email protected]

Now that my tears have abated somewhat (tame geese being behaved in an aircraft and doing as they're told!?)!! whatever next?
Is there someone who can give a simple answer to the initial question as it's now doing my head in with all of the different theories. I agree with 212man at the minute but can be swayed.
carry on.

Now that my tears have abated somewhat (tame geese being behaved in an aircraft and doing as they're told!?)!! whatever next?
Is there someone who can give a simple answer to the initial question as it's now doing my head in with all of the different theories. I agree with 212man at the minute but can be swayed.
carry on.

212 man,

agreed.

everyone else,

prove him wrong, I cant!

Ah Shucks!
thanks, nice to know I'm not alone in my thoughts. I always think that before getting tied up in forgotten 4th form physics, it's a good idea to look at the practicalities of the situation and how they relate to other instances (eg nautical?). Otherwise, I can feel 'Houghton and Carruthers) being pulled from the bookshelf and dusted off.

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Another day in paradise

Air behaves like a liquid.

Imagine a glass with water in it. At the bottom are granuals of sand. The water and sand has a weight. Stir the glass up (supply energy) and the sand becomes suspended in the water.

The sand and water still weight the same. This is despite the heavier granules of sand not touching the bottom.

Essentially the geese are disolved in air after they have supplied there own energy to move around the system.

This is the same as helium balloons and lead bars being essentially dissolved in the same air.

At the surface of Earth air pressure is the result of all the particles above in a unit column of air. If these particles increase in mass or weight then pressure at the surface increases. It's the same inside the aircraft.

Mariner9, I take your point about acceleration being able to change.

Jed A1

Air is a liquid. A liquid by definition is a substance that can resist shear only in motion. A fluid is a substance that deforms continuously under the action of an applied shear force or stress. The process of continuous deformation is called flowing, or as it is applied to air flow.

I would like to commend you on a well thought our and relevant analogy you presented with respect to the sand and the water.

212man

Your analogy of the lead pipes and balloons is again incorrect. If you increase the mass of a control volume the weight increases as weight = mass x acceleration (gravity).

Using your own example, if you were to seal off the container with the lead and balloons in it, and extract all the gas you would agree that the weight would increase. As you know the weight of the extracted gasses would have to added to the tube and lead weight to get the weight of the entire control volume (tube+lead+extracted gas).

:rolleyes:

It seems to me that those suggesting the weight remains the same can only do so by using an analogy that has two different fluids. Jed A1’s glass full of water and sand works with two fluids that have different weights but this is not analogues to the original question. To be so the glass and sand would be in a fish tank full of water, now swirl the sand around and what would happen to the weight of the glass? Having said all that a physicist friend of mine has read this thread and sent me the following:

“
The total mass of the aircraft when in flight is equal to the mass of the aircraft plus , the mass of its :
fuel
passengers
pilot
geese
water
poo in the toilet
and air in the cabin and holds.

The total flying weight of the aircraft is equal to the above total mass multiplied by the gravitational field strength, at that point in space above the Earth. Mass X Gravity (mg)

It does not matter whether the geese are asleep on the floor or on the wing. The total mass of the aircraft and its contents remains the same. Not strictly true because it is using fuel and losing mass and therefore weight every moment.

You might think that flying around the cabin removes their weight but this is an illusion.

Think of the birds as being suspended in a fluid filled tube such as some plastic beads in a jam jar of water.

As the birds fly there are many forces acting upon them:
frictional drag
Lift due to pressure difference as the air flows over their wings
Upthrust from the air due to the displacement of a mass of air
A weak gravitational pull between their own mass and that of the aircraft The forward thrust of their wings on the air

All of these forces will have equal and opposite reaction forces (Newtons ThirdLaw) at some point in this complex system.

They are all, however, relative to the air in the aircraft and the aircraft itself. As such they do not change the overall mass or weight of the system that is the aircraft in flight.

Hope this helps,

Clearly you are not busy enough if matters such as this distract you.”

He has a point in his last paragraph ;)

dragchute

I think 212man was talking to me then!

212man

Sorry, just stirring a little bit, nothing serious!

While I'm here though, you said:

"Alternatively, take your a/c on the ground, with the wheels on scales, and get a load of tame geese to fly into the cabin through an open door. Do you think the weight on the scales will rise with each goose entering? Then close the door, no change. Then get the pilot to ask over the PA that all the geese kindly stop flying and sit down. Then the weight will rise."

So you're saying that should the geese begin to fly again (with all doors and windows closed) the weight will decrease? By the weight of the birds, or are you going to put the change of CoM in there as well? You just described a closed system and said the wieght will change when the geese land.

Is this in the original question (no open door or window) or in your scenario where you opened the window?

Chicken6,
I was intending to refer to the window ope situation, hence my original question about openeing the windo. As soon as you do, it ceases to be a closed system. Quite evidently if you strapped a rocket to the a/c and shot it into space (with the window closed and the a/c pressurised) the whole would act as one. If the windows is open, I feel that the situation changes (apart from a. the geese suffocating b. their blood boils and c. they have nothing to fly in). Seriously, even in the atmosphere I think the situation is different.

I agree with the physicist, it's a bit sad to be dealing with this topic.

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Another day in paradise