Sorry about this, I know the brain teaser style posts have about worn thin the topic of flies in a jar and fluid/gas buoyancy, thrust etc., but this is a very specific question I didn't get a good answer for while reading the other puzzles.

I can believe that a bird or insect in a jar, or a R/C helicopter hovering within a closed container would cause the weight of the container to be the same whether the heli/birds were landed or flying.

I can easily see how a fishtank filled with water would weigh 1 pound more when a 1 pound model boat or submarine was placed in it.

However, its hard to believe that if you take a hydrogen balloon with a 1 gram weight attached and trimmed for neutral buoyancy, placed it in the container, that the container would weigh more. How is the balloon placing any thrust on the container? Is it exactly the same as a submarine in the fishtank?

My question is specifically relating to the balloon, not birds or anything. Only a balloon hovering in the air, not touching anything.

Hmmm... that does tickle my noodle...

The balloon displaces the air it's in with its volume of hydrogen, which is.......... lighter than air! (write that down )
The combination would be ... lighter.
The submarine displaces its volume with the equivalent amount of water.
If the aquarium is filled to the top with water, and the submarine is then placed in it, the excess water slops over.. Eureka!
The weight on the scale changes, relative to the mass of the sub relative to the mass of the displaced water.

Neutral boyancy is = to .00000_+ 0000. No gain or loss in weight.
Congratulations. You stated the question correctly. [sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif]

Actually if the submarine is set to neutral bouancy and you do this the weight on the scale of the full tank before and after the sub is added won't change. You see, if the sub is nuetrally bouyant that means it has the exact same density as the water it displaced over the edge. So the weight of the water over the edge is the same weight at the sub so the scale does not change. Niether will it if the sub is floating on top since it's weight still is reflected by the water pushed over the side. Only if it's heavy enough to sink to the bottom will the scale register a gain in weight. The amount being equal to the difference between the "heavy" sub and the water pushed over the side.

The same will apply to the hydrogen balloon put into the big plastic pail of air. The overall density of the weight and the balloon matches the air it displaces. It has to or it would not float evenly. Put this combo into a bucket with a lid on it and it'll register the same mass as if it was filled with only air since the air you replaced by adding the balloon is the same density as the balloon and weight. And because of this if you suddenly accelerate the bucket the balloon and weight will swing about due to the apparent shift in gravity direction but unless part of it hits the side of the bucket the balloon and bob will tend to try to stay in the same spot within the bucket..... Just like if you slop a jelly fish around in a bucket of water and the water comes to rest the jelly fish is still more or less in the same spot.

To my understanding:

Half full fishtank on scale. Scale reads 10 pounds. 1 pound boat is placed in tank, water level rises, tank does not overflow. Scale reads 11 Pounds.

Same scenario except with neutrally buoyant 1 pound submarine. Same results.

Same 2 scenarios, except all displaced water overflows. Scale still reads 10 pounds.

Empty container on scale. Scale reads 1 pound. Neutral buoyancy hydrogen balloon weighing 1 ounce floats inside. Scale reads 1 pound.

6 ounce hummingbird rests on perch inside 1 pound container on scale. Scale reads 1 pound, 6 ounces. Hummingbird takes flight and hovers. Scale still reads 1 pound 6 ounces when you average any fluctuations. I find this one a little hard to swallow, but it seems to be the popular opinion here.

I understand all these to be true then. Am I correct?

I'm still a bit doubtful about the hummingbird but the rest is spot on.

The catch -- what happens to the energy the bird expends ---------

Forgive me if my thinking is not correct, but is the humming bird not creating lift? Lift has to be equal to the force of gravity on the bird for it to be flying. (unaccelerated flight means the craft, or bird, is in equilibrium) So it is not placing any extra weight on the jar. Just like if a 1600 lbs Cessna skyhawk flys over you head it does not put an extra 1600 lbs on your head to keep it in the air, but if it land on you, you will most certainly be squished.

Make that 1600 lb Cessna a helicopter and place both you and the heli in a jar with it hovering over you. I am sure you'll feel something then. The 1600 lb Cessna is exerting a downward force but that force is not trapped to be just above your head. Just like air within a ballon, it has room to expand sidewards, upwards and downwards. Of course the path of least resistance usually wins out...

After thought: In the fish tank example-

The balloon, boat, and bird are all creating their own "lift or buoyancy" when they are working properly. Take the wings away from the 2oz. bird, and he falls to the floor and adds to the overall weight. Cut the boat and the boat sinks and adds to the weight. Poke a hole in the balloon and it falls to the bottom of the fish tank and adds to the overall weight.

Dick Hanson: Maybe a portion of the birds energy is wasted in all kinds of ways other than thrust on the bottom of the container. Maybe friction and heat. I just got to thinking not all of the force from the birds wings will go in a vector straight down to the bottom of the container. If the sides were made of free hanging newspaper, the bird hovering inside would probably cause them to blow outwards, (or maybe suck inwards?) indicating some force acts on the sides of the container. If any force acts on the sides, wouldnt that prove that the total opposite force of the bird weight CAN't completely be upon the floor, changing the scale setting?

Lomcevak Duck: If you cut the boat and it sinks, the scale shouldn't change much if no water overflowed when the boat was floating right? I say "much" cause I just got to thinking that with the complete sunken hull underwater, the total volume of hull material, batteries, sailor figurines, etc would offer some small buoyancy force. Right?

For example, you have a 1 inch steel ball bearing, suspended from a string attached to a scale. Lower it into water. Is it lighter by the amount a 1 inch sphere of water weighs? I know when I pick up big rocks from the bottom of the lake they feel lighter until they reach the surface. Is this an illusion?

Also when you pop the balloon, the low density gas escapes and goes up to the ceiling. If it stayed in the container, would the weight be the same? Popped balloon on the floor, hydrogen trapped in the lid?

I guess I just need a book on buoyancy.

The "bottle " is a self contained universe --except for -forces IN the bottle which act on the bottle. If the bird slams into the bottle -that force has to go somewhere

Hummingbird or Swallow?

Yes. The hummingbird is acting on the air which is in turn acting on the container. It's no different in effect (barring the bird's momentary acceleration) whether the bird is perched on a stick mounted to the container or it is being held up by its action against the air constrained by the container.

My reasoning is this: If we designed a scale that could capture and measure the force of ALL the air that the bird moved in order to hover, the force would equal the weight-force of the bird, same as if it was perched on the scale. Add to this the weight of a container, put the bird and enough air insided it, and you have the above case.

The bird would indeed set up a circlulation inside a sealed container. The net result of all that air movement would be a weight-force added to the container equal to the bird's weight. Sure there are losses due to friction, heat generation, compressibility, etc, but the net result is the same.

I agree you would feel something- a LOT of wind moving around. But for a wing to create lift, what's underneath it is not a factor until you're very near the ground. I am not sure, but maybe it has something to do with the fact that air compresses easily?

Is that an African Swallow?


It is I, Arthur, son of Uther Pendragon, from the castle of Camelot. King of the Britons, defeater of the Saxons, Sovereign of all England!

Pull the other one!

I am, and this is my trusty servant Patsy. We have ridden the length and breadth of the land in search of knights who will join me in my court at Camelot. I must speak with your lord and master.

What? Ridden on a horse?

Yes!

You're using coconuts!

What?

You've got two empty halves of coconut and you're bangin' 'em together.

So? We have ridden since the snows of winter covered this land, through the kingdom of Mercia, through...

Where'd you get the coconuts?

We found them.

Found them? In Mercia? The coconut's tropical!

What do you mean?

Well, this is a temperate zone

The swallow may fly south with the sun or the house martin or the plover may seek warmer climes in winter, yet these are not
strangers to our land?

Are you suggesting coconuts migrate?

Not at all. They could be carried.

What? A swallow carrying a coconut?

It could grip it by the husk!

It's not a question of where he grips it! It's a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.

King Arthur: Well, it doesn't matter. Will you go and tell your master that Arthur from the Court of Camelot is here?

1st soldier with a keen interest in birds: Listen. In order to maintain air-speed velocity, a swallow needs to beat its wings forty-three times every second, right?

King Arthur: Please!

Am I right?

The balloon has no effect, the bird does.

so -if you put a woodpecker in the bottle and it pecked hard at the bottom of the bottle-- enough to move the bottle (suspended on springs) would that change the weight ?
This is getting goofy ---
As the butcher said - taking a turn for the wurst ---

And here I was just getting ready to tell how I used to have the copilot throw the cat in back to get all the chickens flying so we could get out of that little short strip by the chicken farm.
Between this and the conveyor thread it sure would be a slick way to drastically cut the pile of resumes that needed to be read.
It was beyond goofy from the get go.

Nope. But the weight of the bird does not go away when its flying...inside of a flying airplane...regardless of what its inside....is my point.

Think of a case where there is a treadmill mounted vertically inside the container with perches attached at intervals so the bird can step from one to the other in such a way that it stays in the same position relative to the conveyor apparatus and the container. The weight-force of the bird will be transferred through the conveyor apparatus to the container, just as it would be transferred through the air to the container, and with the same effect as though it was sitting on a stationary perch.

Sorry, I just had to do it.[sm=bananahead.gif]

Don't kill me.

By the way, I love the Swallow Dialogue from Monty Python. I'm glad someone got my hint.

Sorry, no. In all cases, the bottle/tank/etc is capped....closed. So you have closed systems.

The bird's mass is part of the total mass of the container and its contents. Whether it flies or not, that mass is always there. Its flight is just moving the air around that's already inside the container. If the bird quit flying and fell to the bottom, only an acceleration would be noted as the bird hit the bottom.

With a balloon, the envelope, payload, and filling gas all have mass, which is part of the mass of the entire container. You can deflate the balloon and move the items around, but the mass remains the same.

Same with the boat in the sealed tank example. Whether it floats or not, the mass of the system doesn't change.

Weight is nothing more than mass x acceleration of gravity. If mass doesn't change, neither does weight.

You can't make an airplane lighter by having the birds (bees, whatever) fly inside it. Their mass is always part of the total mass of the airplane and cargo until you let them out.

But if the tread mill was set up to exactly match the climb rate of an African Swallow....
Yeah I know it is tempting isn't it?

This thread needs to be dealt with...and I know just the thing.

[Holding the Holy Hand Grenade of Antioch]
King Arthur: How does it... um... how does it work?
Sir Lancelot: I know not, my liege.
King Arthur: Consult the Book of Armaments.
Brother Maynard: Armaments, chapter two, verses nine through twenty-one.
Cleric: [reading] And Saint Attila raised the hand grenade up on high, saying, "O Lord, bless this thy hand grenade, that with it thou mayst blow thine enemies to tiny bits, in thy mercy." And the Lord did grin. And the people did feast upon the lambs and sloths, and carp and anchovies, and orangutans and breakfast cereals, and fruit-bats and large chu...
Brother Maynard: Skip a bit, Brother...
Cleric: And the Lord spake, saying, "First shalt thou take out the Holy Pin. Then shalt thou count to three, no more, no less. Three shall be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, neither count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who, being naughty in my sight, shall snuff it.
Brother Maynard: Amen.
All: Amen.
King Arthur: Right. One... two... five.
Galahad: Three, sir.
King Arthur: Three.

That made my day! Now I'm gonna have to watch it again.

Thanks Liberator for providing the bouyancy this thread needed!

I think most of us "get it " - that is the bird in a bottle --is actually a microcosm of the aircraft - a self conained universe aloft in a larger universe .
Tho it is contained within the larger universe- it-like the airfcraft which is flying around our earth -what happens in it - is affixed to it . (what happens on earth stays on earth)
what happens in the bottle stays in the bottle