Paging all Science nerds

[TheMatt]

So I'm watching Navy Seals last night for about the 40th time (cause I'm a real man)and it finally struck me to ask how they do something that they do.

So, in a submarine, submerged, they exit the sub and swim around front. They access some kind of port and there is an un-inflated zodiac. They take it out and you see a compressed air canister. They turn it on, it starts pumping, fills the zodiac, which then floats to the surface.

My question - How the come the fricken canister of compressed air does not float? For easy math let's say a zodiac takes 5 cubic meters of air to fill. That air, obviously floats when un-compressed as evidenced by the rise of the zodiac.

How come "compressed" air does not float? I mean, does compressing it actually also minimize its buoyancy? If so, how? Even though it occupies less space, is it not the same volume of air with the same properties?

In any event, I would like an actual scientific answer pls. Although I am willing to entertain the idea that this happens because the canister is "made of win", I would like something a little more technical if possible.

 

[traceur_]

The weight of the canister must counteract it's buoyancy to give it neutral buoyancy.

 

[CosmicGrenade]

lets see the canister will be made of metal.... the zodiac is unflated... so thats heavy too and compressed gases are heavy too

 

[TheMatt]

The weight of the canister must counteract it's buoyancy to give it neutral buoyancy.

How then does it counteract the weight of the canister when it is inflated? The gas becomes MORE buoyant when uncompressed? Which leads me back to my question - why/how?

lets see the will be made of metal.... the zodiac is unflated... so thats heavy too and compressed gases are heavy too

Same point how does the air "lose" it;'s buoyancy when compressed? how does it gain it when uncompressed?

 

[Same River New Water]

That's a tricky one. Let me make a totally ignorant hypothesis:

Although the air has the same weight whether it's compressed or uncompressed, in its compressed form there is less upward pressure as it takes up less space. The inflatable life raft has say 4m^2 for the water to push against, compared to only say 20cm^2 for the canister. Since the weight of the water is the same at any point, objects with a larger surface area are more buoyant since they are pushed up by more water; conversely, when the total weight of the air is concentrated in one place, its weight is increased relative to the amount of upward pressure from the water, or more accurately there is less upward pressure of water lifting the air.

It's like diving into a pool: if you pin-drop, you'll sink very far, but if you land flat (belly flop), the water keeps you up, because you have a greater horizontal surface area.

That's my guess. Can anyone confirm or deny?

 

[TheMatt]

That's a tricky one. Let me make a totally ignorant hypothesis:

Although the air has the same weight whether it's compressed or uncompressed, in its compressed form there is less upward pressure as it takes up less space. The inflatable life raft has say 4m^2 for the water to push against, compared to only say 20cm^2 for the canister. Since the weight of the water is the same at any point, objects with a larger surface area are more buoyant since they are pushed up by more water; conversely, when the total weight of the air is concentrated in one place, its weight is increased relative to the amount of upward pressure from the water, or more accurately there is less upward pressure of water lifting the air.

It's like diving into a pool: if you pin-drop, you'll sink very far, but if you land flat (belly flop), the water keeps you up, because you have a greater horizontal surface area.

That's my guess. Can anyone confirm or deny?

Hmmm... Makes sense though doesn't it?

 

[CosmicGrenade]

The weight of the canister must counteract it's buoyancy to give it neutral buoyancy.

How then does it counteract the weight of the canister when it is inflated? The gas becomes MORE buoyant when uncompressed? Which leads me back to my question - why/how?

lets see the will be made of metal.... the zodiac is unflated... so thats heavy too and compressed gases are heavy too

Same point how does the air "lose" it;'s buoyancy when compressed? how does it gain it when uncompressed?

because it is expanding you still have the same amount of gas but it's going from a confined space to a much larger but it's still the same volume.

Edit: think of it like a black hole which has a sun like mass compacted down to a small size and that makes it's more heavy

 

[ontherisess]

When compressed gas stops being compressed it loses a lot of heat. That's why things like deodorant feel very cold even when not kept somewhere chilled. This lose of heat turns into kinetic energy. Also rafts are lighter than metal.

 

[TheMatt]

The weight of the canister must counteract it's buoyancy to give it neutral buoyancy.

How then does it counteract the weight of the canister when it is inflated? The gas becomes MORE buoyant when uncompressed? Which leads me back to my question - why/how?

lets see the will be made of metal.... the zodiac is unflated... so thats heavy too and compressed gases are heavy too

Same point how does the air "lose" it;'s buoyancy when compressed? how does it gain it when uncompressed?

because it is expanding you still have the same amount of gas but it's going from a confined space to a much larger but it's still the same volume.

Edit: think of it like a black hole which has a sun like mass compacted down to a small size and that makes it's more heavy

ahhhh... Now we are into Astro-physics which I understand (Me and Mr. Hawking are boyfriends). As to your blackhole comment - It does not make it more heavy, it makes it more dense. As in, the same weight occupying a smaller amount of space.

 

[Cpt. Red]

If my memory serves me correct what makes an object float is that its density is lower then the water. So when you compress the air its density increases and therefore will have a lower buoyancy...

 

[TheMatt]

If my memory serves me correct what makes an object float is that its density is lower then the water. So when you compress the air its density increases and therefore will have a lower buoyancy...

holy shit does that ever make a lot of sense.

Think of a layered drink... Kahlua at bottom, then bailey's then tequila, each with a lower density then the one before so they don't combine.

Neat. I don;t even care if it's right, it makes sense to me. Thanks dude.

 

[teutonicman]

How dense is the tank that the gas it held in? Also how large is the tank?

 

[fix-the-spade]

More gas + same space = increased pressure.

Compressing a gas increases it's density. The volume of gas inside remains the same but the amount stored inside increases, hence increase in pressure and also in weight of tank.
Also, compressed air bottles are usually made of steel and they weigh a ton.

 

[Caimekaze]

A few people here have mentioned density. That's the crux of the matter.
When people say something is "lighter than air" they mean less dense than air. As the gas is compressed, it becomes quite dense, thus causing it to sink. Once the gas is allowed to expand, it becomes less dense. This makes its effective buoyancy greater, causing it to float.

Says my sleep deprived brain, at least.

 

[Caimekaze]

Jesus Christ people, come on...

Compressed air apparently has higher density than water, so according to Archimedes' law it doesnt float.
That's middle school physics.

density

density

dense

density.

density.dense dense dense.

Density! You're brilliant!

 

[TheMatt]

Jesus Christ people, come on...

Compressed air apparently has higher density than water, so according to Archimedes' law it doesn't float.
That's middle school physics.

According to Archimedes' law you're gay.

haha... ummm, yah.

 

[Zacharine]

Actually density isn't the entire answer.

Buoyancy is a factor of the volume of the object submerged, while gravity (the main force keeping it underwater) is a factor of the object's mass. Denser object = more mass per volume = greater gravitational force while at the same time compressed size = reduced volume = reduced byoyance to act against gravity.

Hope this explains it.