Vacuums and Buoyancy

  • #1

Main Question or Discussion Point

So, it looks like I'm not likely to get an answer to the https://www.physicsforums.com/showthread.php?t=345179", but I figured whilst I was here I'd ask something that's had me curious for years.

Buoyancy causes things to float/fly when the average density of a body is less than that of the density is travelling through, yes? A zeppelin flies (well okay, not so many of them anymore) because the helium or hydrogen inside it is less dense than the air outside.

The least dense thing I can think of is a vacuum. So, if a container could be made that was sufficiently strong so that it wouldn't collapse under the air pressure, and sufficiently large (and light) that the volume of the vacuum it contains is enough to reduce the average densite of the whole body, would it fly? Is there a physical reason why this wouldn't work, or is the reason we don't have vacuum powered airships due to difficulties with engineering - inadequate materials or prohibitive cost?
 
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Answers and Replies

  • #2
f95toli
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So, if a container could be made that was sufficiently strong so that it wouldn't collapse under the air pressure, and sufficiently large (and light) that the volume of the vacuum it contains is enough to reduce the average densite of the whole body, would it fly?
Yes, of course.
But making a container that have walls that are both strong enough to hold a vacuum AND light enough to fly is very difficult, probably impossible with the materials we have today.
 
  • #3
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The athmospheric pressure is very high. That prevents us from making vacuum containing baloons.

On the other hand, a bottle in water is nearly what you are asking about, in a realtive sense.
 
  • #4
uart
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The least dense thing I can think of is a vacuum. So, if a container could be made that was sufficiently strong so that it wouldn't collapse under the air pressure, and sufficiently large (and light) that the volume of the vacuum it contains is enough to reduce the average densite of the whole body, would it fly? Is there a physical reason why this wouldn't work, or is the reason we don't have vacuum powered airships due to difficulties with engineering - inadequate materials or prohibitive cost?
It looks like you're trying to ask why vacuum 'airships" would not be preferable over hydrogen or helium. The reason is very simple, you'd only get about an extra 6% net boyancy from a vacuum as compared with hydrogen and for that small increase you'd pay a huge price in terms of increased weight of the containing vessel to make it strong enough to withstand the enormous pressure.
 
  • #5
Cleonis
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So, if a container could be made that was sufficiently strong so that it wouldn't collapse under the air pressure, and sufficiently large (and light) that the volume of the vacuum it contains is enough to reduce the average densite of the whole body, would it fly?
I concur with the previous answers.

In fact, blimps work the other way round!

In zeppelins the internal helium pressure is equal to the atmospheric pressure. The zeppelin's internal frame must be strong enough to make the Zeppelin able to withstand pummeling by wind, but the atmospheric pressure is dealt with by the Helium.

A blimp on the other hand, has no internal frame; a blimp is skin only. Blimps are pressurized to make them rigid.
 

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