A helium balloon floats...it is lighter then air and a vacuum is lighter then He

[Beamin]

A balloon full of helium floats because it is lighter then air, another way of thinking of buoyancy is density; the density of the balloon is less then air, so what if you took another balloon and blew it up, froze the rubber in its shape and then sucked the air out making it lighter then the he balloon. Would this float? While it would be less dense then air it wouldn't be buoyant like helium balloons are? This also makes me wonder would a he balloon in a free falling box float?

[apis]

Yes, it would float just like a helium/hydrogen balloon. The reason you don't see anyone doing it is that the air pressure would easily crush any such "frozen balloon" unless you made it out of thick steel or concrete, but that would be pretty unwieldy (hmm, I wonder if it would be feasible if you made it large enough though...). Air pressure at sea level is about 100 kilo pascal i.e. about the equivalent of 10 000 kg per square meter (or 15 pound per square inch).

With a helium balloon the pressure inside and outside the balloon is equal so it can be made of a soft material like rubber.

In a free falling box everything floats!

What happens if you try to pump out the air of a railroad tank car:

[0culus]

I mean, even large cathode-ray tubes don't float, so it would have to be *really* big.

[IanB]

if you took another balloon and blew it up, froze the rubber in its shape and then sucked the air out making it lighter then the he balloon. Would this float?

If you took a heavy steel box, filled the middle up with air, and placed it on the sea, would it float? Steel is much denser than water and steel sinks, so what would happen?

[apis]

Buoyancy can be pretty counter intuitive to think about:

(Annotation have been removed by Youtube but there are links to the solution in the description of the video.)

Related: (lead helium balloon)

[IanB]

Buoyancy can be pretty counter intuitive to think about:
...
(Annotation have been removed by Youtube but there are links to the solution in the description of the video.)

Yay! I got it right. Apparently my reasoning ability is at least someone sound 

[T3sl4co1l]

It was at one time proposed to use beryllium spheres (actual ones, not movie props ) to hold vacuum.  The metal shell is much heavier than, say, a rubber balloon, but perhaps the weight can be made low enough that it balances out?

Turns out, nope, it's just not quite good enough.  Not only do you need a very uniform sphere, but no matter how uniform it is, it still needs to be braced against unbalanced external forces -- bumps and bends from mounting and handling.  It only takes a little imbalance to do what happened to the tank car!  So you'd need a sphere that's wrapped in trusses to keep it supported, and the actual spherical walls would really be more like foil windows inbetween those supports.  It might look like a buckyball instead.

Beryllium would be the most likely candidate, for its extraordinary stiffness and strength to weight ratio.  Some ferrous or titanous superalloys, or carbon fiber composite, might have superior strength to weight, but nowhere near the same stiffness.  The stiffness is only superseded by heavier alloys (molybdenum, tungsten carbide..), which are inferior in strength/weight, or far too brittle (even moreso than beryllium itself) to fabricate.

Maybe nowadays with slightly accessible metal 3D printing, it might be possible to revisit, and get comparable performance to traditional materials.  But, suffice it to say: it's damn hard to beat having a gas that provides the same internal pressure, at a fraction of the weight, of the atmosphere outside it!

Tim

[Stray Electron]

I mean, even large cathode-ray tubes don't float, so it would have to be *really* big.

  Not necessarily "really big" but really light for the volume that it occupies.  Having a density (weight divided by volume) less than than air is what makes a hydrogen or helium balloon float.

   In theory; a frozen, evacuated, rubber balloon would float but it would be impossible to make one.

[0culus]

Of course. I was just proffering an example of an everyday (well, only for people who collect old TVs anymore) object with a vacuum in it that is strong enough to support itself against the atmosphere. Pretty sure, as you say with the frozen balloon example, you'd need some kind of ultra featherweight but super strong unobtainium to build a viable vacuum balloon.

[rfeecs]

There was a metal skinned airship, the "tin blimp" ZMC-2 (Zeppelin Metal Clad):
https://en.wikipedia.org/wiki/ZMC-2

But it wasn't a https://en.wikipedia.org/wiki/Vacuum_airship.

Neal Stephenson's (fictional) book "The Diamond Age" featured vacuum airships:
https://en.wikipedia.org/wiki/The_Diamond_Age

[apis]

The thing is, the internal volume of a sphere is proportional to the radius cubed, while the surface area is only proportional to the radius squared. If you can increase the size of the sphere without having to increase the thickness of the shell too much, at some point you will make the total sphere density less than air no matter what material you used in the shell. It's actually the same reason you can make helium balloons lighter than air. To get the shell strong enough and as light as possible, as Tim said, a buckyball is probably the way to do it, if at all possible. No doubt a helium/etc balloon is cheaper so there is no point, but in theory my guess is that it would be possible.

Actually, now that I think about it I believe Bucky even proposed floating (air filled) buckyball cities at one time: https://en.wikipedia.org/wiki/Cloud_Nine_(tensegrity_sphere)
 

[David Hess]

With a helium balloon the pressure inside and outside the balloon is equal so it can be made of a soft material like rubber.

This is also why hot air balloons, blimps, and dirigibles are surprisingly difficult to shoot down.  The pressure difference between the inside and outside is very low so any holes do not leak very much.

[soldar]

If you took a heavy steel box, filled the middle up with air, and placed it on the sea, would it float? Steel is much denser than water and steel sinks, so what would happen?

This is easier than it may seem at first because, being in the atmosphere it would fill with air naturally. I believe they already make something like this.  They call it a "ship" if I am not mistaken.

[MarkR42]

Maybe there is some lightweight uncrushable material (aerogel? micro-structured matrix of some kind?) which can be used to construct a sphere (with a thin shell to keep the air out) which could be buoyant by being evacuated.

It sounds plausible but needs someone who understands the maths to do the sums.

[soldar]

Does not sound plausible at all. The least pressure difference will create forces which will distort and collapse the thing. An spherical cow is more plausible.

[Stray Electron]

The thing is, the internal volume of a sphere is proportional to the radius cubed, while the surface area is only proportional to the radius squared. If you can increase the size of the sphere without having to increase the thickness of the shell too much, at some point you will make the total sphere density less than air no matter what material you used in the shell. It's actually the same reason you can make helium balloons lighter than air. To get the shell strong enough and as light as possible, as Tim said, a buckyball is probably the way to do it, if at all possible. No doubt a helium/etc balloon is cheaper so there is no point, but in theory my guess is that it would be possible.

   The problem with very large balloons and rigid airships is that they're unmanageable in even the lightest wind and even day time solar heating has a drastic effect on them. Go read the history of the USS Akron. Attempts have been made in recent decades to once again use lighter than air ships and to use  modern technology such as weather radar, radios  to avoid bad weather and computers to control the weight and balance and to control lift but none of them have been successful enough to justify them economically.

[apis]

Never claimed it would be practical!

Aeroscraft is a recent airship being developed in the US, (video shows a of a small scale proof of concept model). There haven't been much news about it the last couple of years though. They claim the main problem with cargo airships was that they required ballast and  a large ground crew. This ship is supposed to have internal He compressors so that they can change the buoyancy during flight. (I hope they are better at building airships than making videos).





Another interesting balloon, this one is from the 60s:
https://www.nasa.gov/sites/default/files/thumbnails/image/5277461725_34624f8a73_o.jpg

[Beamin]

Buoyancy can be pretty counter intuitive to think about:

(Annotation have been removed by Youtube but there are links to the solution in the description of the video.)

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A better question why do all three of the videos have the exact same number of views and likes and dislikes? I want to know how smart people are. Sometimes the group psychology of the comments is more interesting then the video. Why don't dislikes count? Ever taken the same comment and put it in multiple videos to see what happens? Or make a comment that you know will get 1k likes get bumped to the top, then a week later edit the comment to say something really unpopular and it looks like 1k people all agree with what ever you wrote. More mysteries for my restless mind. I'm a pathological thinker addicted to new thoughts. Remember before the internet you had to actually travel to the library to find knowledge and if they didn't have the book you would have to wait a week? Such barbaric and primitive times. I always thought the internet would make a generation of jeopardy players but instead we have trump. We are so screwed. 


I also worry we are going to run out of helium right as we find new uses for it as coolant for new technology and we won't have it because helium is too rare.

[paulca]

make a comment that you know will get 1k likes get bumped to the top, then a week later edit the comment to say something really unpopular and it looks like 1k people all agree with what ever you wrote.

This is how Facebook like farms work.  They create non-sense but loveable inspirational quotes, fake competitions to win an iPhone or impostor pages pretending to be Disney or Land Rover and when they get a few 10s of thousands of likes they switch the page to what they really wanted to advertise.  Or, more likely, they sell the page to someone else to advertise on.

When I chastise my friends for liking and sharing scams they say things like, "But what is the harm?", the harm is that once well liked, shared and popular the page then switches over to being a scam conning old people out of their pensions or something similar, all help along by my friends.  Bait, switch and scam.

[Beamin]

make a comment that you know will get 1k likes get bumped to the top, then a week later edit the comment to say something really unpopular and it looks like 1k people all agree with what ever you wrote.

This is how Facebook like farms work.  They create non-sense but loveable inspirational quotes, fake competitions to win an iPhone or impostor pages pretending to be Disney or Land Rover and when they get a few 10s of thousands of likes they switch the page to what they really wanted to advertise.  Or, more likely, they sell the page to someone else to advertise on.

When I chastise my friends for liking and sharing scams they say things like, "But what is the harm?", the harm is that once well liked, shared and popular the page then switches over to being a scam conning old people out of their pensions or something similar, all help along by my friends.  Bait, switch and scam.

Facebook is cancer on the internet and its metastasizing. Why would you give away all your privacy? You can't get it back. 

[apis]

I always thought the internet would make a generation of jeopardy players but instead we have trump. We are so screwed.

Me too. I thought it was the dawn of the information age! Turns out it was the dawn of the disinformation age.

Or, more likely, they sell the page to someone else to advertise on.

That appears to be the case. The owners of the botnets and fake accounts aren't political, they just sell their services to the highest bidder.

[paulca]

Facebook is cancer on the internet and its metastasizing. Why would you give away all your privacy? You can't get it back.

Trust me that is not the main threat caused by Facebook or social media generally.  It's much much worse than that.

[rstofer]

if you took another balloon and blew it up, froze the rubber in its shape and then sucked the air out making it lighter then the he balloon. Would this float?

If you took a heavy steel box, filled the middle up with air, and placed it on the sea, would it float? Steel is much denser than water and steel sinks, so what would happen?

It would probably violate the "Rules of the Road" and run into a container ship.

[m98]

The idea of vacuum airships isn't new. It also isn't physically impossible, but it would require a unique structure and material to work. After all, its just a vacuum chamber that has to be lighter than the volume of air it displaces.

[Bud]

Another silly topic opened by Beamin