From what I understand, balloons float because the helium gas inside of them is less dense than the normal air outside of them, and the buoyancy force is enough to overpower the gravitational force.

But, how exactly does that work? If it's only the density that matters, why can't we just get a really light container which encloses a vacuum? Sure it'd be expensive, but would it float? If not, what exactly is it that makes helium balloons float?

The heavier gases outside the ballon flow down an under it pushing it up out off the way.

why can't we just get a really light container which encloses a vacuum?

Because any container we can create light enough to float when filled with vacuum will implode. And any container rigid enough to withstand pressure isn't light enough.

Edit: video of train cistern implosion: http://www.youtube.com/watch?v=Zz95_VvTxZM

An interesting effect.

If you get a helium filled balloon and tie it down in the back of a van. When you accelerate the van forward the ballon moves forward in the van.

It always confuses me when people call buoyancy a force. I guess it is... but really I think of it as just a different manifestation of gravity. No gravity = no buoyancy.

It all has to do with mass and the effect gravity has on it. Imagine a boulder and a pebble rolling down a hill. Then imagine they both go in like a funnel at the exact same time and only one of them can fit. IE, whichever one fits will be the one that pushes the other out of the way.

The boulder clearly has more mass, and therefore more momentum. So the pebble will get pushed aside.


Air/helium is the same idea. Both want to move down because of gravity... but air, having more mass, pushes the helium out of the way. Therefore the helium floats above it.

If it's only the density that matters, why can't we just get a really light container which encloses a vacuum? Sure it'd be expensive, but would it float?

Theoretically, if it was light enough and strong enough to maintain the vacuum, yes.

The problem is... any material light enough to float in air is likely to be crushed by all the surrounding air pressure. IE: rather than the air pushing the container aside... it would just crush it.

So such a container is implausible.


EDIT: ninja'd by 3 posts! Doh!

@MiiNiPaa: is my scenario mathematically impossible, or just highly impractical? I never mentioned how strong the vacuum is - it doesn't need to be a true vacuum.

@MiiNiPaa: is my scenario mathematically impossible, or just highly impractical? I never mentioned how strong the vacuum is - it doesn't need to be a true vacuum.

If the mass of the container and its contents is less than the mass of the air it displaces, then it will float. It's that simple.

> It always confuses me when people call buoyancy a force. I guess it is...
> but really I think of it as just a different manifestation of gravity.
> No gravity = no buoyancy.

Buoyancy is a force that arises from differences in density; this difference in density (manifested as pressure) may be caused by any force - gravitational, centripetal, magnetic ...

Though experiment: rotate, at high speed, a vessel containing a mixture of helium and nitrogen in a zero gravity field.
Buoyancy will cause the helium molecules to move towards the centre of the vessel (and the nitrogen molecules to move towards the periphery).

This has been experimentally verified at near zero gravity (outer space).

Buoyancy is a force that arises from differences in density; this difference in density (manifested as pressure) may be caused by any force - gravitational, centripetal, magnetic ...

Good point. I hadn't thought about magnetism/centripetal force.

Though it still seems like a side-effect of an existing force and not a force in of itself.


Of course you're right -- and I realize that it is considered a force. It just confuses me to think of it that way.

I suppose it could be considered a force in the same way that centrifugal forces are.

Buoyancy is a force that arises from differences in density; this difference in density (manifested as pressure) may be caused by any force - gravitational, centripetal, magnetic ... Though experiment: rotate, at high speed, a vessel containing a mixture of helium and nitrogen in a zero gravity field. Buoyancy will cause the helium molecules to move towards the centre of the vessel (and the nitrogen molecules to move towards the periphery).

Interesting...So if buoyancy is the opposing force to an accelerating force such as gravity. In your thought experiment centripetal force can not be the force that is opposite to buoyancy.


PS I'm not much of a physicist

These may be of interest:

All fictitious forces are proportional to the mass of the object upon which they act, which is also true for gravity. This led Albert Einstein to wonder whether gravity was a fictitious force as well. He noted that a freefalling observer in a closed box would not be able to detect the force of gravity; hence, freefalling reference frames are equivalent to an inertial reference frame (the equivalence principle). Following up on this insight, Einstein was able to formulate a theory with gravity as a fictitious force; attributing the apparent acceleration of gravity to the curvature of spacetime. https://en.wikipedia.org/wiki/Fictitious_force#Gravity_as_a_fictitious_force

With general relativity, Einstein managed to blur forever the distinction between real and fictitious forces. General relativity is his theory of gravity, and gravity is certainly the paradigmatic example of a "real" force. The cornerstone of Einstein's theory, however, is the proposition that gravity is itself a fictitious force (or, rather, that it is indistinguishable from a fictitious force). Now, some 90 years later, we have innumerable and daily confirmations that his theory appears to be correct. http://www.scientificamerican.com/article/what-is-a-fictitious-force/

PS I'm even less of a physicist.

Though experiment: rotate, at high speed, a vessel containing a mixture of helium and nitrogen in a zero gravity field.

Just wanted to point out that you don't need zero gravity or an ideal model for this experiment. This is the same principle that hospitals use to separate blood cells and platelets from plasma in a centrifuge.

I guess it is... but really I think of it as just a different manifestation of gravity. No gravity = no buoyancy.

The only problem with looking at it this way is that buoyancy isn't specific to gravity. Any acceleration will cause the same type of effect. When you're talking about a system of non-ridged bodies where the angle of incidence doesn't matter then it all comes down to F=m*A, and since all of the elements in the system are accelerating at the same rate, the elements with a higher net mass will displace the lighter ones in the direction of their acceleration.

Let's look at a different system, one where the angle of incidence does matter such as an airplane taking off. In this scenario the wings are shaped to push enough air underneath the wings of the plane so as to create a net force upwards. That is why speed is a key factor and why the goddamn airplane on a treadmill doesn't take off unless it's moving. This is still buoyancy, but it is buoyancy that opposes gravity.

Isn't the airplane example an example of "lift" which is a different force from buoyancy?

But yeah... JLBorges already pointed out that gravity isn't the only way to generate buoyancy. So yeah, you're right.

I thought airplane wings were 'pulled' upwards by the low pressure caused by the shape of the wing? I wouldn't associate that with buoyancy.

Right. If airplanes were buoyant, they wouldn't need to move forward to stay afloat -- they would simply float on their own.

If I remember correctly, it is Bernaulli's principle that is used to lift aeroplanes. The difference in air speeds at the top and bottom of the wings does the trick.

@Disch
Buoyancy has to be defined in terms of a medium. For example, a boat is buoyant in water but not air. (By the same token, could I say I am buoyant on land?)

@chrisname

Technically, a boat is bouyant in any medium you like to pick.

If you go back to first principles then the boat displaces a certain volume of the medium. The medium exerts pressure on the boat (via gravity or whatever). The resultant of these forces, taking into account the weight of the boat (gravity or whatever) can result in a nett uplift/floating effect, 'defying' gravity.

The force exerted by the air on the boat is not sufficient to cause uplift due to the density of the air. ( rho*g*h, rho-water vs rho-air )

Archimedes principle applies whether you are in water or just standing up on dry ground. So, just consider yourself sunk to the bottom in the atmospheric sea because the weight of the air you displace is less than your weight. ( But hopefully you're not drowning - leave that to C++ )

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html