h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I've forgotten most of my high school physics. If a pliable balloon (i.e. not rigid) is filled with helium and released, it rises up. Now, for said balloon to hold its shape, the pressure inside and that outside must be the same. It should rise only if the force at the bottom is not counteracted by that on top. However, if the balloon is being sufficiently compressed by atmospheric pressure to keep it's current shape, why would the force at the bottom be more than that on the top?

Further, is there a molecular/kinetic energy of molecule explanation for this?

Now that I've got this offa my chest, time to go drink beer.

I'm not sure of the exact physics behind it but the balloon rises because the helium is less dense that the air around it, therefore lighter, and so it rises. As to the equal air pressure being equal, well, it isn't. The balloon holds it shape because of the pressure of the helium - outwards - and the elesticity of the balloon - inward - OK ?

Nah, it's got to work against the pressure from outside, too - any anyhow, many helium balloons ain't elasticated.

However - I *think* this is correct - gas particles exert pressure due to the *number of molecules* not the total mass of the molecules. Hence, you have to have the same number of molecules per cm^3 inside the balloon that you have outside - but they can be lighter, and thus less dense.

Probably.

I have absolutely no idea. Whenever I see a helium balloon I deflate it and suck up as much helium as possible. This makes me sound like a complete and utter plonker for up to 30 seconds, and usually gets a few drunken laughs. Anyone know why helium has such an effect on the vocal chords?

It's because sound travels faster through helium, so the pitch is higher.

I've been educated here today. My thanks

A more philosophical question then. Why is someone with a high squeaky voice so fuuny? I just can't explain it. Its one of those things that gets less funny the more you think about it.

The helium is less dense than air theory is right its the same reason why oil floats on water.

*EVERYTHING* gets less funny the more you think about it.

Helium is imune against gravity...

The reason is: although the helium produces the same amount of pressure as the combination of the balloon's elasticity and the air pressure it is less dense. Since the air is more dense, the air falls (due to gravity), pushing the balloon out of the way.

Anyway, to get off the topic a bit, balloon has two consecutive double letters. Can you think of a word with three?

To keep all the ankle-biters happy at the birthday parties and shopping centres.

How about this related question: If you are going along in a car which contains a helium balloon and slam on the brakes, what happens to the balloon?

My beloved Dadda was driving home along the M54 with a bunch of helium balloons in his Fiat Bertone X-19. He saw a blue flashing light in his mirror, heard a siren and slammed on brakes as he pulled over.

The balloons rebounded all over the shop and my Dadda got a stern warning from our boys in blue. Moral is don't speed in a sports car with helium balloons inside.

Really genuinely true story, honest, guv!

Slight diversion. Hot air balloons rise. Warm air rises. Why is the top of a mountain colder than the bottom?

This conversation came up in the pub the other day with an interesting variant. Seeing as Radon is radioactive and not the sort of thing you want to be breathing in too often, would Xenon deepen the voice? I don't have my physics dictionary with me so I don't know the density at STP. If I collected enough light bulbs and amassed the argon therein might it have a similar but less pronounced effect?

Haha. WOOLLEN.

Tricky. Part of it is the reduced air pressure at increasing height, thereby the reduced moisture level of the air and so forth. As water has a much higher latent heat than air, more humid air is capable of retaining more heat than sparse and less-humid air. It's not so much that it gets colder, more that there is less matter about to possess the property of heat at higher altitude. Vacuums aren't so much cold as empty of anything that might be warm.

Triffic reply. You're dead clever and I am going to add you to my list of "7 thankyou's for a bus ticket" forum.

MSSSSPPI


Hmmm.. I thInk I'm mIssIng somethIng.