Re: OT Here is an example of pseudo science.

[Doctor Drivel]

Roger is a babbling idiot. Sad but true.

[Doctor Drivel]

It does. It goes up. An anti-gravity device.

Yep. One end defies gravity and goes up.

[Doctor Drivel]

A baloon is an anti-gravity device, as is a plane. It defies gravity at a point.

[Roger Chapman]

Dribble isn't clever enough to come to that conclusion about anyone but himself.

Some 10 years ago Dribble (then masquerading as Adam) asked on another ng for an explanation of latent heat and at much the same time on this ng asked if metres cubed was a measure of speed. Such a display of ignorance made his parallel claim to be a heating engineer absolutely preposterous while his inability to understand that constantly posting a mixture of garbage and insults reinforces the impression that he is terminally stupid.

[Ronald Raygun]

I think you misunderstand the meaning of "defy". If you have access to a decent dictionary, try looking it up.

[Tim Streater]

Quite right. If the earth suddenly vanished and left the air behind, a balloon would no longer rise in the air. [1]

Objects in a gravitational field not in free fall (e.g objects on the surface of the earth) can be said to be *resisting* gravity, by doing so they experience weight. Something in free fall, such as a satellite or a person on the ISS, is not resisting gravity and so experiences no weight. It still has mass, of course. The word "defy" does not apply in any relevant context.

[1] Actually this is not quite true. There would be a tiny residual gravitational field due to the mass of the atmosphere, in a thin shell (until it dispersed of course). The field would act as if all the mass were concentrated at the centre of the shell.

[Andy Champ]

Surely if the air stayed still the balloon would still rise, because of the (gravitationally induced) pressure difference across its height? And in fact would rise faster, as there would now be no gravity holding it back?

Of course the pressure difference would vanish as the air scattered rather rapidly...

Andy.

[Tim Streater]

But next-to-no gravity to pull the air down more than the balloon.

[Doctor Drivel]

Roger is a babbling idiot. Sad but true.

[Doctor Drivel]

If men did not put gas in the balloon it would not resist gravity,the fabric of the balloon would go the other way - DOWN towards the pull of gravity. So it is an anti-gravity machine.

[Rick Cavallaro]

No need to put gas in the balloon to make it rise. I once made a hot air balloon by simply taping 6 big trash bags together into a big tetrahedron. I put nothing but air inside. The sun warms it up, and it goes thousands of feet into the air. You could call it "defying" gravity if you like, but it defies gravity to pretty much the same extent that you defy gravity when you get out of bed.

[Gib Bogle]

OK, I stand corrected.

[Roger Chapman]

Dribble isn't clever enough to come to that conclusion about anyone but himself.

Some 10 years ago Dribble (then masquerading as Adam) asked on another ng for an explanation of latent heat and at much the same time on this ng asked if metres cubed was a measure of speed. Such a display of ignorance made his parallel claim to be a heating engineer absolutely preposterous while his inability to understand that constantly posting a mixture of garbage and insults reinforces the impression that he is terminally stupid.

Dribble seems under the impression that if he edits out of his replies facts he is unable to dispute they disappear from the public record but all the potentially endless repetition merely gives extra publicity to the original facts.

I don't intend to bother reinstating my original comments again unless Dribble adds something new.

[Ronald Raygun]

Agreed.

What pressure difference? If you're saying the air pressure outside the balloon is lower near the top of the balloon than near the bottom of the balloon, isn't this compensated for by the gas pressure inside the balloon also being higher at the bottom than the top? The motivating force would continue to be entirely due to density difference between gas and air.

Eh? Gravity never did hold it back, it was what *caused* it to go up! It would rise slower because there would be less gravity.

The pressure of both air and gas would decrease so much, that the balloon would soon burst. But if the balloon were strong enough not to burst, the tension in its material would at a certain size prevent the gas pressure inside from dropping further. In due course the gas's density would exceed that of the air outside, which is still expanding, and then the balloon would "sink" to the middle of the air cloud.

[Rick Cavallaro]

If we're still imagining the air cloud to be a shell with nothing in the middle (i.e. the place where the earth is missing), objects would not sink to the middle of that void. There's no gravity inside a uniform shell of any thickness.

I still owe you a response on your most recent analysis. Sorry for the delay.

[Roger Chapman]

You sure about that?

[Alan Braggins]

He ought to be, it's a standard result:

a spinning atmosphere with weather patterns will remain as approximately a spherical shell for long when the earth disappears is another question, but he did say "If we're still imagining ..."

[Roger Chapman]

Thanks for the pointer. It is something you either know or don't know as it is not exactly obvious that the sum of the gravitational forces on a body within a planetary shell is zero regardless of position, but I take a crumb of comfort in the fact that the thought that first came to mind was not of a planetary shell at all but of a man sized sphere with Dribble weightless within it 'defying gravity'.

That lack of knowledge reminds me of a put-down by a climbing acquaintance some 44 years ago, when discussing the dynamic waist belay: some thing like "call yourself an engineer and you don't know that the capstan effect is exponential" and also a few years later, when the boot was on the other foot my less than complimentary remarks to another acquaintance who was a geologist and sadly lacking in any knowledge of Radon and the danger it posed to householders in certain parts of the country. However expert you are in a subject you can't possibly know it all.

[The Natural Philosopher]

Hmm. there's a nasty integral to be done there..Or some tensor calculus..

I would not be surprised if it were true.

[Rick Cavallaro]

It's true, and the integral is not pretty. It's more easily proved by simulation (in my opinion).