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DF3VI > INFO 21.07.02 12:52l 100 Lines 4655 Bytes #999 (180) @ WW
BID : L7CDB0OVN00S
Read: DB0FHN GUEST
Subj: Re^2: Balloons/Hot Air
Path: DB0FHN<DB0ZWI<DB0HDF<DB0ERF<DB0FBB<DB0GOS<DB0OVN
Sent: 020721/1032z @:DB0OVN.#NRW.DEU.EU [BCM West<>Link - Neuss] bcm1.46 LT:180
From: DF3VI @ DB0OVN.#NRW.DEU.EU (Patrick)
To: INFO @ WW
X-Info: Sent with login password
Alan (G0FUM) wrote:
> The easiest way to assimulate Balloons rising etc is down to
>pressure basicaly.
>
> Atmospheric pressure reduces the higher you go due to the gravitational
>pull of the earth on the air surrounding our planet.Air has density
>therfore weight/mass.
>
> Hot air is more rarified than cold air so is lighter, as the molecules
>are more widely spaced (Less dense). therefor the average atmospheric
>pressure is greater than the hot air, so it will rise to find an equal
>pressure.
If it would be the difference of pressure that makes a balloon
rising, then how can an open hot air balloon rise?
The inside pressure is exactly like the outside pressure, because
this balloon is OPEN on the bottom side. A higher inside pressure
cannot be held up.
No, what makes a balloon rising is the DENSITY of the inside media
versus the outside media. Or to be more exact, the DIFFERENCE
of the density, which results in a mass difference. The bigger the mass
difference, the bigger the lifting force.
This is why a balloon floats in air. Just like a ship in water...
Actually, the theoretically best media for a balloon would be a vacuum.
NOT because it has no inside pressure, but because it has NO MASS.
The only trouble is, it takes some very rigid material around to keep
the shape, and this is why a vacuum balloon wouldn't float in air
(and why vacuum tubes don't fly around, hihi).
So it needs the pressure of an inside media to keep the enevlope material
light and easy, not exceeding the lifting force of the inside gas.
Think about why a helium balloon floats in air why an air balloon with
the same inside pressure stays on ground...
The reason why a balloon on the moon falls down is not because of the
missing air pressure around but because of the missing surrounding media.
There is nothing that a balloon could float in. The air pressure around
a balloon on earth is the same all around and gives no effective lifting
or sinking force.
Air pressure just determines how big a balloon will be: The inside and outside
pressure difference gives a force to the envelope and will stretch it until
the tension of the envelope material equals that pressure difference.
If a balloon rises, the outer air pressure reduces, and thus the balloon
widens until the material cannot stand the force anymore and will burst...
> Now moving on to another aspect. If you blow up a balloon then let the
>spout go the escaping air will propel the baloon like a jet engine.
>
> Now what is it that does the propelling,
>
> Is it the jet of air ejecting from the balloon? NO
>
> It is the pressure inside the balloon that creates the motive force.
>
> A sealed balloon has an equal pressure on all its inside surface. If one
>point on that surface is opened then the pressure on that side is reduced
>.
>
>The unequal pressure on the inside of the balloon will cause the side with
>the higher pressure to move away from the side with the lower pressure.
>
If your theory of pressure difference would be true, that balloon must move
in opposite direction than it does, because the moving force is lower where
the pressure is lower.
> This is how a Jet Engine works.
> A rocket works by the same principal.
That can't be correct either. If it would, the rocket engine wouldn't work
in space. Actually it IS the propelled MASS which drives a rocket forward,
according to the old Newton law "Actio = Reaction".
And it is the same for that above mentioned balloon....
Nice experiment: Take a boat full of stones. Now throw away one stone after
another to the back. You will see that the boat will move slowly forward
(into the opposite direction), though there is no pressure difference.
And even if you take the stone from one side only, it has no effect on
the direction (but the boat might sink because being unballanced, hihi).
PS: Why doesn't a jet engine work in space? Because there is no air for
combustion! If you supply that air, it will give a certain force. But the
main force of a jet engine is not the exhausted gas, but the huge mass of
COLD air which is moved backwards by the surrounding turbine!
73
y
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