On Jun 4, 9:15 pm,

In sci.physics PD <TheDraperFam...@gmail.com> wrote:

On Jun 4, 5:35 pm,

In sci.physics PD <TheDraperFam...@gmail.com> wrote:

On Jun 4, 1:55 pm,

No, the point is the weight of the balloon equals the weight of the
contained gas plus the weight of the balloon.

The density of the balloon equals the total weight divided by the
volume.

If the density of the balloon is less than the surrounding air, it
goes up.

If the density of the balloon is greater than the surrounding air, it
goes down.

That's it. Pressures around the balloon have nothing to do with the
balloon going up or down.

This is just plain wrong, Jim. If you immerse a cinderblock in water
by a rope, it will still go down, but its apparent weight (the tension
you feel in the rope) will decrease. This decrease is not because
gravity has changed (it hasn't). It is because there is a NEW force
acting on the block, and that is the buoyancy force upwards. Now,
where the hell did the buoyancy force come from? What could be acting
on the cinderblock to produce the buoyancy force? There is only one
thing available, and that's the surrounding water. But the only way
that water can exert a force on the brick is through its pressure
acting on the block, and the fact that this pressure results in a
buoyancy force UPWARDS tells you that there MUST be a gradient in the
pressure of the water on the cinderblock.
If you have a can of Campbell's Cream of Buckshot Soup, and a can of
Campbell's Cream of Styrofoam Soup, there is NO difference whatsoever
in the buoyancy force between the two cases. (How could it be
different? The water can't see the contents of the can.) However, the
downward gravitational force on the Cream of Buckshot Soup is greater
than this buoyancy force (and so it goes down), and the gravitational
force on the Cream of Styrofoam Soup is less than this same buoyancy
force (and so that can goes up).

So, by that logic, if take a 10 foot round balloon, fill it with
hydrogen, tie it off, then attach a short hose to the static port
of an altimeter and stick the hose next to the bottom, then the
top of the balloon, I will see something other than a 10 foot
difference in altitude because the buoyancy force increases the
air pressure on the bottom of the balloon?

Now I take a 10 lead sphere and hang it from a support and do the same
thing with the altimeter. Does the altimeter now the top of the lead
sphere much lower than the bottom of the sphere because of the
negative buoyancy force increasing the air pressure at the top?

--
Jim Pennino

Remove .spam.sux to reply.

No, you still didn't get it quite right. A 10 ft radius balloon has a
volume of 4200 cubic feet or 119 cubic meters.
The density of air is 1.3 kg/cubic meter. From the top to the bottom
of either the hydrogen balloon or the lead balloon, the pressure
difference is
(1.3 kg/m^3)(9.8 m/s^2)(6.1 m) = 78 N/m^2, about 7.7E-4 atm.
If you integrate the pressure difference over the surface of the
balloon, you find that the buoyant force upwards is 1500 N. This
buoyant force is the *same* for the lead balloon and for the hydrogen
balloon and points upwards in both cases. [This integral is left as an
exercise for the reader.]
Now, the graviational weight of the hydrogen in a 10 ft radius balloon
is 105 N. The weight of the lead balloon is 13E6 N. It is pretty easy
to see that 1500 N > 105 N, which is why the hydrogen balloon rises.
It is also pretty easy to see that 1500 N < 13E6 N, which is why the
lead balloon falls.
As I said, the gravitational forces acting on the 10 ft hydrogen and
lead balloons are dramatically different. The buoyant force, due to
the small atmospheric pressure gradient from top to bottom of both
balloons, is the same.

So, you spent 5 paragraphs to say that air pressure changes by
about 1" Hg per 1000 feet?

I believe I said that about 3 or 4 posts ago.

I agreed with you that the pressure difference (and hence the buoyancy
force) is too small to affect the flight of a tennis ball in any
significant way.
You also said that the gradient in pressure had nothing whatsoever to
do with why a balloon rises, and I've just spent 5 paragraphs
explaining to you why what you said is a load of hooey.

And you spent several posts to say there are no "magic" pressure
differentials due to the force of buoyancy and the only pressure
change is that due to altitude?

No, I didn't say anything of the kind. I said that the force of
buoyancy is due *precisely* to pressure differentials, though no magic
of any kind is required.

I believe I also said that about 3 or 4 posts ago.

--
Jim Pennino

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On Jun 4, 9:15 pm,

In sci.physics PD <TheDraperFam...@gmail.com> wrote:

On Jun 4, 5:35 pm,

In sci.physics PD <TheDraperFam...@gmail.com> wrote:

On Jun 4, 1:55 pm,

No, the point is the weight of the balloon equals the weight of the
contained gas plus the weight of the balloon.

The density of the balloon equals the total weight divided by the
volume.

If the density of the balloon is less than the surrounding air, it
goes up.

If the density of the balloon is greater than the surrounding air, it
goes down.

That's it. Pressures around the balloon have nothing to do with the
balloon going up or down.

This is just plain wrong, Jim. If you immerse a cinderblock in water
by a rope, it will still go down, but its apparent weight (the tension
you feel in the rope) will decrease. This decrease is not because
gravity has changed (it hasn't). It is because there is a NEW force
acting on the block, and that is the buoyancy force upwards. Now,
where the hell did the buoyancy force come from? What could be acting
on the cinderblock to produce the buoyancy force? There is only one
thing available, and that's the surrounding water. But the only way
that water can exert a force on the brick is through its pressure
acting on the block, and the fact that this pressure results in a
buoyancy force UPWARDS tells you that there MUST be a gradient in the
pressure of the water on the cinderblock.
If you have a can of Campbell's Cream of Buckshot Soup, and a can of
Campbell's Cream of Styrofoam Soup, there is NO difference whatsoever
in the buoyancy force between the two cases. (How could it be
different? The water can't see the contents of the can.) However, the
downward gravitational force on the Cream of Buckshot Soup is greater
than this buoyancy force (and so it goes down), and the gravitational
force on the Cream of Styrofoam Soup is less than this same buoyancy
force (and so that can goes up).

So, by that logic, if take a 10 foot round balloon, fill it with
hydrogen, tie it off, then attach a short hose to the static port
of an altimeter and stick the hose next to the bottom, then the
top of the balloon, I will see something other than a 10 foot
difference in altitude because the buoyancy force increases the
air pressure on the bottom of the balloon?

Now I take a 10 lead sphere and hang it from a support and do the same
thing with the altimeter. Does the altimeter now the top of the lead
sphere much lower than the bottom of the sphere because of the
negative buoyancy force increasing the air pressure at the top?

--
Jim Pennino

Remove .spam.sux to reply.

No, you still didn't get it quite right. A 10 ft radius balloon has a
volume of 4200 cubic feet or 119 cubic meters.
The density of air is 1.3 kg/cubic meter. From the top to the bottom
of either the hydrogen balloon or the lead balloon, the pressure
difference is
(1.3 kg/m^3)(9.8 m/s^2)(6.1 m) = 78 N/m^2, about 7.7E-4 atm.
If you integrate the pressure difference over the surface of the
balloon, you find that the buoyant force upwards is 1500 N. This
buoyant force is the *same* for the lead balloon and for the hydrogen
balloon and points upwards in both cases. [This integral is left as an
exercise for the reader.]
Now, the graviational weight of the hydrogen in a 10 ft radius balloon
is 105 N. The weight of the lead balloon is 13E6 N. It is pretty easy
to see that 1500 N > 105 N, which is why the hydrogen balloon rises.
It is also pretty easy to see that 1500 N < 13E6 N, which is why the
lead balloon falls.
As I said, the gravitational forces acting on the 10 ft hydrogen and
lead balloons are dramatically different. The buoyant force, due to
the small atmospheric pressure gradient from top to bottom of both
balloons, is the same.

So, you spent 5 paragraphs to say that air pressure changes by
about 1" Hg per 1000 feet?

I believe I said that about 3 or 4 posts ago.

I agreed with you that the pressure difference (and hence the buoyancy
force) is too small to affect the flight of a tennis ball in any
significant way.

You also said that the gradient in pressure had nothing whatsoever to
do with why a balloon rises, and I've just spent 5 paragraphs
explaining to you why what you said is a load of hooey.

And you spent several posts to say there are no "magic" pressure
differentials due to the force of buoyancy and the only pressure
change is that due to altitude?

No, I didn't say anything of the kind. I said that the force of
buoyancy is due *precisely* to pressure differentials, though no magic
of any kind is required.

I believe I also said that about 3 or 4 posts ago.

--
Jim Pennino

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