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A question on density
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mike
science forum beginner


Joined: 07 Jun 2005
Posts: 3

PostPosted: Sat Jun 11, 2005 8:51 pm    Post subject: A question on density Reply with quote
Something that has on occasion come to mind since my schooldays.
But I'm just too thick to be able to decide on the answer.
Perhaps someone here might be able to put it to rest for me?

If I had a large hollow sphere *, and removed the air within to have a
vacuum.
Would it lift into the atmosphere?

*Made of a lightweight plastic. For arguments sake tough enough to
hold its form.
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PD
science forum Guru


Joined: 03 May 2005
Posts: 4363

PostPosted: Sat Jun 11, 2005 9:07 pm    Post subject: Re: A question on density Reply with quote
mike wrote:
Quote:
Something that has on occasion come to mind since my schooldays.
But I'm just too thick to be able to decide on the answer.
Perhaps someone here might be able to put it to rest for me?

If I had a large hollow sphere *, and removed the air within to have a
vacuum.
Would it lift into the atmosphere?

If the weight of the plastic alone is less than the weight of the
entire sphere's worth of air, yes.

A hot-air balloon is precisely this principle. A hot air balloon
doesn't rise because the air is hot. The balloon rises because hot air
is less dense (closer to a vacuum) than the surrounding air. Replacing
the hot air with a vacuum is just the limit of that principle.

Quote:

*Made of a lightweight plastic. For arguments sake tough enough to
hold its form.

PD
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Clemens W
science forum Guru Wannabe


Joined: 17 May 2005
Posts: 172

PostPosted: Sun Jun 12, 2005 7:34 am    Post subject: Re: A question on density Reply with quote
mike wrote:
Quote:
Something that has on occasion come to mind since my schooldays.
But I'm just too thick to be able to decide on the answer.
Perhaps someone here might be able to put it to rest for me?

If I had a large hollow sphere *, and removed the air within to have a
vacuum.
Would it lift into the atmosphere?

*Made of a lightweight plastic. For arguments sake tough enough to
hold its form.

Theoretically: Yes. Air has a density of about 1.225 kg/m^3, so if you
take a evacuated, hollow sphere with a volume of 1 m^3 (which would
mean a diameter of about 1,24 m, since V=4/3 * PI * r^3) with a weight
of less than the above mentioned 1.225 kg, then the sphere would be
lighter than air and lift into the athmosphere.

Practically: No. The same sphere would have to withstand athmospheric
pressure (about 10^5 Pa, please consult local barometer for exact
values). Since S=4 * PI * r^2, a sphere with a volume V of 1m^3 has a
surface S of 4,84 m^2. The total force exerted by air pressure
therefore becomes 4,84*10^5 N, equivalent the weight of about 490
metric tons.

The term "flat as a pancake" crosses my mind all of a sudden ;-)

Good luck,

A. Friend
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mike
science forum beginner


Joined: 07 Jun 2005
Posts: 3

PostPosted: Sun Jun 12, 2005 2:27 pm    Post subject: Re: A question on density Reply with quote
Thanks to all for the replies, wasn't too sure if i was right or wrong
on that, i cant even recall why the question ever came to me!
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Mike Yarwood
science forum Guru Wannabe


Joined: 02 May 2005
Posts: 117

PostPosted: Mon Jun 13, 2005 6:47 am    Post subject: Re: A question on density Reply with quote
"mike" <mike@invariant.freeserve.co.uk> wrote in message
news:mgooa19q9lspnoqb3h91l7i1t0b5j6ie88@4ax.com...
Quote:

Thanks to all for the replies, wasn't too sure if i was right or wrong
on that, i cant even recall why the question ever came to me!

You maybe remember "Tarzan at the Earths Core" by Edgar Rice-Burroughs

(might have been called Pelucidar - I can't really remember) where they
travelled by sepellin to one of the large holes at the earth's poles where
you can enter Pelucidar. The zepellin did not use hydrogen but instead
relied on evacuating the interior of a shell made from a new, light and
incredibly strong alloy.

Best of Luck - Mike
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tadchem
science forum Guru


Joined: 03 May 2005
Posts: 1348

PostPosted: Mon Jun 13, 2005 11:42 am    Post subject: Re: A question on density Reply with quote
Mike Yarwood wrote:

Quote:
You maybe remember "Tarzan at the Earths Core" by Edgar Rice-Burroughs

http://www.ac.wwu.edu/~stephan/Tarzan/tarzan13.html
"Tarzan at the Earth's Core", Edgar Rice-Burroughs, (1930) Metropolitan
Books, Inc.

Quote:
(might have been called Pelucidar - I can't really remember)
Story summary:

http://www.erblist.com/erblist/taecsummary.html
The domain visited was named "Pellucidar."

Quote:
where they
travelled by sepellin

Zeppelin
http://spot.colorado.edu/~dziadeck/zeppelin.html

Quote:
to one of the large holes at the earth's poles where
you can enter Pelucidar.

The "hollow earth" theory is old
http://www.unmuseum.org/hollow.htm
and thoroughly discredited, but there are still some cranks who can't
let go of it.

Quote:
The zepellin did not use hydrogen but instead
relied on evacuating the interior of a shell made from a new, light and
incredibly strong alloy.

....a light metal called 'Harbenite'.

The gain in buoyancy would be problematical. Which would weigh more,
the hydrogen in a full Zepplin or the excess weight of a hull stiff
enough to resist crushing under 1 atmosphere pressure?

[remember the well-known 'crush-the-can-with-air-pressure' demo?]
http://www.meigsmagnet.org/~franklint/geewhiz.html#Crushing%20a%20Can%20with%20Air%20Pressure

Tom Davidson
Richmond, VA
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PD
science forum Guru


Joined: 03 May 2005
Posts: 4363

PostPosted: Mon Jun 13, 2005 11:49 am    Post subject: Re: A question on density Reply with quote
Clemens W wrote:
Quote:
mike wrote:
Something that has on occasion come to mind since my schooldays.
But I'm just too thick to be able to decide on the answer.
Perhaps someone here might be able to put it to rest for me?

If I had a large hollow sphere *, and removed the air within to have a
vacuum.
Would it lift into the atmosphere?

*Made of a lightweight plastic. For arguments sake tough enough to
hold its form.

Theoretically: Yes. Air has a density of about 1.225 kg/m^3, so if you
take a evacuated, hollow sphere with a volume of 1 m^3 (which would
mean a diameter of about 1,24 m, since V=4/3 * PI * r^3) with a weight
of less than the above mentioned 1.225 kg, then the sphere would be
lighter than air and lift into the athmosphere.

Practically: No. The same sphere would have to withstand athmospheric
pressure (about 10^5 Pa, please consult local barometer for exact
values). Since S=4 * PI * r^2, a sphere with a volume V of 1m^3 has a
surface S of 4,84 m^2. The total force exerted by air pressure
therefore becomes 4,84*10^5 N, equivalent the weight of about 490
metric tons.

The term "flat as a pancake" crosses my mind all of a sudden ;-)


And of course this got me thinking about how a hot air balloon, which
has a shell of absolutely non-rigid structure, stays intact. So, for
the sake of completeness, the reason why a hot air balloon rises is
because the effective density is less than the surrounding air. The
reason why it stays inflated is because the air inside is hotter.

PD
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Mike Yarwood
science forum Guru Wannabe


Joined: 02 May 2005
Posts: 117

PostPosted: Tue Jun 14, 2005 8:56 pm    Post subject: Re: A question on density Reply with quote
"tadchem" <thomas.davidson@dla.mil> wrote in message
news:1118670179.377886.122360@g47g2000cwa.googlegroups.com...
Quote:


Mike Yarwood wrote:

You maybe remember "Tarzan at the Earths Core" by Edgar Rice-Burroughs

http://www.ac.wwu.edu/~stephan/Tarzan/tarzan13.html
"Tarzan at the Earth's Core", Edgar Rice-Burroughs, (1930) Metropolitan
Books, Inc.

(might have been called Pelucidar - I can't really remember)
Story summary:
http://www.erblist.com/erblist/taecsummary.html
The domain visited was named "Pellucidar."

where they
travelled by sepellin

Zeppelin
http://spot.colorado.edu/~dziadeck/zeppelin.html

to one of the large holes at the earth's poles where
you can enter Pelucidar.

The "hollow earth" theory is old
http://www.unmuseum.org/hollow.htm
and thoroughly discredited, but there are still some cranks who can't
let go of it.

The zepellin did not use hydrogen but instead
relied on evacuating the interior of a shell made from a new, light and
incredibly strong alloy.

...a light metal called 'Harbenite'.

The gain in buoyancy would be problematical. Which would weigh more,
the hydrogen in a full Zepplin or the excess weight of a hull stiff
enough to resist crushing under 1 atmosphere pressure?

[remember the well-known 'crush-the-can-with-air-pressure' demo?]
http://www.meigsmagnet.org/~franklint/geewhiz.html#Crushing%20a%20Can%20with%20Air%20Pressure

Ah yes but it was a ripping yarn! and just as good as lighter than air

craft filled with special gasses illuminated by (was it the eighth?)
barsoomian ray.

Harbenite - you have a much better memory than me.

Best of Luck - Mike
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tadchem
science forum Guru


Joined: 03 May 2005
Posts: 1348

PostPosted: Wed Jun 15, 2005 12:02 pm    Post subject: Re: A question on density Reply with quote
Mike Yarwood wrote:

<snip>

Quote:
... you have a much better memory than me.

Credit for that goes to Deja Thoris, a true memeory motivator...

<grin>

Tom Davidson
Richmond, VA
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