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How does buoyancy work?

J

josephkk

Jan 1, 1970
0
Strictly speaking, that's neutral buoyancy. A floating object must
displace an amount of water that weighs more than it does.


Mark L. Fergerson

Did you mean a forcibly submerged object?

?-)
 
R

rickman

Jan 1, 1970
0
Polyethelene and polypropylene are the only ones that come to mind.

You mean polyethelene but not high density polyethelene. The stuff milk
bottles are made of does not float until they are made into plastic
kayaks and filled with paddlers.
 
R

rickman

Jan 1, 1970
0
Strictly speaking, that's neutral buoyancy. A floating object must
displace an amount of water that weighs more than it does.

If it displaced an amount of water that weighed more than the object, it
not only would float, it would rise until the rule Jim stated was met.
I think you mean if the object is totally submerged it would displace
water of a greater weight than itself, but that would require you to
hold it down. The rule is about the equilibrium condition.

One thing folks have not mentioned that is important. When that water
is displaced, it is pushed into the body of water which causes the level
of the surface of water to rise. This rise of the water level is what
stores the energy of the object lowering in the gravitational field.
 
M

MrTallyman

Jan 1, 1970
0
Did the paisons miss the adjective "gradient" ?>:-}

...Jim Thompson


Never saw his post, but what exactly does he think one would "float on"
in space?

Without a gravitational attractor, all one does is traverse through
space, and one would punch through anything one encountered in the form
of gasses. With liquids, The results of that impact would depend on
several factors with one of the main ones being relative velocity. What
liquids exist in space, and how does it stay glommed together? Lotta
water ice there, but any glomming was likely while it was solid ice.

But no floating.
 
S

Spehro Pefhany

Jan 1, 1970
0
You've obviously never heard of "floating through space".

Oh yes, they float, they all float up there. and when you're up there
YOU'LL FLOAT TOO!"


Best regards,
Spehro Pefhany
 
P

Phil Allison

Jan 1, 1970
0
"Pimpom"
It will still float if the material is less dense than water, which may
well be the case although I'm not sure either way (wish I had a ping-pong
ball I could test that with). Some common plastics *are* less dense than
water.

** A sometimes missed fact is that when an object IS more dense than water -
it sinks all the way to the bottom.

IOW, objects either sink or float.

Fish and submarines etc can maintain a given depth, but that takes
engineering and the ability to adjust buoyancy continuously.

A sub that has negative buoyancy will sink until the hull is crushed and
then drop like a stone.

Also, as water gets deeper, its density does not change due to increasing
pressure but only temp variations.


.... Phil
 
U

Uwe Hercksen

Jan 1, 1970
0
Strictly speaking, that's neutral buoyancy. A floating object must
displace an amount of water that weighs more than it does.

Hello,

no, that is wrong. A swimming ship sinks so deep in water that it
displaces an amount of water that weighs the same than the ship does.
The ship sinks if it is not able to displace so much weight of water
that the ship weighs.

Neutral bouyancy has a submarine when it is completely under the water
and stays in every depth without sinking or rising. No propulsion active
of course.

Bye
 
U

Uwe Hercksen

Jan 1, 1970
0
Phil said:
Also, as water gets deeper, its density does not change due to increasing
pressure but only temp variations.

Hello,

water is compressible too, using 4000 bar the density is doubled.

Bye
 
U

Uwe Hercksen

Jan 1, 1970
0
Phil said:
Hello,

you should read more carefully what you cite.

"The low compressibility of non-gases, and of water in particular, leads
to their often being assumed as incompressible. The low compressibility
of water means that even in the deep oceans at 4 km depth, where
pressures are 40 MPa, there is only a 1.8% decrease in volume."

I was writing about a pressure of 4000 bar or 400 MPa.

Bye
 
S

Spehro Pefhany

Jan 1, 1970
0
P

Phil Allison

Jan 1, 1970
0
"Spehro Pefhany"
qrk
Uwe Hercksen


Thing is, it's not linear, thus the bulk modulus is not a constant- it
increases with pressure.

Water has about 11% volume change at 4000 bar:
http://image.thefabricator.com/a/articles/photos/1333/fig1.jpg

** Correct, 4000 bar corresponds with 60,000 psi.

Water is extraordinarily incompressible, less so than steel.

Diamond and few modern materials are less compressible than water.



..... Phil
 
G

George Herold

Jan 1, 1970
0
"Spehro Pefhany"








  ** Correct,  4000 bar corresponds with 60,000 psi.

Water is extraordinarily incompressible, less so than steel.

Diamond and few modern materials are less compressible than water.

....  Phil- Hide quoted text -

- Show quoted text -

I'd call that the bulk modulus
http://en.wikipedia.org/wiki/Bulk_modulus
At one atm. at least steel is ~50 times larger.

George H.
 
J

Jasen Betts

Jan 1, 1970
0
On Feb 19, 8:41 am, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-

Strictly speaking, that's neutral buoyancy. A floating object must
displace an amount of water that weighs more than it does.

The bit that's above the water isn't diplacing any water.
so ignoring boyancy in the atmosphere the volumne of wter displaced
has the mass of the whole floating object.

See "Archimedes's principle"
 
Hello,

no, that is wrong. A swimming ship sinks so deep in water that it
displaces an amount of water that weighs the same than the ship does.
The ship sinks if it is not able to displace so much weight of water
that the ship weighs.

Neutral bouyancy has a submarine when it is completely under the water
and stays in every depth without sinking or rising. No propulsion active
of course.

Again, buoyancy is about density. All other things being equal, a
submarine-shaped volume of water does not rise or sink because it has
the same density as the water surrounding it (it displaces a volume of
water that has its exact weight).

For a submarine to occupy that space without rising or sinking in
must have the exact same density as the sub-shaped volume of water; it
will therefore by definition *weigh* the same.

Submarines *do not* need active "propulsion" to change depth;
water's density conveniently increases with depth so they fill or
flood their ballast tanks which changes the sub's overall *density*,
forcing it to rise or sink to a depth where the water's density
matches its own.


Mark L. Fergerson
 
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