| Topic: |
Science > Physics |
| User: |
"James" |
| Date: |
29 Mar 2005 03:32:11 AM |
| Object: |
What is buoyancy? |
Is buoyancy the net force of all fluid pressures (as vectors)
perpendicular to all the surface on an object? Or, is buoyancy a true
calculation of fluid displacement? I am reading various descriptions of
it as an "up force based on fluid displacement" but that wouldn't make
nearly as much sense to me.
James
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| User: "Schoenfeld" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 03:45:28 AM |
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James wrote:
Is buoyancy the net force of all fluid pressures (as vectors)
perpendicular to all the surface on an object? Or, is buoyancy a true
calculation of fluid displacement? I am reading various descriptions
of
it as an "up force based on fluid displacement" but that wouldn't
make
nearly as much sense to me.
James
It's just the weight of the displaced medium. (Weight is a force).
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| User: "Sam Wormley" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 07:36:38 AM |
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James wrote:
Is buoyancy the net force of all fluid pressures (as vectors)
perpendicular to all the surface on an object? Or, is buoyancy a true
calculation of fluid displacement? I am reading various descriptions of
it as an "up force based on fluid displacement" but that wouldn't make
nearly as much sense to me.
http://scienceworld.wolfram.com/physics/Buoyancy.html
http://scienceworld.wolfram.com/physics/BuoyancyForce.html
http://scienceworld.wolfram.com/physics/ArchimedesPrinciple.html
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| User: "Sam Wormley" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 10:08:29 AM |
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James wrote:
Is buoyancy the net force of all fluid pressures (as vectors)
perpendicular to all the surface on an object? Or, is buoyancy a true
calculation of fluid displacement? I am reading various descriptions of
it as an "up force based on fluid displacement" but that wouldn't make
nearly as much sense to me.
James
http://scienceworld.wolfram.com/physics/Buoyancy.html
http://scienceworld.wolfram.com/physics/BuoyancyForce.html
http://scienceworld.wolfram.com/physics/ArchimedesPrinciple.html
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| User: "tj Frazir" |
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| Title: Re: What is buoyancy? |
30 Mar 2005 11:25:09 AM |
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Gravity is pushing evrything into the same space.
This mole is trying to role and push the next mole out of its way .
two moles of equal mass push against each other and dont move .
But if one mole is 1 pound and the next mole is 1 oz then the big one
will push it out of the way and if its between 2 moles a pound each
then it will be squashed up and sit on top of the two moles. if its a 3
pound mole then it will push the two moles apart and fall between
pushing one pound moles out of the way.
I have a balloon and if its full of big moles it will push moles out
of its way but if a foot of balloon is less wieght than a foot of air
the air will push together and push the balloon up out of its way .
Gravity pushes the balloon up.
Gravity pushes the sea together ,,remove the psi and it come apart and
be a gas.
Gravity pushes down on the water I displace and pushes the water
together .
So gravity is pushing up on my ship.
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| User: "Andy Resnick" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 09:00:50 AM |
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James wrote:
Is buoyancy the net force of all fluid pressures (as vectors)
perpendicular to all the surface on an object? Or, is buoyancy a true
calculation of fluid displacement? I am reading various descriptions of
it as an "up force based on fluid displacement" but that wouldn't make
nearly as much sense to me.
Both are correct- the first definition you gave is a bit mangled-
pressure is a scalar, but pressure is also the normal component of the
stress tensor. So, by definition, the pressure (defined at a point on a
dividing surface) is associated with a vector normal to the surface at
that point. Integrating over the entire surface will give a directed
force, and it can be shown that the net force is directed either up or
down (depending on relative densities)- the sideways components cancel.
The magnitude of the force is given by the difference in weights (not
masses!) of the object and displaced fluid.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Andy Resnick" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 11:23:05 AM |
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Andy Resnick wrote:
James wrote:
Is buoyancy the net force of all fluid pressures (as vectors)
perpendicular to all the surface on an object? Or, is buoyancy a true
calculation of fluid displacement? I am reading various descriptions of
it as an "up force based on fluid displacement" but that wouldn't make
nearly as much sense to me.
Both are correct- the first definition you gave is a bit mangled-
pressure is a scalar, but pressure is also the normal component of the
stress tensor.
Oops- I meant "the isotropic part of the stress tensor". Sorry.
So, by definition, the pressure (defined at a point on a
dividing surface) is associated with a vector normal to the surface at
that point. Integrating over the entire surface will give a directed
force, and it can be shown that the net force is directed either up or
down (depending on relative densities)- the sideways components cancel.
The magnitude of the force is given by the difference in weights (not
masses!) of the object and displaced fluid.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Randy Poe" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 09:09:38 AM |
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Andy Resnick wrote:
James wrote:
Is buoyancy the net force of all fluid pressures (as vectors)
perpendicular to all the surface on an object? Or, is buoyancy a
true
calculation of fluid displacement? I am reading various
descriptions of
it as an "up force based on fluid displacement" but that wouldn't
make
nearly as much sense to me.
Both are correct- the first definition you gave is a bit mangled-
pressure is a scalar, but pressure is also the normal component of
the
stress tensor. So, by definition, the pressure (defined at a point
on a
dividing surface) is associated with a vector normal to the surface
at
that point. Integrating over the entire surface will give a directed
force, and it can be shown that the net force is directed either up
or
down (depending on relative densities)- the sideways components
cancel.
The magnitude of the force is given by the difference in weights (not
masses!) of the object and displaced fluid.
Wasn't there a heated discussion here a month or so
ago about whether the buoyant force would be different
for an object resting on the bottom because you'd be
missing one surface of integration?
How did that resolve? I guess for most real objects
with rough surfaces, there would be plenty of
fluid on the bottom as well and so the usual
buoyancy model would apply.
- Randy
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Andy Resnick" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 11:31:58 AM |
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Randy Poe wrote:
Andy Resnick wrote:
<snip>
Wasn't there a heated discussion here a month or so
ago about whether the buoyant force would be different
for an object resting on the bottom because you'd be
missing one surface of integration?
How did that resolve? I guess for most real objects
with rough surfaces, there would be plenty of
fluid on the bottom as well and so the usual
buoyancy model would apply.
I don't recall that discussion (I tend to ignore a lot of things in this
group), but it doesn't present any difficulty. Cauchy's law still
applies to a solid-solid dividing surface, one is not missing a surface
of integration. The specifics are probably nasty due to the more
complex nature of a solid than a simple fluid, and even worse if one
considers a multi-phase surface (water-logged sand, or mud, for example,
but the end result should still be something like the upward force of
the surface on the object is Mg, where M is the difference in mass
between the object and the displaced fluid.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Schoenfeld" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 10:21:29 PM |
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Andy Resnick wrote:
Randy Poe wrote:
Andy Resnick wrote:
<snip>
Wasn't there a heated discussion here a month or so
ago about whether the buoyant force would be different
for an object resting on the bottom because you'd be
missing one surface of integration?
How did that resolve? I guess for most real objects
with rough surfaces, there would be plenty of
fluid on the bottom as well and so the usual
buoyancy model would apply.
I don't recall that discussion (I tend to ignore a lot of things in
this
group), but it doesn't present any difficulty. Cauchy's law still
applies to a solid-solid dividing surface, one is not missing a
surface
of integration. The specifics are probably nasty due to the more
complex nature of a solid than a simple fluid, and even worse if one
considers a multi-phase surface (water-logged sand, or mud, for
example,
but the end result should still be something like the upward force of
the surface on the object is Mg, where M is the difference in mass
between the object and the displaced fluid.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
Are you saying that the top face of a cube resting in the bottom of a
pool has buyoant force acting on it?
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| User: "Andy Resnick" |
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| Title: Re: What is buoyancy? |
30 Mar 2005 07:26:24 AM |
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Schoenfeld wrote:
Andy Resnick wrote:
<snip>
Are you saying that the top face of a cube resting in the bottom of a
pool has buyoant force acting on it?
I'm not sure what you are asking. If an object is at rest on a surface,
then there is a net downward force exerted by the object onto the
surface caused by gravity, and a reaction force from the surface onto
the object, keeping the object at rest.
Does that help?
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Schoenfeld" |
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| Title: Re: What is buoyancy? |
30 Mar 2005 12:04:00 PM |
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Andy Resnick wrote:
Schoenfeld wrote:
Andy Resnick wrote:
<snip>
Are you saying that the top face of a cube resting in the bottom of
a
pool has buyoant force acting on it?
I'm not sure what you are asking. If an object is at rest on a
surface,
then there is a net downward force exerted by the object onto the
surface caused by gravity, and a reaction force from the surface onto
the object, keeping the object at rest.
Does that help?
Yes, thanks.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Schoenfeld" |
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| Title: Re: What is buoyancy? |
30 Mar 2005 04:54:28 PM |
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Andy Resnick wrote:
Schoenfeld wrote:
Andy Resnick wrote:
<snip>
Are you saying that the top face of a cube resting in the bottom of
a
pool has buyoant force acting on it?
I'm not sure what you are asking. If an object is at rest on a
surface,
then there is a net downward force exerted by the object onto the
surface caused by gravity, and a reaction force from the surface onto
the object, keeping the object at rest.
Does that help?
Yes, thanks.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Schoenfeld" |
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| Title: Re: What is buoyancy? |
29 Mar 2005 10:02:44 PM |
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Randy Poe wrote:
Andy Resnick wrote:
James wrote:
Is buoyancy the net force of all fluid pressures (as vectors)
perpendicular to all the surface on an object? Or, is buoyancy a
true
calculation of fluid displacement? I am reading various
descriptions of
it as an "up force based on fluid displacement" but that wouldn't
make
nearly as much sense to me.
Both are correct- the first definition you gave is a bit mangled-
pressure is a scalar, but pressure is also the normal component of
the
stress tensor. So, by definition, the pressure (defined at a point
on a
dividing surface) is associated with a vector normal to the surface
at
that point. Integrating over the entire surface will give a
directed
force, and it can be shown that the net force is directed either up
or
down (depending on relative densities)- the sideways components
cancel.
The magnitude of the force is given by the difference in weights
(not
masses!) of the object and displaced fluid.
Wasn't there a heated discussion here a month or so
ago about whether the buoyant force would be different
for an object resting on the bottom because you'd be
missing one surface of integration?
How did that resolve? I guess for most real objects
with rough surfaces, there would be plenty of
fluid on the bottom as well and so the usual
buoyancy model would apply.
- Randy
Do you mean like a cube resting on the floor of a pool? The buoyant
force has no components on the side faces and the bottom face has no
contact with the medium. This means there is no buoyancy, right? Even
if it's filled with Helium for example, would it float to the surface?
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| User: "Randy Poe" |
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| Title: Re: What is buoyancy? |
30 Mar 2005 09:20:26 AM |
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Schoenfeld wrote:
Do you mean like a cube resting on the floor of a pool?
Yes.
The buoyant
force has no components on the side faces and the bottom face has no
contact with the medium.
There is no "buoyant force" acting on top, sides and
bottom. There is fluid pressure, acting on all surfaces.
The "buoyant force" is the sum of the pressure forces
on all sides. There is pressure on the sides, on all
sides, but it cancels out so there is no net horizontal
force.
When completely surrounded by fluid, the fluid at
the bottom is under higher pressure than on the
top (in the presence of gravity), so it pushes
harder against the bottom of the object than
the fluid at the top. Net effect: buoyancy.
What I forgot and what Andy Resnick has corrected
me on, is that even with a cube resting on the
bottom of a pool, there is pressure against
the bottom. It's just that it's coming from the
solid pool bottom. The net effect still follows
Archimedes' Law.
This means there is no buoyancy, right?
No.
Even if it's filled with Helium for example,
would it float to the surface?
Uh, really no. The net "buoyant force" points up,
against gravity. If there is no buoyancy, there's
no upward force. Swimming in weightless conditions
would probably feel very different than on earth due
to the lack of buoyancy. It depends on differential
pressure, which depends on gravity.
We are buoyant in air. We aren't flying.
- Randy
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| User: "Schoenfeld" |
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| Title: Re: What is buoyancy? |
30 Mar 2005 12:04:36 PM |
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Randy Poe wrote:
[snip]
GOt it. Thanks for the reply.
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