How does buoyancy work?

[Tom Del Rosso]

Laws of physics.....
Leftists like to change Laws.

Progressive laws and sustainable technology. Ass backwards.

 

[TunnelRat]

H ow do you define 'pressure differential of the surrounding atmosphere'?



Like starting an upside-down siphon, almost.

Ever see that detective show about the UK Underground fire that killed
31 people, that raced up the 30 degree inclined escalator trenches in
mere seconds?

They discovered a lot about how fires work in enclosed 'trenches' or
'raceways'.

 

[mpm]

I know this isn't the right group for this question, but I want

an answer an electrical engineer can understand.



I understand the simple model of buoyancy. If something weighs

less than the medium it displaces, it floats.



The question is, "why?"

If you stick a ping pong ball under water and let go, it floats.

What's the mechanism? The water has no idea what's inside the

ball. We have water molecules banging into the ball on all sides.



Is there a simple explanation, in layman's terms, of what's actually

going on to force the ball up?



I really want to understand how convection works in a chimney,

but water should be easier to grasp.

Hummm..... depends on how deep.
I'm pretty sure if you submerge an ordinary ping-pong ball deep enough, it will never submerge. Pressure would eventually crush the ball into a tiny blob of whatever ping-pong balls are made of - likely letting the air out in the process. Or, the ball ruptures and fills with water.

 

[SoothSayer]

That's one of those laws of nature that seems to 'cause' things to happen as
if they understand the law.

When they were so ecstatic about the video of their "discovery" of
particles glomming together "in space", in a baggie on the space station,
I wondered if they considered the fact that there would also likely be an
electrostatic effect working.

 

[Jasen Betts]

Thanks, guys, for the inputs.
Those are the classical arguments. I accept them as true and useful
models for everyday use.

But, I'm still not satisfied.

Throw a sheet of thin plywood into the swimming pool.
If you put sufficient force on it, it sinks.
Now turn it vertical and push it down.
The force to submerge it should not change.
But the direction of force applied by all the molecules banging
on the side do not obviously have a net upward force.

so now you want to discuss viscosity?

The thing I'm trying to get my brain around is convection.
If I have a 20 foot chimney and build a fire in the firebox,
air gets sucked in the box and goes up the chimney.
If I cut a hole in the bottom and stack it on top of a
similar chimney (close up the front), does the same amount
of air get sucked into the bottom (cold) fireplace?
My intuition says it does.

no it won't. friction in the cold chimney will reduce the flow.
also the lower air pressure at the higher fireplace will hinder
combustion slightly.

OK, so now, put an S-curve in the pipe, kinda like a P-trap.
Can you make a passive thermostat out of it? Suck air
only when the sun shines?

that can probably be made to work.

 

[Jeroen Belleman]

The sinking, specifically.

Jeroen

One might guess that the sinking would be exactly the same.

I suppose it pretty much would be exactly the same sinking.

But NO! The gravity is less, so it would *have* to sink less, and the
rule includes that the water is standard Earth sea level density water.
[...]

Let's stay within the confines of normal atmospheric pressure and
temperature. Just considering the effects of gravity, of course, on
the moon the ball weighs less, but so does the water, and by the
same proportion. The sinking is therefore the same.

An interesting case to think about is what would happen in perfect
weightlessness: What's the final state if a free floating blob of
water touches a free floating ping pong ball? (Hint: Surface
tension reigns.)

Jeroen Belleman

 

[SoothSayer]

On 2013-02-18 16:29, SoothSayer wrote:
On Mon, 18 Feb 2013 14:50:23 +0100, Jeroen Belleman

On 2013-02-18 14:35, SoothSayer wrote:
On Mon, 18 Feb 2013 23:41:00 +1100, Sylvia Else


So how much net force is there? Well, if the pingpong ball were not
there, then in its place would be a sphere of water. Since the water
wouldn't go anywhere, the net force on that sphere must be equal to its
weight. Thus the force on the pingpong ball is equal to the weight of
the water that would have been there had the ping pong ball not been.


In other words, it only sinks a small bit because that small bit of
volume, in the form of water, would weigh the same as the ping pong ball.

So, if it displaces one teaspoon (for the sake of the analogy), then the
weight of one teaspoon of water and one ping pong ball would balance on a
scale (be the same).

On the Moon, it would sink far less. And the water wouldn't last long
either! (unless you were aboard The Eagle).

You may want to think about that last statement a little longer....

Jeroen Belleman


About the sinking, the water, or the Eagle?

The sinking, specifically.

Jeroen

One might guess that the sinking would be exactly the same.

I suppose it pretty much would be exactly the same sinking.

But NO! The gravity is less, so it would *have* to sink less, and the
rule includes that the water is standard Earth sea level density water.
[...]

Let's stay within the confines of normal atmospheric pressure and
temperature. Just considering the effects of gravity, of course, on
the moon the ball weighs less, but so does the water, and by the
same proportion. The sinking is therefore the same.

Ahh, so on the surface of Jupiter, it would sink completely!
Oh...

An interesting case to think about is what would happen in perfect
weightlessness: What's the final state if a free floating blob of
water touches a free floating ping pong ball?
kerplop.

(Hint: Surface
tension reigns.)

Unless the ping pong ball is coated with RainX.

 

[josephkk]

Thanks, guys, for the inputs.
Those are the classical arguments. I accept them as true and useful
models for everyday use.

But, I'm still not satisfied.

Throw a sheet of thin plywood into the swimming pool.
If you put sufficient force on it, it sinks.
Now turn it vertical and push it down.
The force to submerge it should not change.
But the direction of force applied by all the molecules banging
on the side do not obviously have a net upward force.

The thing I'm trying to get my brain around is convection.
If I have a 20 foot chimney and build a fire in the firebox,
air gets sucked in the box and goes up the chimney.
If I cut a hole in the bottom and stack it on top of a
similar chimney (close up the front), does the same amount
of air get sucked into the bottom (cold) fireplace?
My intuition says it does.
And that means that if you're interested in ventilation,
it doesn't matter where the heat gets into the pipe.
Solar collector should work for that. And the distance
to the space to be ventilated should be irrelevant???

OK, so now, put an S-curve in the pipe, kinda like a P-trap.
Can you make a passive thermostat out of it? Suck air
only when the sun shines?

I am not sure but i suspect what is called a gravity ventilator may what
you are asking for. Something that may be what you are after.

?-)

 

[Pimpom]

Hummm..... depends on how deep.
I'm pretty sure if you submerge an ordinary ping-pong ball
deep
enough, it will never submerge. Pressure would eventually
crush the
ball into a tiny blob of whatever ping-pong balls are made
of -
likely letting the air out in the process. Or, the ball
ruptures and
fills with water.

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.

 

[JW]

On Mon, 18 Feb 2013 17:26:18 +0100, Jeroen Belleman

On 2013-02-18 16:29, SoothSayer wrote:
On Mon, 18 Feb 2013 14:50:23 +0100, Jeroen Belleman

On 2013-02-18 14:35, SoothSayer wrote:
On Mon, 18 Feb 2013 23:41:00 +1100, Sylvia Else


So how much net force is there? Well, if the pingpong ball were not
there, then in its place would be a sphere of water. Since the water
wouldn't go anywhere, the net force on that sphere must be equal to its
weight. Thus the force on the pingpong ball is equal to the weight of
the water that would have been there had the ping pong ball not been.


In other words, it only sinks a small bit because that small bit of
volume, in the form of water, would weigh the same as the ping pong ball.

So, if it displaces one teaspoon (for the sake of the analogy), then the
weight of one teaspoon of water and one ping pong ball would balance on a
scale (be the same).

On the Moon, it would sink far less. And the water wouldn't last long
either! (unless you were aboard The Eagle).

You may want to think about that last statement a little longer....

Jeroen Belleman


About the sinking, the water, or the Eagle?

The sinking, specifically.

Jeroen


One might guess that the sinking would be exactly the same.

I suppose it pretty much would be exactly the same sinking.

But NO! The gravity is less, so it would *have* to sink less, and the
rule includes that the water is standard Earth sea level density water.
[...]

Let's stay within the confines of normal atmospheric pressure and
temperature. Just considering the effects of gravity, of course, on
the moon the ball weighs less, but so does the water, and by the
same proportion. The sinking is therefore the same.

Ahh, so on the surface of Jupiter, it would sink completely!
Oh...

An interesting case to think about is what would happen in perfect
weightlessness: What's the final state if a free floating blob of
water touches a free floating ping pong ball?
kerplop.

(Hint: Surface
tension reigns.)

Unless the ping pong ball is coated with RainX.

Or even better:

 

[Uwe Hercksen]

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.

Hello,

but we are talking about solid plastics, no foamed plastics with gas
filled bubbles inside. If the density is less than the density of water,
it will float, if the density is larger, it will sink.

Ping-pong balls are made of Celluloid which seems to have a density of
about 1.67. If they are crushed and filled with water, they will sink.

Bye

 

[SoothSayer]

Or even better:

FAR OUT!!!

I wonder if the Earth were coated with that if we would not have a Moon
right now, 'cause the impactor slid right past...

 

[Spehro Pefhany]

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.

Polyethelene and polypropylene are the only ones that come to mind.

 

[SoothSayer]

FAR OUT!!!

I wonder if the Earth were coated with that if we would not have a Moon
right now, 'cause the impactor slid right past...

That's it! If we go coat Apophis with it, it will just slip right by!

 

[halong]

I know this isn't the right group for this question, but I want
an answer an electrical engineer can understand.

I understand the simple model of buoyancy.  If something weighs
less than the medium it displaces, it floats.

The question is, "why?"
If you stick a ping pong ball under water and let go, it floats.
What's the mechanism?  The water has no idea what's inside the
ball.  We have water molecules banging into the ball on all sides.

Is there a simple explanation, in layman's terms, of what's actually
going on to force the ball up?

I really want to understand how convection works in a chimney,
but water should be easier to grasp.

I guess without gravity the ball will not move in any direction -
agree?

So gravity is the main factor to make the ball floating-up

....One now can imagine, gravity's pulling the liquid down better than
pulling the ball, hence no room for the ball at the bottom... it must
go up to yield the space

In fact all of these factors involve: Gravity, density, viscosity ...

 

[Pimpom]

Hello,

but we are talking about solid plastics, no foamed
plastics with gas
filled bubbles inside. If the density is less than the
density of
water, it will float, if the density is larger, it will
sink.

I was also talking about solid plastics. The range of
plastics with a density less than that of water may not be
very wide - I don't know, I'm no expert on plastics - but
products made from them are quite common.

Ping-pong balls are made of Celluloid which seems to have
a density of
about 1.67. If they are crushed and filled with water,
they will sink.

I'll take your word on that.

 

[[email protected]]

Ignoring gravity gradient effects, isn't the simple buoyant rule that
a floating object displaces an amount of water equal to the object's
weight?

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


Mark L. Fergerson

 

[mpm]

I was also talking about solid plastics. The range of

plastics with a density less than that of water may not be

very wide - I don't know, I'm no expert on plastics - but

products made from them are quite common.








I'll take your word on that.

I've never seen a solid ping-pong ball.

Maybe the "real" question is: "What kind of sound would they make underwater?"

 

[MrTallyman]

Ignoring gravity gradient effects, isn't the simple buoyant rule that
a floating object displaces an amount of water equal to the object's
weight?

Without gravity, there is no "floating".

 

[[email protected]]

Without gravity, there is no "floating".

You've obviously never heard of "floating through space".