Sudden and uncontrolled acsent.

[lamont]

blackwood is right.

what everyone refers to as the "buoyancy" of an object is just the net force on the object when it is submerged (gravitational force minus buoyancy force). when you breathe down a tank its displacement and buoyancy don't change, but its mass changes, hence the gravitational force changes, hence the net force changes.

 

[pants!]

what everyone refers to as the "buoyancy" of an object is just the net force on the object when it is submerged (gravitational force minus buoyancy force).

Right.. in this definition, an emtpy tank is more buoyant than a full tank. That's how everyone uses the term buoyancy, and it's technically correct.

Blackwood is trying to use buoyancy and buoyant force interchangably, which is why everyone is all confused. Technically that is correct as well, but that's not what everyone means when they say buoyancy.

 

[lamont]

Right.. in this definition, an emtpy tank is more buoyant than a full tank. That's how everyone uses the term buoyancy, and it's technically correct.

Blackwood is trying to use buoyancy and buoyant force interchangably, which is why everyone is all confused. Technically that is correct as well, but that's not what everyone means when they say buoyancy.

*shrug* that dictionary definition posted earlier agrees with me and blackwood, so does wikipedia:

In physics, buoyancy is an upward force on an object immersed in a fluid (i.e. a liquid or a gas), enabling it to float or at least to appear to become lighter. If the buoyancy exceeds the weight, then the object floats; if the weight exceeds the buoyancy, the object sinks. If the buoyancy equals the weight, the body has neutral buoyancy and may remain at its level.

imo, the terminology is just kinda crappy.

 

[H2Andy]

Fb = Vol x rho x g

[snip]

Fr = W - V x rho x g

ah... so.... the bouyancy of the tank doesn't change. what changes is
the weight of the tank, which affects how the tank will act HAVING
THE SAME BOUYANCY

so why do they teach "positive" and "negative" bouyancy, which implies
a change in the bouyancy?

 

[The Kraken]

The buoyancy of the tank doesn't change, its specific gravity does.

Eureka !!!! I have found it !!!!

the K

 

[JeffG]

The buoyancy of the tank doesn't change, its specific gravity does.

Eureka !!!! I have found it !!!!

the K

No, its because the martians hate water and they think we are nuts to go into it. So after a bit of time they get uncomfortable and they use a tractor beam and try to pull us out of the water.

 

[Soggy]

so why do they teach "positive" and "negative" bouyancy, which implies a change in the bouyancy?

Because that's the net effect, it's simpler to understand, and it's a nice susinct summary that doesn't require explaining how buoyancy (meaning the buoyant force) works.

 

[Blackwood]

ah... so.... the bouyancy of the tank doesn't change. what changes is
the weight of the tank, which affects how the tank will act HAVING
THE SAME BOUYANCY

so why do they teach "positive" and "negative" bouyancy, which implies
a change in the bouyancy?

What Soggy said. It's just an easy way to express the relationship between the two forces.

Neutral buoyancy is used to mean weight=buoyant force.
etc.

there's you're mistake -
The formula you've quoted is the DISPACEMENT of the tank!!!

If the tank is submerged, its displacement = its volume. Displacement has nothing to do with density or gravity.

The formula I wrote is not for the displacement of the tank. It's for the weight of the fluid displaced by the tank.

Buoyancy of the tank = (the volume of the tank * the density of the fluid in which it is submerged * the acceleration of gravity) - ( density of the tank * the acceleration of gravity).

That formula = [buoyant force] - [weight of the tank] (I assume that's what you were getting at... you forgot the volume of the tank in your second term, though)...

Which is what I've said all along... Back on page 5: "You are talking about the relationship between weight and buoyancy."

At that point, I thought a discussion about the forces themselves would be relevant.

 

[MikeFerrara]

there's you're mistake -
The formula you've quoted is the DISPACEMENT of the tank!!! that for the most part is a constant, there will be a little change as pressure changes because of 'stretch'.


The buoyancy = Displacement - weight of the tank( and it's contents). It's that simple!



Buoyancy of the tank = (the volume of the tank * the density of the fluid in which it is submerged * the acceleration of gravity) - ( density of the tank * the acceleration of gravity).

so if the tank displaces 5 gals of water and water is 8 lbs/gal that's 40 lbs displacement, right? now an emptyl tank weighs 39 lbs, it floats, right?
If you fill that same tank with 4 lbs of air it now weighs 43 lbs, eh? what's it do?

If you said it floats, you need to try this -

fill an AL80, take it out to a lake and see if it floats ( just the tank and valve nothing else), now let (or do a dive and breath) it down to 100 psi, retest it in the same lake. Guess what it'll float!

Old Archemedies (did I come close on the spelling?) remember?

An object is buoyed by a farce equal to (the weight of the water/fluid the object displaces) - the weight of the object.

 

[pants!]

Old Archemedies (did I come close on the spelling?) remember?

An object is buoyed by a farce equal to (the weight of the water/fluid the object displaces) - the weight of the object.

That only applies to an object that's floating. Objects that are submerged are not necessarily buoyed by this.. er, farce.