# Buoyant force? Say what?

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#### SaintJude

##### Full Member
Two identical balloons of negligible mass are tethered at altitudes of 2000 meters and 2600 meters, respectively. The balloons are filled with helium gas to equal volumes. Which of the following statements is true concerning the buoyant force acting on each balloon?

The buoyant force on the balloon at 2600 meters will be greater.
The buoyant force on the balloon at 2000 meters will be greater.
The buoyant forces on the two balloons will be equal.
The relationship between the buoyant forces cannot be determined

#### chiddler

##### Full Member
10+ Year Member
Density of air decreases as we go higher. That's why it is hard to breathe while mountain climbing right?

Buoyant force, Fb = rho * V * g

so higher altitude = less density of air which makes the higher altitude variant have less buoyant force.

#### circulus vitios

##### Full Member
10+ Year Member
7+ Year Member
Density of air decreases as we go higher. That's why it is hard to breathe while mountain climbing right?

Buoyant force, Fb = rho * V * g

so higher altitude = less density of air which makes the higher altitude variant have less buoyant force.

That's true, but balloons expand as they rise due to decreased atmospheric pressure. So you're looking at increasing balloon volume versus decreasing air density. I have no idea how we're expected to balance those changing variables. It's a terrible and ambiguous question.

#### chiddler

##### Full Member
10+ Year Member
That's true, but balloons expand as they rise due to decreased atmospheric pressure. So you're looking at increasing balloon volume versus decreasing air density. I have no idea how we're expected to balance those changing variables. It's a terrible and ambiguous question.

baloons are filled to equal volumes, according to the question.

#### pfaction

##### Full Member
10+ Year Member
So if the density decreases as we go high, I guess the buoyant force would be greatest 2000 meters?

#### SaintJude

##### Full Member
I think what happened was that I (haphazardly) applied what I knew about buoyant force in liquids to that of air without taking into account the difference.

I remember that in an incompressible liquid, buoyant force will be the same at every depth b/c density of liquid is the same at every point in the liquid. But in air, that's not the same scenario here.

##### Full Member
I think what happened was that I (haphazardly) applied what I knew about buoyant force in liquids to that of air without taking into account the difference.

I remember that in an incompressible liquid, buoyant force will be the same at every depth b/c density of liquid is the same at every point in the liquid. But in air, that's not the same scenario here.

No, in water the deeper you go the greater the density. So if the baloons were in water, the greatest Fb would be the deepest one. Idk abt air though, I woulda guessed C

#### chiddler

##### Full Member
10+ Year Member
No, in water the deeper you go the greater the density. So if the baloons were in water, the greatest Fb would be the deepest one. Idk abt air though, I woulda guessed C

no i think you're mistaking density with pressure. density stays the same more or less because fluids are incompressible unless under very high P's.

##### Full Member
no i think you're mistaking density with pressure. density stays the same more or less because fluids are incompressible unless under very high P's.

Are you sure? I swore I read in tbr density increases the lower you go, I could be wrong though I'm a newb w/ fluids

#### pm1

##### Full Member
Are you sure? I swore I read in tbr density increases the lower you go, I could be wrong though I'm a newb w/ fluids

I also just read in TPR yesterday that for MCAT purposes we assume that the density stay the same in liquid, because it is an incompressible fluid. Hence, when in a liquid the buoyant force is independent of the depth of the object.

#### SaintJude

##### Full Member
I also just read in TPR yesterday that for MCAT purposes we assume that the density stay the same in liquid, because it is an incompressible fluid. Hence, when in a liquid the buoyant force is independent of the depth of the object.

Yup EK says the same thing

##### Full Member
Yup EK says the same thing