# Buoyant force

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#### chiddler

##### Full Member
10+ Year Member
As a cork rises through a viscous liquid by buoyant forces, its acceleration:

A. Stays constant.
B. Decreases from initial value due to viscosity
C. Increases from initial value due to increasing kinetic energy
D. Decreasing from initial value due to decreasing potential energy

I typed this, walked to bathroom and realized the answer. The realization is that viscosity is just like air resistance. More air speed entails more air resistance which lowers acceleration.

So. Enjoy.

##### Full Member
As a cork rises through a viscous liquid by buoyant forces, its acceleration:

A. Stays constant.
B. Decreases from initial value due to viscosity
C. Increases from initial value due to increasing kinetic energy
D. Decreasing from initial value due to decreasing potential energy

I typed this, walked to bathroom and realized the answer. The realization is that viscosity is just like air resistance. More air speed entails more air resistance which lowers acceleration.

So. Enjoy.

Yeah but the viscosity is constant. Ohhh wait, I got dis yo. It's because Fb=pVg right? Welll fluids at a greater depth have a higher density. Soo as you ascend, the density
decreases, so your Fb decreases, so your acceleration does too... I think. Good Q yo

#### MrNeuro

##### Full Member
7+ Year Member
As a cork rises through a viscous liquid by buoyant forces, its acceleration:

A. Stays constant.
B. Decreases from initial value due to viscosity
C. Increases from initial value due to increasing kinetic energy
D. Decreasing from initial value due to decreasing potential energy

I typed this, walked to bathroom and realized the answer. The realization is that viscosity is just like air resistance. More air speed entails more air resistance which lowers acceleration.

So. Enjoy.

i thought of it as its NOT an ideal fluid b/c its viscous...
density of fluid greatest at high depths = greatest buoyant force but then again if its not a rigid volume then the volume will decrease and probably cancel out this effect....

so your explanation is better...what book is that question from?

Yeah but the viscosity is constant. Ohhh wait, I got dis yo. It's because Fb=pVg right? Welll fluids at a greater depth have a higher density. Soo as you ascend, the density
decreases, so your Fb decreases, so your acceleration does too... I think. Good Q yo

assuming the volume doesn't change

#### dmf2682

##### Membership Revoked
Removed
i thought of it as its NOT an ideal fluid b/c its viscous...
density of fluid greatest at high depths = greatest buoyant force but then again if its not a rigid volume then the volume will decrease and probably cancel out this effect....

so your explanation is better...what book is that question from?

assuming the volume doesn't change

I like how you think. Cork is compressible and will grow in volume and buoyant force as it ascends. But if it's from left field like that we shouldn't over think it. Not sure where to draw the line about overthinking stuff. I guess if the problem alludes to it we can use it.

##### Full Member
assuming the volume doesn't change

Wait, how would the volume change?

And yeah that is my biggest problem, knowing when I'm not supposed to overthink problems b/c its really super easy;/

#### dmf2682

##### Membership Revoked
Removed
Wait, how would the volume change?

And yeah that is my biggest problem, knowing when I'm not supposed to overthink problems b/c its really super easy;/

Well the idea is that at lower depths your pressure is greater from all the water above you. So pressure multiplied by surface area is the force compressing the body. It's why submarines have double hulls. Ever see hunt for red October where they are afraid of getting crushed? Same effect. So at shallower depths the pressure is lower, less compression and volume goes up.

#### TinaBina22

##### Full Member
So if it's an ideal fluid, acceleration stays constant? But because it's a viscous fluid, acceleration is decreasing right? What's the answer?

#### chiddler

##### Full Member
10+ Year Member
There is no indication that the cork is compressible so you cannot assume that. All that we're looking at is how the speed of the cork changes as it rises in a viscous fluid. This is just like viscosity: you assume ideal fluid unless told otherwise. Similarly, you assume ideal solid unless told otherwise.

Acceleration should be decreasing because the faster it goes, the more resistance it encounters. Therefore, acceleration should be decreasing as it goes faster and faster.

#### pfaction

##### Full Member
10+ Year Member
I got this wrong too, screw that. I didn't even read it was a viscous fluid in the passage and didn't equate it to the way you did with air resistance, so I was like, well derp, FB is constant as it's submerged, and W is always constant, so duh, it's constant.

But for the sake of my knowledge, if it's a nonviscous fluid, acceleration is constant no?

Question is from TPR passage, I forget which.

10+ Year Member
yes

#### BerkReviewTeach

##### Company Rep & Bad Singer
Vendor
10+ Year Member
Wait, how would the volume change?

And yeah that is my biggest problem, knowing when I'm not supposed to overthink problems b/c its really super easy;/

It's a problem that can be solved though. There is no better hint as to how deeply you're suppose to think than the four answer choices given to you. The person who wrote the question and decided what the best answer is wrote all four answer choices. The detail of their choices will match the detail they expect in your choice of answers. That's why POE is so important on the MCAT.

For this question, changing fluid density because of non-ideal compression and thermal gradients within the medium coupled with variations in the gas solubility within the cork and the compressibility of air pockets impacting the cork volume are all well and nice for round table discussions at a physics party, but the answer choices don't address any of them.

The choices are basically:
A. No variation
B. Variation because drag force is not constant
C. Variation because kinetic energy is not constant
D. Variation because potential energy is not constant.

C and D are ruled out, because energy and force are different topics. The question is reduced to whether or not viscous drag is constant. The force of firction varies with speed and always opposes the direction of the velocity. The cork is going straight up, so it comes down to the speed. The cork is accelerating up, so it has increasing speed. As speed increases, the impact of friction with the medium (viscosity) will increase. Choice B is the one that fits this deviation.

The key point here is that you need to use the answer choices more, to give you insight into how detailed the answer needs to be.