Acceleration in a fluid

[happyfellow]

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There is a cork submerged to the bottom of a 3 meter column of fluid. The fluid is much more dense than the cork. When the cork is let loose and rises to the top, its acceleration:

A. remains constant
B. decreases from its initial value due to viscosity
C. increases from its initial value due to the increasing kinetic energy
D. decreases from its initial value due to the decreasing potential energy

Apparently the answer is B. However, the passage mentions nothing about viscosity of the fluid or any drag force, etc. I would think that the acceleration would increase because there is less pressure as the cork rises because the fluid above it is decreasing. Can anyone help me out?

 

[loveoforganic]

Bouyant force is proportional to the volume displaced alone. Depth submerged is unrelated to buoyant force (as long as the object is already fully submerged). That said, you're right, it says nothing about the viscosity of the fluid.

 

[phltz]

As the previous poster said, the buoyant force does not change as the cork rises. So it has the same upwards force acting up on it the whole time. When it is first released, it is at rest, and so has no drag force on it. As it picks up speed, though, the drag force becomes more and more. Eventually, the drag force downwards cancels the buoyant force acting upwards, and terminal velocity is reached.

So the acceleration starts out upwards, becoming less and less, and eventually is zero when the cork is at its terminal velocity. Once it's at terminal velocity, it's at a constant velocity, and there's no further acceleration.

 

[surag]

Nvm

 

[surag]

I would say this....process of elimination shows that b is right. It can't be increasing in acceleration since it stops eventually once it reaches top-if it were to do so. If not then it would keep going out of the water! So it must be going to zero. Also since potential energy rises as body goes up...mgh it can't b D. That leaves b

 

[Phantastic]

I would say this....process of elimination shows that b is right. It can't be increasing in acceleration since it stops eventually once it reaches top-if it were to do so. If not then it would keep going out of the water! So it must be going to zero. Also since potential energy rises as body goes up...mgh it can't b D. That leaves b

I concur. It might not have mentioned whether or not this was an ideal fluid, but POE had me picking B even though I didn't know.