beech ball buoyancy

[thull]

Questions 26 & 27 of mechanical comprehension in the ASTB study guide. Does a beach ball have more buoyancy near the surface of the water or deep down?

well, since water does not increase in density, a beach ball would have the same buoyancy near the surface as it does deeper down.

in fact if you were to get technical, the pressure would shrink the size of the beach ball as you took it further down, lowering the displacement of the water, making it less buoyant than near the surface when more fully expanded.

but the answer sheet says it feels more buoyant deep down.

I assume the answer sheet is wrong?

 

[Brett327]

Questions 26 & 27 of mechanical comprehension in the ASTB study guide. Does a beach ball have more buoyancy near the surface of the water or deep down?

well, since water does not increase in density, a beach ball would have the same buoyancy near the surface as it does deeper down.

in fact if you were to get technical, the pressure would shrink the size of the beach ball as you took it further down, lowering the displacement of the water, making it less buoyant than near the surface when more fully expanded.

but the answer sheet says it feels more buoyant deep down.

I assume the answer sheet is wrong?

Any compressible vessel containing air will shrink with depth and become less buoyant. Less volume = less water displaced = less buoyant. Any SCUBA diver is familiar with this, as you must continue to fill your BC as you go down to maintain neutral buoyancy.

Brett

 

[thull]

Any compressible vessel containing air will shrink with depth and become less buoyant. Less volume = less water displaced = less buoyant. Any SCUBA diver is familiar with this, as you must continue to fill your BC as you go down to maintain neutral buoyancy.

Brett

that's what I thought. don't know why the answer sheet would say otherwise. unless they just screwed up, maybe thinking intuitively since it's hard to push a beach ball underwater, it must be REALLY hard to hold it even deeper down....which doesn't actually make any sense. oh well..thx

 

[Brett327]

that's what I thought. don't know why the answer sheet would say otherwise. unless they just screwed up, maybe thinking intuitively since it's hard to push a beach ball underwater, it must be REALLY hard to hold it even deeper down....which doesn't actually make any sense. oh well..thx

It's easier to hold it under when it's deeper - it's less buoyant. Live by the gouge, die by the gouge.

Brett

 

[IrishNavy05]

Maybe the question was worded poorly, and what they were going for was an object AT the surface. Buoyancy depends on the volume of an object submerged, and only a tiny portion of the beach ball would be submerged.

 

[scoober78]

well, since water does not increase in density

Actually it does...just not much in response to pressure. This is a key element in the design of every water-moderated nuclear reactor in the world.

 

[jg5343]

It's easier to hold it under when it's deeper - it's less buoyant.
Brett[/QUOTE

This is definitely true. When using lift bags for salvage you only need to put enough air in them to get it to slowly lift off the bottom because by the time it hits the surface the air has expanded and the bags are screaming up. On controlled lifts you have to vent air to get the bags to ascend slowly, like the BC Brett was talking about.

 

[Conaway]

but the answer sheet says it feels more buoyant deep down.

My interpretation:
The key word is that it "feels" more bouyant deep down, hence it is harder to keep submerged because it "feels" like it is acting against you with greater force the deeper you go.

 

[Brett327]

My interpretation:
The key word is that it "feels" more bouyant deep down, hence it is harder to keep submerged because it "feels" like it is acting against you with greater force the deeper you go.

Are we talking about a ball floating vs. submerged, or a ball submerged at 1 ft vs. one submerged at 20 ft?

Brett

 

[Conaway]

Are we talking about a ball floating vs. submerged, or a ball submerged at 1 ft vs. one submerged at 20 ft?

Brett

I'm assuming we're talking about a ball floating on top of the water vs. at the very minimum, completely submerged. I think as it's floating on top of the water the amount of water around the bottom of the ball is negligible and basically resting on the surface.

 

[Brett327]

I'm assuming we're talking about a ball floating on top of the water vs. at the very minimum, completely submerged. I think as it's floating on top of the water the amount of water around the bottom of the ball is negligible and basically resting on the surface.

At any rate, it sounds like a poorly thought out and phrased question.

Brett

 

[ch1]

Just to clarify.
the actual question is.
26. When is it more difficult to hold an inflated beach ball under water?
A. Close to the surface
B. Down deep
C. The difficulty is the same either depth
Correct answer according to the answer sheet is B

27. When is the same beach ball seem more buoyant?
A. Close to the surface
B. Down deep
C. The buoyancy is the same at either depth
Correct answer according to the answer sheet is B


Both answers seem to contradict what was said earlier.

So is the answer sheet right because of the statement / word "more difficult" and "seem"?
Or is the answer sheet wrong?

Could someone help me on clarification of the question?


Thanks

 

[eddie]

Just to clarify.
the actual question is.
26. When is it more difficult to hold an inflated beach ball under water?
A. Close to the surface
B. Down deep
C. The difficulty is the same either depth
Correct answer according to the answer sheet is B

27. When is the same beach ball seem more buoyant?
A. Close to the surface
B. Down deep
C. The buoyancy is the same at either depth
Correct answer according to the answer sheet is B


Both answers seem to contradict what was said earlier.

So is the answer sheet right because of the statement / word "more difficult" and "seem"?
Or is the answer sheet wrong?

Could someone help me on clarification of the question?


Thanks

Those aren't contradictory at all. If something seems more bouyant (more likely to float) at depth, then the logical conclusion is that it would be harder to hold it down at depth.

 

[scoober78]

Those aren't contradictory at all.

And that isn't what he said.

Both answers seem to contradict what was said earlier.

 

[tiger84]

The question is definitely just poorly worded, as you can make a case for any of the answers being correct. Brett's explanation of decreasing bouyancy as a compressible vessel is submerged works. The explanation that water is nearly incompressible and thus bouyancy remains constant, assuming a constant volume pressure vessel, would explain choice C. Finally, if you assume we're talking about seawater or a very deep body of water that experiences drastic changes in temperature or salinity with depth, then the density does in fact increase with depth and bouyancy, again assuming constant volume, would increase the farther down you go. I wouldn't get too worked up over what's correct. It's more important to focus on the examples that aren't open to interpretation.