Marine Gouge ASTB Practice Test: Weird Questions

[Jon Scarfo]

Hey all,

I just took the practice ASTB on the Chicago Marine Officer site. THe link is here: http://www.chicagomarineofficer.com/Downloads/ASTB/ASTBMarineGouge.pdf

I have some beef with a couple of the questions, and was looking for some clarification.

These two were the most puzzling to me

#26, Mechanical Comprehension

When is it more difficult to hold an inflated beach ball below water?
A. When its close to the surface
B. Down deep
C. It is equally difficult at either depth

Archimedes principle, in a nutshell, states that the buoyant force is equal to the weight of fluid displaced by the submerged object. This counteracts the force of gravity on the object, so if the object is of a lower density than the fluid, the buoyant force will overcome gravity and will tend to rise.
In light of this, the answer should be C. Depth does not matter. However, the answer key states that the answer is B. The only way this could make sense is if by "close to the surface", they mean that the ball is only partially submerged (floating). However, this seems to be ruled out in the question itself when they use the words "under water". This question seems a bit fuzzy.

Am I missing something here, or is the answer key just wrong?

#27

This is essentially the same question, with the same fundamental problem.

When is the beach ball seem more buoyant? (Note the excellent grammar :icon_tong)
A. When its close to the surface
B. Down deep
C. It is equally difficult at either depth

Again, the answer should be C, however the answer key states B. What gives here?

Another question which might suffer from too-little-information is
#5 in the mech. comp. section. The center of rotation is not defined here. Although chances are that one would assume that it is C, its not immediately made clear.

My general concern is, are ambiguous questions like these rampant in the tests, or am I just getting worked up over nothing?

Thanks

 

[jmcquate]

The weight of the water (water pressure) increases with depth and would assist in keeping the object submerged.

 

[scarfus]

....which would lead to the conclusion that the ball becomes EASIER (Choice A) to hold underwater as depth increases, because the water pressure exerts a downward force.

scratching head

 

[HueyHornet75]

My $.2. as you stated:"The Archimedes principle states that the buoyant force is equal to the weight of fluid displaced by the submerged object". Let's think about it from a common sense standpoint. If you had a ball in a swimming pool and you submerged it just under the surface, it would pop out-no big deal. Now, if you submerged that ball and stood on it, then let it go, it would probably launch out of the air? This begs for you to choose B. Since it's been a long time since I took Physics, and it was never my strongest subject, let's look at it another way (very simplistically). The weight of the water near the surface that the ball would displace is X. The amount of water over the ball contributes a weight of Y. So when you submerge the ball 6 inches (Y is weight at 6 in of water over the ball), the weight of the water displaced by the ball would be X+ Y(6 in). now if you submerge the ball 72", the weight of the water displaced by the ball would be X+Y(72in). so it will have 6 feet of heavy water on top of the weight merely displaced by the ball. so since there is more weight on top of the ball deeply submerged, the buoyant force will increase. hope that helps...

 

[Swanee]

Nerds!

I'm not sure about beachballs, but Manswers had a segment about the buoyancy of fake vs real boobs. Real boobs are more buoyant.
So just write in boobs and you'll be good.

 

[HueyCobra8151]

I remember also being frustrated by that question.

If anything it should be more buoyant near the surface, because as the beach ball goes to a lower depth, the air inside of it (which is inflated at sea level pressure), will be compressed, making the beach ball smaller.

A smaller beach ball displaces less water, hence less buoyant.
--
In the case of a more incompressible object however, buoyant force does increase with depth because the water will compress.

Buoyancy = (density)(Volume)(gravity)

So with a beach ball, the decrease in volume would be greater than the very gradual increase in fluid density.

 

[scarfus]

HueyHornet,

I have to respectfully disagree.The depth, if anything, will increase the net downward force on the ball. The volume of water being displaced is essentially the same at any depth (its just the volume of the ball). So, the buoyant force remains constant (as long as temperature-dependent density changes are ignored; there is not enough information in the question to suggest that we should even consider this, so we wont). As depth increases, the weight of water acting downward onto the ball increases, counteracting the buoyant force.

Swanee,

If boobs was choice D, I wouldnt have bothered posting. The answer would have been clear.

HueyCobra,

Exactly! If we decide to throw out the reasonable engineering assumption that all densities will remain constant between the two reference points (deep and shallow), then it shouldnt matter. If you try to factor temperature/pressure-dependent density changes as depth increases, then your assessment would be correct. Higher temperatures/lower water pressures at the surface would cause the ball to maintain a certain volume which should be higher than lower temperatures/higher water pressures subjected to the ball at greater depths, which translates to a higher buoyancy (greater volumetric displacement).

Also, given the nature of water and air, the density of water will change at a much lower rate than the density of air will as temperature changes. Thus, as temperature drops, the air in the ball will decrease in density MORE than the surrounding water, thus causing it to displace a lesser mass of water, DECREASING buoyancy as depth increases.

But seriously, do they REALLY expect us to make such assumptions based on such a simple question? Are ambiguous questions like this rampant? Or was this just a mistake on the practice test?

 

[Treetop Flyer]

You're going to love API

 

[scarfus]

You're going to love API

Haha care to elaborate?

All sarcasm aside, I'm sure I will. After all thats why I'm doing this!