Physics question from DM exam

[LowVizWiz]

"What do you want me to do? LEAVE? Then they'll keep being wrong!"

That's funny stuff

The fact that the wrong answer has been propagated by an OW instructor (and many divers) makes me sad.

Do I get extra credit for jumping the gun now?

 

[Divrdug]

Part of the problem is that the question, as it is phrased here, can be read two different ways. Add to that, an object with 3lbs of bouyancy wouldn't be at a depth of 56 ft. It would be at the surface.

 

[SparticleBrane]

Do I get extra credit for jumping the gun now?

I suppose so. At least you were right...


By my calculations if you take into effect the water buoying the mass of lead, you would need to add 58.27lbs of lead to make the object 50lbs negative...anyone else get this?

Add to that, an object with 3lbs of buoyancy wouldn't be at a depth of 56 ft. It would be at the surface.

There's also nothing that says the object isn't tied down or being held in place. The depth of the object or the fact that it is underwater while being positively buoyant is irrelevant...

 

[NudeDiver]

Add to that, an object with 3lbs of bouyancy wouldn't be at a depth of 56 ft. It would be at the surface.

Unless it is tethered via a chain to a rock on the bottom. Then it's just floating there, desperately trying to reach the surface and the light of day, so that it can look up and see the sun and smile, knowing that, at last, it is safe - safe from the dangers of narcosis and oxygen toxicity and silly divers finning around trying to snort powdered coral. Oh, glory be to that which is floating on the surface, waves slashing over it, yielding a sweet, yet salty flavour.

By my calculations if you take into effect the water buoying the mass of lead, you would need to add 58.27lbs of lead to make the object 50lbs negative...anyone else get this?

By the way, the question wants to know how to "ADD 50 lbs of negative buoyancy" not how to give it a NET negative buoyancy of 50 pounds (i.e. make it 50 pounds negative). If you want to ADD 50 pounds negative buoyancy and are disregarding the buoyancy of what you are adding, then you simply add 50 pounds of lead. The object's current buoyancy doesn't matter in the slightest.

 

[LowVizWiz]

Part of the problem is that the question, as it is phrased here, can be read two different ways. Add to that, an object with 3lbs of bouyancy wouldn't be at a depth of 56 ft. It would be at the surface.

That type of questioning is all over the PADI DM exams you have to figure out what they are really asking and ignore the rest. There were at least 4 or 5 questions that you honestly could answer 2 different ways and be correct, but if it's not what the answer key says you are wrong

 

[kadleck]

There's also nothing that says the object isn't tied down or being held in place. The depth of the object or the fact that it is underwater while being positively buoyant is irrelevant...

True that it's irrelevant (and I agree that 53lbs is the answer), but the question is very badly worded (though I suppose it could have been remembered wrongly). "Lying at a depth of x" is completely different to "tethered at a depth of x". Ah, the semantics of the English language!.

 

[Gombessa]

It sounds to me like people are answering completely different questions here.

If you disregard the buoyancy of the lead, 50# of lead is 50# negatively buoyant. As SB said, this conveniently simplifies the math needed either way.

The question as it was asked by the OP was: "What amount of lead (disregarding the water displaced by the mass of the lead) would be required to add 20 kg/50 lbs of negative buoyancy to the object?"

The answer to that question is 50# of lead. Admittedly, it's possible to also read "add 50lb of negative buoyancy" as meaning the same thing as below, but the ambiguity in the wording makes it a prime candidate for a valid challenge. You simply can't argue that one interpretation is correct to the exclusion of the other.

What Sparticle is doing, is assuming (probably correctly) that either the OP incorrectly recited the question or that the question was originally framed poorly in the exam. The question SB answers is, and what is probably intended by the original question stem is: "What amount of lead (disregarding the water displaced by the mass of the lead) would be required to make the object a total of 20 kg/50 lbs negatively buoyant?"

So, those of you who answered 50#, you pass the reading comprehension portion of the exam. For those who answered 53#, you pass the deductive reasoning section.

Does that sound reasonable?

 

[SparticleBrane]

By the way, the question wants to know how to "ADD 50 lbs of negative buoyancy" not how to give it a NET negative buoyancy of 50 pounds (i.e. make it 50 pounds negative). If you want to ADD 50 pounds negative buoyancy and are disregarding the buoyancy of what you are adding, then you simply add 50 pounds of lead. The object's current buoyancy doesn't matter in the slightest.

If you are adding 50lbs of negative buoyancy to something that is already 3lbs positive, you have to add 53lbs of negative buoyancy (again disregarding the force of water acting on the weights)...

If you add 50lbs of negative buoyancy to something that is 3lbs positive, the object will only be 47lbs negative.



Here, let me give you an example
Neutral diver: someone hands you 50lbs of lead, "adding 50lbs of negative buoyancy to you" -- the diver is now 50lbs negative (and quickly sinking to the bottom).

Diver 3lbs positive: Someone hands you 50lbs of lead, again, "adding 50lbs of negative buoyancy to you" -- now you are 47lbs negative (and once again, probably spiraling towards the bottom).

 

[NudeDiver]

If you are adding 50lbs of negative buoyancy to something that is already 3lbs positive, you have to add 53lbs of negative buoyancy (again disregarding the force of water acting on the weights)...If you add 50lbs of negative buoyancy to something that is 3lbs positive, the object will only be 47lbs negative.

Oh come on man. It's just negative numbers. Basic math.

If you add -50 to +3 you may end up with -47, but you still added -50.

The question, as posted, clearly asked how to you ADD -50 lbs of buoyancy, not how do you RESULT in -50 lbs of buoyancy.

The question asked about adding, it didn't ask about the final buoyancy of the object.

 

[kadleck]