NAUI Advanced Diver Written Exam
Alan Wiemann
(One of the rec.scuba community requested that I post this since it had been
some time since he had taken the NAUI Advanced Diver written exam.)
Having recently taken this exam, I clearly recall one of the questions whose
"correct" answer isn't. Here's the question:
How much lift should be attached to a submerged object to bring it to the
surface?
a) Just a little more than the object weighs.
b) Twice as much as the object weighs.
c) Ten times as much as the object weighs.
d) None of the above.
The "correct" answer is a. I answered d.
Suppose I have a 1000 pound object (that's a heavy object for all of our
metric readers) whose density is just slightly greater than sea water, say 1.01
times that of sea water. This object is submerged in sea water. To make this
object neutrally buoyant, I must attach 1000 / 1.01 ~= 10 pounds of lift. If
I attach 1010 pounds of lift to this object (answer a), it will probably rocket
to the surface and launch itself. Answers b and c have equally catastrophic
consequences. So I chose d. I'd like to attach just a little greater than 10
pounds of lift to this object.
The correct answer, as explained to me, is a, because the object weighs
less underwater. I argued unsuccessfully against this that F = ma (where, in
this case, F is the force due to gravity and a is the acceleration due to
gravity), even underwater. What NAUI surely meant is that weight doesn't
refer to force due to gravity, but rather that force minus the buoyant force
to which it is subjected while submerged. They have their own definition of
weight.
There were other questions of dubious merit. Here's my favorite:
When using a drysuit for the first time, training in buoyancy control is
a) recommended.
b) required.
c) essential.
d) a and b but not c.
Think hard about this one before reading further.
I answered c. Here's my (obviously flawed) reasoning. Every good diver
knows that use of a dry suit presents buoyancy control problems not present in
a wet suit, and that training in buoyancy control while using a drysuit is a
must. So, I reasoned, it is certainly recommended (a). It is so recommended
that I consider it required (b). Perhaps essential (c) is NAUI's way of saying
really, really, really required. Sound sort of like synonyms to me. Anyways,
I answered c. d is obviously wrong because b and c mean the same thing.
The "correct" answer is d.
I suspected that this would be an interesting test when the instructions
stated that "to pass you must answer 75% (38 questions) correctly." The test
had 100 questions.
Even thought some parts of the written exam need serious rewriting, the
course as a whole was good (really depends on the individual instructor) and
I'd recommend it to anyone who wants to sharpen their skills and learn all
about their unsafe diving habits.
Alan Wiemann
Nicholas J. Simicich
For some reason, I thought that the definition of weight was different
than the definition of mass. Mass will stay constant, whereas weight
is dependent on the local conditions, that is, what a spring scale
will read based on things like gravity, SD and displacement. Thus, by
the definition I mislearned so so many years ago in high school
physics, the object really does weigh less under water, just as it
would weigh less on the moon, but it will still mass the same.
Actually, I agree with you because I tend to put a little less than
the object "weighs" into the lift bag and start it up by hand until
expansion takes over and sends it up.
--
Nick Simicich - uunet!bywater!scifi!njs - n...@watson.ibm.com
SSI #AOWI 3958, HSA 318
Alan Wiemann
> For some reason, I thought that the definition of weight was different
> than the definition of mass. Mass will stay constant, whereas weight
> is dependent on the local conditions, that is, what a spring scale
> will read based on things like gravity, SD and displacement.
> [ stuff deleted ]
Nick is partly right. Weight and mass *are* different. Mass is a quantity
of matter, expressed in kilograms (or slugs), and weight is a quantity of
force, expressed in newtons (or pounds). Unfortunately, popular usage does not
make the distinction between mass and weight, and kilograms are used
(incorrectly) for both weight and mass. This works in recipes, for example,
because most people do their cooking on earth, and gravity is pretty much equal
over the inhabited surface of the earth.
Mass will stay constant, above water, below water, in outer space, and
throughout the universe. Weight is dependent on *one* local condition:
gravity.
The mass of an object can be measured by comparing it with an object of
known mass. This is done with a *balance*. The object of unknown mass is
balanced against a combination of known masses. Most people have had an
opportunity to do this in high school chemistry lab. A balance will indicate
the same mass on the earth, on the moon, anywhere.
The weight of an object can be measured by comparing the gravitational force
exerted on the object with a known force, such as that exerted by a spring.
This is done with a *spring scale*. The upward force exerted by the
displacement of the spring equals the downward force exerted by gravity on the
object whose weight is to be determined. Some sort of indicator attached to
the spring points to a graduated scale from which the weight can be read. The
weight of the object will different when measured with different gravity.
Alan Wiemann
Dave Waller
> n...@scifi.uucp (Nicholas J. Simicich) writes:
>
> > For some reason, I thought that the definition of weight was different
> > than the definition of mass. Mass will stay constant, whereas weight
> > is dependent on the local conditions, that is, what a spring scale
> > will read based on things like gravity, SD and displacement.
> > [ stuff deleted ]
>
> Nick is partly right. Weight and mass *are* different. Mass is a
> quantity of matter, expressed in kilograms (or slugs), and weight is a
> quantity of force, expressed in newtons (or pounds). Unfortunately,
> popular usage does not make the distinction between mass and weight,
> and kilograms are used (incorrectly) for both weight and mass. This
> works in recipes, for example, because most people do their cooking on
> earth, and gravity is pretty much equal over the inhabited surface of
> the earth.
>
> Mass will stay constant, above water, below water, in outer space,
> and throughout the universe. Weight is dependent on *one* local
> condition: gravity.
Well, since we *are* being pedantic (:-)), I must throw in that Alan
is also partly right (more appropriately, _mostly_). However, just to
make this as correct as possible, _mass_ is a measure of the inertia
of a material abject, not its quantity of matter. _Interia_ is the
property of matter whose characteristic is to resist a change in its
velocity vector (this change can either be a change in magnitude --
i.e. it speeds up or slows down, or the direction of the vector --
i.e. a deviation from a linear path). Mass IS NOT, however,
constant, even though the _quantity_ of matter can remain unchanged.
Mr. Einstein demonstrated this through his theory of Special
Relativity, and it has been confirmed countless times with high-energy
physics experiments in particle accelerators, to name one example.
In fact, mass *is* affected by gravitational feilds. An object
subjected to a gravitational force increases in mass, yet the "amount"
of matter present remains unchanged. However, the change for a tiny
gravitational feild such as the Earth's is so tiny as to be completely
negligible for everyday experience, and therefore can be ignored (i.e.
the mass of a particular 5_lb lead weight is unchanged on the Earth's
surface vs. deep space, for all practical purposes). However, near a
very strong gravitational feild, as would be found near a black hole,
the change in mass can be substantial.
Further, mass is a relative quantity, depending on the relative motion
between object and observer/measurer (hence the term _Relativity_).
If it was possible to measure the mass of that 5_lb weight as it sped
past you at close to the speed of light, you would find that its mass
measured considerably greater than 5_lbs, yet you would also find no
additional matter that makes up for this discrepancy. What gives?
The kinetic energy of its motion relative to you adds inertia, and
hence mass.
But what of my use of lbs to express mass, and the general usage of kg
for the same? While Alan is correct in his assertion that kg is
formally a measure of mass ad lbs is a formal measure of force, both
actually _are_ "official" units of force and mass, respectively, and
their use of such is not incorrect. These units are called
"kilogram-force" and "pound-mass" (a.k.a. kgf lbm), which are used
quite extensively in engineering. They are defined as one would
expect: 1 kgf is the force required to accelerate the SI standard
1_kg mass at a rate of 1 m^2/s^2, which happens to also be 1_N
(Newton). A lbm is defined similarly.
Dave Waller
David Duis
>
> Mass will stay constant, above water, below water, in outer space, and
>throughout the universe. Weight is dependent on *one* local condition:
^^^^^^^
Jeez, I thought this had all been settled. Hadn't everyone agreed
that weight was the force required to lift an object? Given that most
dictionary writers don't weigh things in the water, it is
understandable that common definitions would confuse mass and weight.
But all good divers know that when Bernoulli's Principle is taken into
account, the force required to lift an object is reduced by its volume
times the density of the surrounding medium (air, water, mercury, etc.).
Thus weight depends on NOT ONLY the local force of gravity, BUT ALSO
the object's volume and the density of the surrounding medium.
Perhaps a more interesting question is: "Does an object at sea level
weigh more or less in a vacuum chamber than in air?"
This might make the "matter" clearer to those confused by the H2O.
Cheers,
Dave Duis NAUI AI Z9588, PADI DM 43922, EMT
du...@bent.esd.sgi.com Opinions my own, and subject to dispute.
David Duis
>
>But all good divers know that when Bernoulli's Principle is taken into
>account, the force required to lift an object is reduced by its volume
>times the density of the surrounding medium (air, water, mercury, etc.).
Ooops -- I meant Archimedes' Principle, not Bernoulli's. Thanks to
Mike Oliver for pointing this out.
Cheers,
Dave Duis NAUI AI Z9588, PADI DM 43922, EMT
du...@bent.esd.sgi.com Better diving through drugs.
Dave Waller
>
> Now now the question becomes which definition should NAUI use when
> writing tests. I believe that they should use the scientific
> definition but regardless of which definition they choose to use, they
> should clearly define what they mean by weight either on the test or
> in the class.
> ----------
In my opinion, NAUI should just be more careful in their wording,
thereby eliminating the ambiguity of definitions. For example, a the
following question is ambiguous, based upon the discussion we have been
having:
"How large a lift bag is required to lift a 200 pound object submerged
in 100 feet of water, to the surface?"
There are many problems with this question:
* What do they mean by weight (this is the one we are dealing
with here)?
* Does the lift bag have a vent? Is it sealed shut?
* Is it in fresh water or sea water? Could it be in the Great
Salt Lake?
* "How large" means what? Volume? Physical dimensions?
* Does "lift" mean establishing neutral buoyancy and then giving
the object/bag union a push? Or do they mean establishing
slight positive buoyancy at 100 feet depth, and then letting
the system rise by itself?
The question could be reasonably rephrased so as to clear up these
ambiguities as follows:
"How large (in cubic feet) of a vented lift bag is necessary to lift
an object weighing 200 pounds on land on an ordinary scale, and
displacing a volume of 1 cubic foot, to the surface from a depth of
100 feet in sea water? Assume the lift system and object rise to the
surface unaided with a buoyant force of 1 pound."
This wording leads virtually everyone to the same answer.
An interesting twist on the question, that results in a different answer,
is the following
"How large (in cubic feet) of a sealed lift bag is necessary to lift
an object weighing 200 pounds on land on an ordinary scale, and
displacing a volume of 1 cubic foot, to the surface from a depth of
100 feet in sea water? Assume the lift system and object rise to the
surface unaided."
Okay, gang, let's have your answers.
Dave Waller
Nicholas J. Simicich
>pwei...@shearson.com (Philip Weissman) writes:
>>
>> Now now the question becomes which definition should NAUI use when
>> writing tests. I believe that they should use the scientific
>> definition but regardless of which definition they choose to use, they
>> should clearly define what they mean by weight either on the test or
>> in the class.
>> ----------
>
>In my opinion, NAUI should just be more careful in their wording,
>thereby eliminating the ambiguity of definitions. For example, a the
>following question is ambiguous, based upon the discussion we have been
>having:
>
>"How large a lift bag is required to lift a 200 pound object submerged
>in 100 feet of water, to the surface?"
>
>There are many problems with this question:
>
> * What do they mean by weight (this is the one we are dealing
> with here)?
As we've learned, there is one unambiguous meaning for weight. There
are lots of other opinions, but the correct definition seems to be
that the object would deflect a spring scale by that amount, under
local conditions.
> * Does the lift bag have a vent? Is it sealed shut?
Have you ever seen a lift bag that was sealed at the bottom? I can't
recall seeing such an animal. Certainly not in common use, although
there may be some specialized applications that call for sealed lift
bags, perhaps with overpressure vents. But common sport diver salvage
procedures, as I recall, call for open bottom lift bags.
> * Is it in fresh water or sea water? Could it be in the Great
> Salt Lake?
Since we've learned that the definition of weight already takes this
into account, it is immaterial.
> * "How large" means what? Volume? Physical dimensions?
Lift bags are commonly measured in pounds of lift capacity.
> * Does "lift" mean establishing neutral buoyancy and then giving
> the object/bag union a push? Or do they mean establishing
> slight positive buoyancy at 100 feet depth, and then letting
> the system rise by itself?
Now, this sems to be of little import. You have a 200 lb object. You
need a lift bag with slightly more than 200 lbs of capacity. If the
bag has exactly 200 lbs of capacity, you'll have to swim it up. I
think that the 100 feet of sea water is a smoke screen, unless a
subsequent question says something like, "How many cubic feet of air
(surface equivalent) will it take?"
Ian Dall
>In article <845...@hpcupt1.cup.hp.com> wie...@hpcupt1.cup.hp.com (Alan Wiemann) writes:
>>throughout the universe. Weight is dependent on *one* local condition:
>>gravity. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> ^^^^^^^
>the object's volume and the density of the surrounding medium.
I would have called that *effective* weight. The weight of an object
is the force due to gravity. *Archimedes* principle states that
the effective weight is the weight minus the weight of the water
displaced.
According to your definition, (floating) ships weigh 0.
--
Ian Dall I'm not into isms, but hedonism is the most
harmless I can think of. -- Phillip Adams
ACSnet: i...@sibyl.eleceng.ua.oz
internet: i...@sibyl.eleceng.ua.oz.au
Dave Waller
> >"How large a lift bag is required to lift a 200 pound object submerged
> >in 100 feet of water, to the surface?"
> >
> >There are many problems with this question:
> >
> > * What do they mean by weight (this is the one we are dealing
> > with here)?
>
> As we've learned, there is one unambiguous meaning for weight. There
> are lots of other opinions, but the correct definition seems to be
> that the object would deflect a spring scale by that amount, under
> local conditions.
I disagree completely. Regardless of what we have determined here in
this forum to be the clear, unambiguous meaning of weight, the
discussion indicates to me, at least, that the definition of "weight"
is certainly not unambiguous to the general diving public. By
ignoring this difficulty you are assuming that everyone taking the
test has studied, and knows, the "one unambiguous meaning for weight".
This is clearly not the case.
Again, what I am doing here is expressing my opinion; I just want to
make that clear. To me, the most important issue on a test is, what
is the knowlege that the test is attempting to determine? If it is
the definition of terms, then the questions should clearly test for
this:
"Define the term 'weight', as it applies to the SCUBA diving
environment."
This would be a reasonable question, which should yeild an answer that
matches the "one unambiguous meaning for weight" that we have hashed
out here. However, if the term "weight" is an ancillary portion of
another question, where the knowlege being tested is the candidate's
understanding of lift bag systems, it is deceptive to make the
assumption you have made, Nick, unless "weight" is clearly and
unambiguously defined as part of the instruction, and emphasis is
placed on its proper definition. This has not been the case, in my
experience.
> > * Does the lift bag have a vent? Is it sealed shut?
>
> Have you ever seen a lift bag that was sealed at the bottom? I can't
> recall seeing such an animal. Certainly not in common use, although
> there may be some specialized applications that call for sealed lift
> bags, perhaps with overpressure vents. But common sport diver salvage
> procedures, as I recall, call for open bottom lift bags.
Yes, I have seen all manner of lift bags. You are correct, however,
in your concern: I have never seen one that is commercially perchased
that was not either open on the bottom, or vented in some other
manner. However, I have seen all sorts of homegrown bags, sometimes
sealed, every one referred to as a "lift bag". Again, ambiguity
exists here. However, I will concede that this is a nitpicky point,
and that "common knowlege" will probably not include Rube Goldberg
lift bags. Still, there is no harm in being very specific, thereby
eliminating possible misinterpretation by the candidate. Again, it is
important to remember that the purpose of the test is to test specific
knowlege, not the ability of the candidate to "correctly" interpret
the questions.
> > * Is it in fresh water or sea water? Could it be in the Great
> > Salt Lake?
>
> Since we've learned that the definition of weight already takes this
> into account, it is immaterial.
See my comments above regarding "weight". It is hardly immaterial.
> > * "How large" means what? Volume? Physical dimensions?
>
> Lift bags are commonly measured in pounds of lift capacity.
So what? What is "common" is not necessarily a dependable paradigm
for interpreting the question, as the whole weight issue has shown.
If the "common" usage of the term "weight" is used, as we have seen,
incorrect interpretation can easily result. There is no harm in being
specific about what answer is being sought. However, if left
ambiguous, the candidate can easily misinterpret the question, using
"common" definitions that may deviate from the "common" definition
used by the question writer.
> > * Does "lift" mean establishing neutral buoyancy and then giving
> > the object/bag union a push? Or do they mean establishing
> > slight positive buoyancy at 100 feet depth, and then letting
> > the system rise by itself?
>
> Now, this sems to be of little import.
Hardly. I *have* taken a test where two of the multiple choice
answers were (adjusted for the parameters of this hypothetical
question) 3.125 ft^3, and 3.14 ft^3. Which is the right answer? I
answered 3.14, the "right" answer was 3.125. Exercise left to the
reader to figure out what's going on here.
> You have a 200 lb object. You
> need a lift bag with slightly more than 200 lbs of capacity.
Note that I agree completely with this analysis, as did my regional
director. Yet, the test writer was looking for the other answer.
Therefore I got the question wrong. Was I wrong? NO! The question
was unclear. Both answers are "correct", given the scant wording of
the question. In my opinion, the "wrong" answer is more correct than
the "right" one.
> If the
> bag has exactly 200 lbs of capacity, you'll have to swim it up. I
> think that the 100 feet of sea water is a smoke screen, unless a
> subsequent question says something like, "How many cubic feet of air
> (surface equivalent) will it take?"
Yes, the 100 feet is a smokescreen, deliberately thrown in to test the
logic of the candidate. A deliberate inclusion of an irrelevant piece
of information is, in my opinion, acceptable (even a good idea).
However, a requirement of such an inclusion MUST BE that it does not
cause the relevant information to become ambiguous. Note that in the
second form of my question (in the original post), it is modified such
that the depth becomes important.
Dave Waller
David Duis
>In article <1991Jun10....@odin.corp.sgi.com> du...@sgi.com (David Duis) writes:
>>
>>Thus weight depends on NOT ONLY the local force of gravity, BUT ALSO
>>the object's volume and the density of the surrounding medium.
>
>I would have called that *effective* weight. The weight of an object
>is the force due to gravity. *Archimedes* principle states that
>the effective weight is the weight minus the weight of the water
>displaced.
>
>According to your definition, (floating) ships weigh 0.
Yup, but only if completely immersed in a single medium -- e.g., a submarine.
Another definition of effective weight (good term!) is "the amount of
force required to lift the object." In the case of a surface ship,
the force required to lift it is not zero, since you are moving
between two different density media.
Cheers,
Dave Duis NAUI AI Z9588, PADI DM 43922, EMT
du...@bent.esd.sgi.com Better diving through drugs.
Ed Falk
>>
>>According to your definition, (floating) ships weigh 0.
I assume this argument over terminology is why ships are described
in terms of displacement instead of weight.
-ed falk, sun microsystems
sun!falk, fa...@sun.com
In the future, somebody will quote Andy Warhol every 15 minutes.