Two more dive physics problems

[Akimbo]

After looking at it again, the wording of both problems is pretty appalling.

 

[rhwestfall]

did I not read somewhere the level this stuff is written for is pretty low???????

 

[Akimbo]

did I not read somewhere the level this stuff is written for is pretty low???????

Low for the student or low for the people who are supposed to be teaching? Why not:

1. An object is a five ft. [-]square and five ft. high[/-] cube and is floating in the ocean. Six inches of the object is above the waterline. What is the [-]minimum line strength that would be needed to lift the object out of the water[/-] gross weight of the object? (Of course you would need to include the correct value.)

2. [-]If a diver takes a sealed, rigid, cubical container measuring 10 inches per side to a depth of 20 ft. in the ocean, the total crushing force (assume the wall thickness is zero) on the container would be about:[/-] What is the total hydrostatic force acting on a hallow sealed 10" cube with one atmosphere inside at an average depth of 20 FSW (Feet of Sea Water)?

The total force is different if the maximum or minimum depth is 20'. The average depth compensates for the 5" above and below the average. The wall thickness is irrelevant because you are only interested in the external surface area and the differential pressure.

 

[archiebald]

I don't answer these questions for people because;

a) they should be working out their homework themselves,
b) I live in the part of the world that uses sensible units (metric)

 

[Rich Keller]

I don't answer these questions for people because;

a) they should be working out their homework themselves,
b) I live in the part of the world that uses sensible units (metric)

He did work out his own homework and he got it wrong, that is part of the learning process, all we are doing is showing him where and how he got it wrong so he will learn from that mistake. Yes these questions are poorly worded and lack some needed information from our perspective but they were not intended for us, they were meant to be simple math problems based only on what a new diver has been told. We are over thinking what were meant to be simple but poorly asked questions.

 

[Akimbo]

I don't answer these questions for people because;

a) they should be working out their homework themselves…

True enough, except all the multiple choice answers are wrong, technically and practically!

Edit: OK #2 had a correct answer but painfully miserable phrasing.

…
b) I live in the part of the world that uses sensible units (metric)

I completely agree except I have seen a lot of European divers get lazy and just use 1 Metric Ton/Meter³ or 1 Kg/Liter for both salt and fresh water. Have you found that to be true in your area?

For reference, the average weight of seawater is rounded to about 1028.9 Kg/M³ of Seawater or 1.0289 Kg/Liter. That nearly 3% can really matter in salvage calculations.

 

[scubastingray]

I don't answer these questions for people because;

a) they should be working out their homework themselves,
b) I live in the part of the world that uses sensible units (metric)

Well damn, how will OP ever manage without you?



Seriously, why say anything?

 

[BRT]

These are just badly phrased questions. In the first case you don't have enough info because you don't know if they want to know what the object weighs or want to know what line strength would actually be safe to lift it with. And the exact number for the weight would vary depending on just what density of seawater you were working in. The second problem doesn't tell you if the inside of the cube is at atmospheric pressure or no pressure and doesn't specify top, bottom or center of the cube at 20 feet or the density of the seawater. They are questions written by people who don't understand the physics.

 

[kmurray719]

So which answer would you pick? How much margin do I need to add for line safety? Are there specific regulations on this?

---------- Post added August 14th, 2014 at 05:48 PM ----------



If the book explained how to solve them, then I would have obviously solved them myself. The book doesn't even say what total crushing force is! I've done actual physics courses in university, and it was never this frustrating to figure out what rules we are playing by. SSI is simplifying physics, supposedly to make it more approachable, but it's often hard to tell what the simplifications actually mean. Not to mention the specifics of the dive industry regulations, which, for some reason, they don't always deem necessary to include in the printed material.

The point of learning dive physics isn't to be able to regurgitate exact information you've received, it's to be able to apply it when you need to. If you understand the principles you were supposed to learn in the course you should be able to figure out how to solve the problems. These are the exact two problems I struggled with on this exam, and they're not easy, but the information you need to solve them is in the book. Nowhere is the question asking you to come up with the "real life" answer including safety regulations, it's asking for the math answer to the question.

He did work out his own homework and he got it wrong, that is part of the learning process, all we are doing is showing him where and how he got it wrong so he will learn from that mistake. Yes these questions are poorly worded and lack some needed information from our perspective but they were not intended for us, they were meant to be simple math problems based only on what a new diver has been told. We are over thinking what were meant to be simple but poorly asked questions.

These aren't homework questions, they're from the final exam of the course, which can (and often is in my experience) given as a take home exam. If he's not able to work them out on his own, that's information that would be relevant to an instructor certifying that the student has passed the physics session of the course or not. Unless the exam has already been turned in (in which case the instructor should have/would have gone over the questions students got wrong) he's basically cheating on the exam.

 

[agilis]

... if only there were people out there with knowledge of diving greater than that of a students' who could help to guide eager minds to an understanding. We should endeavor to create this leadership role that can provide knowledge and wisdom. We shall call them instructors and we shall see that it is Good ...

Instructors with an advanced understanding of diving technology, or of science beyond a high school level, are rare. Many instructors pretend to have scientific knowledge that they do not possess. Some of the scientific principles they pass on to their students are essentially medieval, and the technology, borrowed from the texts they used, or from the imperfectly remembered data they learned from their own instructors, is sometimes based on misunderstood principles of physics, chemistry, and biology.

One frequent result that I have seen or been told about is that these instructors tend to avoid theoretical discussion and focus on rules and the memorization of charts. They waste student's time with foolishly complex calculations, totally irrelevant to the level of certification or to useful skills, thereby adding a fradulent flavor of technical complexity that neither the instructor nor their student actually understand and certainly do not need.

There are so many truly vital skills that are not taught in modern scuba instruction that wasting time with irrelevant math problems is unforgivable.

A few years ago I was certified for Nitrox. A very simple, effortless certification. The instructors loaded the course with a great deal of very badly explained math, levels of needlessly complex calculation that might be appropriate to deep mixed gas diving, but utterly useless and probably counterproductive to learning the relatively straightforward issues involved in diving with mixes of Nitrogen and Oxygen that differ to a limited extent from those found in natural air. The math problem tests were not only useless, but gave the instructor team lots of free time while their students struggled with pocket calculators and decimals.

I actually began teaching maths to some of my fellow students.

I was much older, generally better educated, and much more experienced than the other students. I did not hesitate to point out the occasional errors, contradictions, and misdirections contained in test problems that had been awkwardly adapted from other tests and from sources beyond the competence of the instructors.

The instructors reacted with hostility when called on an error, acted as if I were a disloyal troublemaker, and sulked when they were proven incorrect. I get the impression that this is not an unusual phenomenon.