Quiz - Physics - Minimum Amount of Water That Must Be Displaced

[TMHeimer]

Honestly, I don't think the DM or Instructor class is meant to be restricted to only those things YOU are interested in.

Point taken. You could word your reply differently.

 

[saxman242]

I suppose someone will point out that having this on the tip of the tongue is vital in some way for being a divemaster

I think it's more about demonstrating an understanding of the difference between weight and buoyancy, which is most definitely appropriate to a DM class.

 

[TMHeimer]

I think it's more about demonstrating an understanding of the difference between weight and buoyancy, which is most definitely appropriate to a DM class.

I agree. I guess what I was trying to say is this can just be explained and maybe one example given-- rather than all the time we spent doing many of these problems 10 years ago.

 

[boulderjohn]

I think it's more about demonstrating an understanding of the difference between weight and buoyancy, which is most definitely appropriate to a DM class.

It is really important, but I would argue that by focusing on examples like this, things that the DMC can dismiss from the mind after certification, they somehow miss the practical side that really does matter to a DM.

• The diver is using a steel tank for the first time--how will the diver's weighting needs change?
• The diver sinks below the surface to dive easily enough but later on keeps bobbing to the surface. Why is adding more weight the wrong solution?

 

[TMHeimer]

This is not a practical question--it's a theory question. How often do you think you will be called upon to lift something off the floor and know what its weight and displacement are?

When you actually lift something, it's a guessing game. You guess what size bag(s) you will need, and you make sure you bring enough. Then you add air a little at a time until you can move it.

Absolutely agree here too. Some practical experience with a lift bag makes sense. I suppose a DM may be a little involved with that if assisting on an AOW course. And you would be very interested in using a lift bag if that is something that you are interested in. Either way, working the math is interesting, but probably won't be necessary to help you lift the thing.
I'm sure somewhere the is an example of a DM having to know the math involved, but have a hard time thinking of where.
Also agree with your follow up post above about practicality. I agree with PADI in adding practical stuff to the DM course-- like the Deep & S&R aspects. But am unsure what theory has been "dumbed down" as they say. Was wondering if it was these weight/displacement questions.
Dismissing stuff from the mind after DM certification because you just don't ever do some of these things is an interesting point. I wonder how many of the 15 or instructors I assisted could do these problems without at least looking something up. Probably some of them. I'm sure they all had a clear idea of weight vs. displacement, though.

 

[boulderjohn]

It is really important, but I would argue that by focusing on examples like this, things that the DMC can dismiss from the mind after certification, they somehow miss the practical side that really does matter to a DM.

• The diver is using a steel tank for the first time--how will the diver's weighting needs change?
• The diver sinks below the surface to dive easily enough but later on keeps bobbing to the surface. Why is adding more weight the wrong solution?

I should have added this question to the list, since something like it appears regularly on ScubaBoard.

• At the beginning of a dive Diver A and Diver B are both 9 pounds negatively buoyant. Diver A's steel cylinder weighs 5 pounds more than Diver B's aluminum cylinder. During the dive, both divers consume 5 pounds of air. What are their buoyancies at the end of the dive?

It would be interesting to see the answers for this question.

 

[tursiops]

I should have added this question to the list, since something like it appears regularly on ScubaBoard.

• At the beginning of a dive Diver A and Diver B are both 9 pounds negatively buoyant. Diver A's steel cylinder weighs 5 pounds more than Diver B's aluminum cylinder. During the dive, both divers consume 5 pounds of air. What are their buoyancies at the end of the dive?

It would be interesting to see the answers for this question.

Why are they carrying so much weight?

 

[boulderjohn]

Why are they carrying so much weight?

THat might be a nice topic for discussion, but I just threw out numbers so we would have a math problem.

 

[-JD-]

I should have added this question to the list, since something like it appears regularly on ScubaBoard.

• At the beginning of a dive Diver A and Diver B are both 9 pounds negatively buoyant. Diver A's steel cylinder weighs 5 pounds more than Diver B's aluminum cylinder. During the dive, both divers consume 5 pounds of air. What are their buoyancies at the end of the dive?

It would be interesting to see the answers for this question.

Why are they carrying so much weight?

The guy with the steel is diving an HP120 and has a final buoyancy of -4lbs but he could have stayed down down for another 20 minutes or so.

The gal with the aluminum is -2. She's an instructor with an Al80 and handed off 2lbs to the student who was chilled and added a vest after his last dive without adjusting his weighting and couldn't quite stay down on the SS. She also likes to be just a tad on the heavy side in case she needs to lend a little extra negativity to another student She has a better RMV than the guy with the steel, but with all of the airshares with students (this was pre-covid) she burned the tank a little fast.

 

[CT-Rich]

This is the least practical question you could ever ask. Knowing the volume of an irregularly shaped object is almost impossible. More realistically you could know the mass of anchor and extrapolate it’s volume by its density. Of course, knowing it density and weight makes it’s volume irrelevant.