Quiz - Physics - Minimum Displacement

[Pedro Burrito]

From the Physics Section of the PADI Dive Theory Exam:

A 600 kg/1350 lb concrete block lies in 19 m/63 ft of fresh water. The block displaces 300 l/11cf of water. To move the block off the bottom, what is the minimum amount of water that must be displaced from a lifting device?

a. 291.26 l/10.32 cf

b. 318 l/11.26 cf

c. 282.5 l/10 cf

d. 300 l/10.63 cf


I will post a daily question from my exams to help newer divers and to encourage more experienced divers to interact gracefully and helpfully with the newer divers.

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Thank you for your patience while we try to give people something to discuss other than Covid-19 and/or Politics. I will post the answer covered by the spoiler tag later today.

 

[HKGuns]

Well, this wasn't easy, but at least I'm falling in line with the most popular choice after forgetting 1 ATM yesterday.

 

[Scraps]

This one's simple if you stay metric. Imperial makes you do some arithmetic.

 

[Storker]

Is the density of freshwater assumed to be exactly 1 kg/L?

Besides, meesa like that the question gives more info than is needed to answer. At least I think I'm getting some surplus info. Which makes it more difficult for the student to reverse-engineer the question.

 

[BlueTrin]

This one's simple if you stay metric. Imperial makes you do some arithmetic.

I am wondering if there is a trick, this sounds too easy

 

[HKGuns]

This one's simple if you stay metric. Imperial makes you do some arithmetic.

Great point. I always go right to imperial and gloss right over the metric numbers. Good lesson for me to look more broadly.

 

[TMHeimer]

I am wondering if there is a trick, this sounds too easy

Yeah, I'm too lazy to do the math. Maybe a dumb question, but why isn't it d. 300l/11cf, not 10.63cf?

 

[CT-Rich]

Is the density of freshwater assumed to be exactly 1 kg/L?

Besides, meesa like that the question gives more info than is needed to answer. At least I think I'm getting some surplus info. Which makes it more difficult for the student to reverse-engineer the question.

At least I wasn’t figuring, that pressure on the cylinder would only increase by about 60 psi, which on 3000 psi cylinder would be almost nothing....

 

[MargaritaMike]

What are the dimensions of the block? What is the lift point? How far off of the bottom satisfies "To move the block off the bottom"? Is the 300l/11cf displaced at exactly 19m/63ft?

With the answers already given, I guess it doesn't make a difference. It's a theoretical block that weighs x and displaces y amount at exactly z depth.

I guess my biggest question is why do you want to lift it in the first place? It's just concrete.

Cheers,

 

[CT-Rich]

What are the dimensions of the block? What is the lift point? How far off of the bottom satisfies "To move the block off the bottom"? Is the 300l/11cf displaced at exactly 19m/63ft?

With the answers already given, I guess it doesn't make a difference. It's a theoretical block that weighs x and displaces y amount at exactly z depth.

I guess my biggest question is why do you want to lift it in the first place? It's just concrete.

Cheers,

You need to do something with the 40 liter cylinder from yesterday...