Quiz - Physics - Minimum Displacement

[-JD-]

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

I'm gonna go with: "I want to move my boat mooring ..." Did I win?

 

[wetb4igetinthewater]

I'm gonna go with: "I want to move my boat mooring ..." Did I win?

When I moved the cement blocks for the mooring line at my parent's beach house, you can bet your patootie I wasn't doing these silly calculations. I just used lift bags that totaled more lift than the weight out of the water. Rolled the weights down the beach. Let the tide come in. Inflated the lift bags until the nb was achieved.

I don't see the point of some of these questions. How do you apply them in real life?

 

[tursiops]

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

Because 11 cuft is over 311 liters.

 

[TMHeimer]

Because 11 cuft is over 311 liters.

Thanks. Thought it was something like that.

 

[-JD-]

When I moved the cement blocks for the mooring line at my parent's beach house, you can bet your patootie I wasn't doing these silly calculations. I just used lift bags that totaled more lift than the weight out of the water. Rolled the weights down the beach. Let the tide come in. Inflated the lift bags until the nb was achieved.

I don't see the point of some of these questions. How do you apply them in real life?

I this case I actually see it being useful, to determine the resources to complete the task.

Not everybody has 300+ l (almost 700 lbs lift) of liftbags sitting in their SCUBA crap pile ...

 

[TMHeimer]

When I moved the cement blocks for the mooring line at my parent's beach house, you can bet your patootie I wasn't doing these silly calculations. I just used lift bags that totaled more lift than the weight out of the water. Rolled the weights down the beach. Let the tide come in. Inflated the lift bags until the nb was achieved.

I don't see the point of some of these questions. How do you apply them in real life?

I was thinking that 10 years ago when taking the (pre 2010) DM course. I'm still waiting for that guy to offer me $500 to retrieve the outboard motor he dropped into 100' sw. I think it is important to know all you have to know to dive as safely as possible. A lot of the explanations of "why" is simply interesting information. Much like maybe 80% of the stuff I learned doing my 2 music degrees. No student ever asked me about the influence Mozart & Haydn had on Beethoven's symphonies or how to write effective species counterpoint.

 

[NAM001]

I like the question it is one to make you think how big a lift bag is needed to move something underwater.

 

[wetb4igetinthewater]

I this case I actually see it being useful, to determine the resources to complete the task.

Not everybody has 300+ l (almost 700 lbs lift) of liftbags sitting in their SCUBA crap pile ...

please explain. Not looking to argue, but if you have something heavy to lift, you are not going to measure its dimensions and Google the density of whatever it is (assuming it is homogenous material). You will simply try to figure out how much it weighs on dry land and then use enough lift to bring it to the surface. Lift bags have buoyancy ratings.

Now if lift bags were rated by the amount of water being displaced, then we'd have to do some math. But that's ridiculous.

Again, I don't see the practicality of the question.

What I'm getting at is information retention and interference theory. While I know how to perform such calculations from my physics/engineering background, if I were from a liberal arts background for example, I'd probably have to learn it.

I don't like learning things just for the sake of passing an exam and never using it afterwards.

 

[CT-Rich]

Now if lift bags were rated by the amount of water being displaced, then we'd have to do some math. But that's ridiculous.

Again, I don't see the practicality of the question.

What I'm getting at is information retention and interference theory. While I know how to perform such calculations from my physics/engineering background, if I were from a liberal arts background for example, I'd probably have to learn it.

I don't like learning things just for the sake of passing an exam and never using it afterwards.

They are rated by the water they displace, but if you use the imperial system the math is a PIA. A 100 liter lift bag would displace 100 kg in fresh water or about 220 lbs. when you see them rated by weight, someone else has already done the math for you. Understanding the underlying principles saves you from doing the really dumb stuff because you have an intuitive understanding of the principle. This is only a theory, I have seen plenty of people who should know better do some pretty dumb stuff.

The scary thing about this question is that they are implying an OW certified diver should even toy with the idea of trying to raise an object that weighs more than a half ton with a lift bag.

 

[wetb4igetinthewater]

They are rated by the water they displace, but if you use the imperial system the math is a PIA. A 100 liter lift bag would displace 100 kg in fresh water or about 220 lbs. when you see them rated by weight, someone else has already done the math for you. Understanding the underlying principles saves you from doing the really dumb stuff because you have an intuitive understanding of the principle. This is only a theory, I have seen plenty of people who should know better do some pretty dumb stuff.

The scary thing about this question is that they are implying an OW certified diver should even toy with the idea of trying to raise an object that weighs more than a half ton with a lift bag.

But I see lift bags with lbs/kilogram ratings, not volume.

I believe these questions are for a DM course. When it comes to recreational S&R, the training is just for 25 lb objects.

One could argue that one should figure out how much gas one will need to actually lift an item, but my guess is most would just bring more than enough gas to do the job.