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Spider Crabs can just f*ck off
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Discussion Starter · #1 ·
I have to bite the bullet soon and sit my DM physics exam. Now, I'm not a stupid person (I like to think) but I cannot grasp the whole 'volume/displacement/air needed to lift' concept. I've had it explained to me by a Course Director and I'll ask Turbanator to go through it again (poor sod) so I'm not looking for a lesson here.

What I need are some examples to work through. I'll do the ones in the PADI DM workbook but are there any anywhere else that I can use?

I'm more of a language/arts/literature person (physics, maths and I have a very poor history), and I'm getting myself tied up in knots about this, so any help would be appreciated. :redface:

Thank you.
 

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All hail the mighty ZOM
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Right. It's dead simple. Immerse somethign in a liquid and you get an upthrust equal to the weight of liquid displaced.

So if you have a 10x10x10 cm cube of something and stick it in say water that has a density of 1 kg/litre then you'll get an upthrust of 1kg. Now- if your 10x10x10 cube weighs less than 1kg (say it's polystyrene which weighs sod all) then it will float as the 1kg upthrust is much bigger than its weight. If the 10x10x10 cube is heavier, say lead, then it will sink as 10x10x10 of lead weighs about 11kg. However it will still have the same 1kg of upthrust so it will actually "weigh" on the bottom of your sink 10kg.

For denser liquids the upthrust is greater- see Digger's post on filling the SETT with mercury.
 

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Finless: You couldn't invent him...
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Examples/schmegamples!

Volume Displacement

Volume Displacement. Archimedes was a Greek mathematician, inventor and physicist who lived from 287 - 212 B.C. A king once gave him a difficult task. ...
www.rit.edu/~vjrnts/courses/matter/labs/volume_displacement/ - 8k - Cached - Similar pages - Note this
Volume Displacement

Volume Displacement. Question: If you were in a boat in a small swimming pool (so that you could measure the water level) and there were rocks in the boat, ...
www.newton.dep.anl.gov/newton/askasci/1993/physics/PHY23.HTM - 16k - Cached - Similar pages - Note this
Volume Displacement Measurement

Examples such as this illustrate MicroXAM optical profiler's efficacy in providing failure analysis, wear analysis, and volume displacement. ...
www.phase-shift.com/products/volume-displacement-measurement.shtml - 14k - Cached - Similar pages - Note this
Analysis of volume displacement and length changes of the ...

The fraction of total rib cage volume displacement (delta Vrc) contributing to abdominal displacement is predicted from anatomic considerations and ...
jap.physiology.org/cgi/content/abstract/53/3/750 - Similar pages - Note this
 

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Finless: You couldn't invent him...
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So ....................... when you pass .......................... you'll be a dive mistress?

:)
 

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PADI Internet Specialty Diver
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The man himself sat in the bath. There's a good start. As you get in the water level goes up. If the bath is full the water comes over the side. If whatever is in the bath (you in this case) weighs less than the water that comes over the side it will float. If it weighs more (e.g. Turb's PSP) it will sink.

Chris
 

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Spider Crabs can just f*ck off
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Discussion Starter · #6 ·
Right. It's dead simple. Immerse somethign in a liquid and you get an upthrust equal to the weight of liquid displaced.

So if you have a 10x10x10 cm cube of something and stick it in say water that has a density of 1 kg/litre then you'll get an upthrust of 1kg. Now- if your 10x10x10 cube weighs less than 1kg (say it's polystyrene which weighs sod all) then it will float as the 1kg upthrust is much bigger than its weight. If the 10x10x10 cube is heavier, say lead, then it will sink as 10x10x10 of lead weighs about 11kg. However it will still have the same 1kg of upthrust so it will actually "weigh" on the bottom of your sink 10kg.

For denser liquids the upthrust is greater- see Digger's post on filling the SETT with mercury.
:frown: I think I understand that bit. It's the order of the calculations that throws me. You know, the multiplying by 1.03 for sea water and 1 for fresh water, the volume of the object minus the displacement, the age of Archimedes divided by the gross annual olive crop etc etc.

Surely, in the real world, if you have to lift an object from the seabed, you take down the biggest liftbag you have, fill it completely full of air and let it go? Do people really sit about calculating how much air they need to the nearest bar? Do they buggery!
 

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Finless: You couldn't invent him...
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23,946 Posts
The man himself sat in the bath. There's a good start. As you get in the water level goes up. If the bath is full the water comes over the side. If whatever is in the bath (you in this case) weighs less than the water that comes over the side it will float. If it weighs more (e.g. Turb's PSP) it will sink.

Chris
The "it" in your example being Kirstie? :)

And the rest of you can stop perving .......... I'm sure Kirstie is fully clad in the bath!!!

:)
 

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All hail the mighty ZOM
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:frown: I think I understand that bit. It's the order of the calculations that throws me. You know, the multiplying by 1.03 for sea water and 1 for fresh water, the volume of the object minus the displacement, the age of Archimedes divided by the gross annual olive crop etc etc.

Surely, in the real world, if you have to lift an object from the seabed, you take down the biggest liftbag you have, fill it completely full of air and let it go? Do people really sit about calculating how much air they need to the nearest bar? Do they buggery!
Teh problem with lifting stuff isn't getting it to the surface but overcoming the "sticktion" of the seabed.

Anyway. The upthrust of an object is it's volume x density of the liquid. Take that away from it's weight in air and you have the required upthrust you need to make it neutrally buoyant.

So- say you have a 10x10x10 cm lump of lead you want to lift. Weight in air = 11 kg. Upthrust in fresh water = 1 kg. Weight in water = 10 kg. Volume of air needed to make it neutral = 10/1 = 10 litres.

Or for seawater:
Weight in air = 11 kg
Upthrust = 1.03 kg
Weight in water = 9.97 kg
Volume of air needed to make it neutral = 9.97 / 1.03 = 9.67 litres.
 

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Darren of the Deep
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The easy way 1KG =1liter so if you had to lift 1KG from 40m you would need 5 liters of gas.
 

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All hail the mighty ZOM
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Now let's be a bit silly and assume we are diving in pure alcohol.

10x10x10 cube of gold.

Weight in air = 19.3 kg
Upthrust in alcohol = 0.78 kg
Weight in alcohol = 18.52 kg
Volume of air needed to make neutral = 18.52/0.78 = 23.74 litres

So you need more air to lift in booze than in water as the upthrust is lower as the alcohol is not as dense.

BTW for you pedants out there I am assuming that the weight of air is negligible. We could be really silly and lift with argon or something which is a bit heavier so you would need more.
 

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Spider Crabs can just f*ck off
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Discussion Starter · #11 ·
The "it" in your example being Kirstie? :)

And the rest of you can stop perving .......... I'm sure Kirstie is fully clad in the bath!!!

:)
I always wear my swimmies to bath in.

What's a PSP? I haven't seen that, either in the bath or anywhere else.
 

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PADI Internet Specialty Diver
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Spider Crabs can just f*ck off
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Discussion Starter · #14 ·
That's me in my dive kit in Tenerife! :)
 

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Registered
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Kirstie

I'm sorry I don't have any more examples to point you to so you can work through them. I post merely to offer my condolences for you're asking the question and getting so much accurate...yet somehow unhelpful...information. :p
 

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Spider Crabs can just f*ck off
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Discussion Starter · #17 ·
Kirstie

I'm sorry I don't have any more examples to point you to so you can work through them. I post merely to offer my condolences for you're asking the question and getting so much accurate...yet somehow unhelpful...information. :p
LOL, to be fair a lot of it is helpful - I've printed off a few attachments which I'll read through in the hope that the penny drops :)

My problem is that I convince myself I'm not going to understand before I've even tried.
 

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Ginger, Irish, sometimes stroppy
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I find it easier to do it in steps

Work out the volume in litres of the thing to move, that'll be the volume of water thats displaced. U

Using 1kg/l (or 1.03 kgl) the weight of the water displaced is approximately the same as the volume of the water displaced.

To be neutral the object displacing the water should eb the same weight.

To go up it should be lighter, to go down it should be heavier.

Thats the basic bit, so onwards to lifting something.

If you have something at the bottom you want to lift that ways 2kg and occupies 1kg of volume, according to the above it is 1kg negative.

Therefore to make it neutral you need to displace a further 1kg of water so you would need a container of about 1 liter (in fresh water etc) filled with air.

The amount of gas needed to fill the object would be determined by depth, so at the surface you need 1 surface equivalent litres of air, at ten meters you need 2 surface equivalent litres of air, and so on.

The reason for 'surface equivalence' is that at the respective depths they are always 1 litre.

So for an example, an object weighing 1000kg with a volume of 700L is sunk in fresh water at 30m.

What size lifting bag would be required to make it neutral?
What volume of gas would be required to fill the bag at that depth?
If you have a 12L cylinder with you for lifting the object, what pressure would it need to have to contain enough gas to do the lift?

Sorry if all that is useless but it was interesting for me :)
 

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You could also try here. It may be another way of trying to explain something that doesn't "stick" very well in the way you think.

Quite a few graphics for explanations and I like the term "Bobbies". ;)
 

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Finless: You couldn't invent him...
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LOL, to be fair a lot of it is helpful - I've printed off a few attachments which I'll read through in the hope that the penny drops :)

My problem is that I convince myself I'm not going to understand before I've even tried.
Just think .......... when it drops you'll know if it will be buoyant and, if not, how much gas to put in a lift bag to recover it! Incentive to learn if I've ever heard of it? :)
 
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