DM Physics

[richarddean]

I cannot get my head around the physics in the divemaster course and would be grateful if someone could talk me through an example

An object weighs 237kg and displaces 123 lts of water. How much additional sea water do you need to displace th give the object 40kg positive bouyancy.

I came up with the answer of 150.4 but I am sure this is wrong, can someone show me how I would work this out if my answer is wrong please

I know seawater weighs 1.03kg


thanks

 

[MikeFerrara]

If sea water is 1.04 kg/lt

For an object that weighs 237 kg to be buoyed up by a force of 40 kg it must displace a volume of water that weighs 277 kg.

How many liters of displacement is that? 277 kg / 1.03 kg/lt
= 268.93Lt

The object already displaces 123 lt so how much additional volume is needed?
268.93 lt - 123 lt = 145.93 lt

 

[CrawfishDiver]

Darn metric people

 

[amascuba]

Darn metric people

It's been a while and I don't have any reference material, but let's see if I can give a similar example, but in imperial.

Salt water weighs 8.6lbs/gallon
Fresh water weighs 8lbs/gallon

For an object that weighs 130lbs, and displaces 6 gallons of salt water, to be buoyed up by a force of 40lbs it would need to displace a volume of water that weighs 170lbs.

How many gallons of salt water is that? 170 / 8.6 = 19.77gallons of salt water.

If the object already displaces 6 gallons of water then you would need to subtract 19.77 - 6 = 13.77gallons.

It's the same math, but in imperial rather than metric.

 

[Rick Murchison]

If sea water is 1.04 kg/lt

For an object that weighs 237 kg to be buoyed up by a force of 40 kg it must displace a volume of water that weighs 277 kg.

How many liters of displacement is that? 277 kg / 1.03 kg/lt
= 268.93Lt

The object already displaces 123 lt so how much additional volume is needed?
268.93 lt - 123 lt = 145.93 lt

So which is it, Mike? 1.04 or 1.03? (or could it be closer to 1.025???? on average, of course )
Rick

 

[Rick Murchison]

I cannot get my head around the physics in the divemaster course and would be grateful if someone could talk me through an example

An object weighs 237kg and displaces 123 lts of water. How much additional sea water do you need to displace th give the object 40kg positive bouyancy.

I came up with the answer of 150.4 but I am sure this is wrong, can someone show me how I would work this out if my answer is wrong please

I know seawater weighs 1.03kg


thanks

By the way, this is one of those really silly questions... they ought to just stick with pure water for these (which would be 237-123+40=154) both for ease of calculation and for accuracy. The density of "sea water" varies considerably, depending on where you are in the world, and shaving an answer to a tenth or hundredth of a Kg per 150 Liters is akin to "measuring with a micrometer, marking with chalk and cutting with an ax."
Silly.
Both your answer of ~150 and Mike's of ~145 are "correct" - depending on where you are
I wouldn't even hazard a guess at what the "book" answer they want is.
Rick

 

[Meng_Tze]

.."measuring with a micrometer, marking with chalk and cutting with an ax."
Silly.
Rick

People still do it though.......

 

[Rick Murchison]

People still do it though.......

Indeed they do! Aggravates the dickens outa me
Rick

 

[rawls]

I'm coming up with the same answer as Mike. Negative displacement of 106.8lsw (110kg/1.03 kg). Add 106.8lsw plus 38.8lsw (40kg/1.03kg) = 145.6lsw.

 

[richarddean]

thanks for the valuable info Mike, perhaps you can show me how to work another one out, not the brightest tool in the box me lol

A diver consumes 7 bar per minute at 33m in seawater with a given cylinder. Using the same cylinder what is the divers consumption at 25m


thanks