Physics Rant: The Truth to Buoyancy in Diving

[Gombessa]

Neither. You spelled both incorrectly. Now let's start a spelling and grammar rant.

Its principle[,] not principal. never mind the missing apostrophes.

Why can't Americans distinguish "loose" from "lose"? :mooner:

I dunno, but I sure don't loose any sleep over it.

 

[eponym]

The above is why spelling corrections and the like should be off-limits . . . otherwise nothing of substance ever gets discussed. Oh, wait . . . we were way off topic and into satire already, weren't we?

-Bryan

PS. Forget "Possession is nine-tenths of the law." Instead, think "Possessives are a lost art."

 

[Diver Lori]

Well, I just read the OP to my dive buddy physicist. He said you are right for the most part, and some parts not, but the overwhelming crux of your post is that you really need to find something to do.

 

[J.R.]

The above is why spelling corrections and the like should be off-limits . . . otherwise nothing of substance ever gets discussed. Oh, wait . . . we were way off topic and into satire already, weren't we?

I thought we *started* with satire...

 

[LarryHinDC]

Sometimes I wonder how we even manage to not go crazy in a sport in which volume is measured in pounds, as in "How much air do you have left?" -"700 pounds"

Uh, that may be because we always have the same volume of air in our tank, regardless of the pressure.

 

[builder58]

No, I meant F=ma.

Neglecting things like scooters, upwelling currents, etc., if you have positive buoyancy, your acceleration is positive (upwards), but with no other information, no conclusions can be drawn about your velocity (i.e. it could be downwards).

It's been a while, and feel free to correct me. But a positive acceleration would indicate an increase in velocity (regardless of vector) whereas a negative acceleration would be an reduction of velocity. So, in this case, if you have a positive acceleration you are increasing your velocity and in order to have positive buoyancy (displacing a volume of material (in this case, water) that has a mass greater than that of the object in question and therefore "floaty"), the vector of your velocity must be up?

--Jeez, I need to find something better to do too.

 

[batmanusa]

First off, let me admit that, as Scott Adams put it, I have 'The Knack'. I'm an aerospace software engineer, and I was always the kid in science class that would mess up the curve for everybody else. What's worse, I'm always "that guy" who spoils the movie by always pointing out the plot flaws and physical impossibilities in every film I watch with my wife (lucky her). I know we can be a pain to be around, but just remember that is it wasn't for us (and her), you wouldn't even have computers right now.

So if you know anyone like me (maybe a step-brother you keep locked in the basement), you know the best way to get under our skin is to use technical terms, in randomly incorrect ways, or even better: apply the laws of physics in a way that, while not apparent to the more attractive of our species, will bug the bee-jesus out of us more than the platypus annoyed taxonomists.

So, with this in mind, let me say that I am really tired of hearing the term Negative Buoyancy. Buoyancy is a force. Being a force, it has both Magnitude, and Direction. By definition, a force cannot have a negative magnitude, although its magnitude can be in direct opposition to another force.

Furthermore, Forces are additive, but discrete. That is to say that, if two forces act in opposing directions in equal amounts, they will counter-balance one another (ie, no work will be performed), but they will both continue to exist in their own right. This is fairly obvious, as when you lay down in bed, you still weigh the same amount even though the mattress is now supporting (counter balancing) that weight.

So now for those who haven't added me to their ignore list for mentioning physics, lets get back to diving...
There are pretty much two forces at play while we are diving that contribute to what we refer to as 'buoyancy': Gravity, and Buoyancy. For simplicity, we will define Gravity as the 'downward' magnetic pull of the Earth on any object with Mass (ie, how much something weighs). Buoyancy we will define as the 'upward' force imparted by water to a submerged object (Air also provides a buoyant force, but we won't get into that for now.)

As any aging woman can attest, Gravity is at work on us at all times. We cannot even escape it's clutches underwater (it may actually increase the farther we get below sea level.) Disappointing as it is to me, if I'm 10lbs overweight on the surface, I'm still 10lbs overweight underwater. When we submerge our bodies, however, the buoyant forces increase, and, if managed correctly, can be used to counter-balance the gravitational forces, and achieve equilibrium. However, even if managed incorrectly, divers will always have some amount of buoyancy underwater, and that that buoyancy will ALWAYS be directed "up". So terms like "Positively Buoyant", "Neutral Buoyancy" and "Negative Buoyancy" are either misnomers, redundant, or just plain wrong.

Positive Buoyancy was a term invented by the diving community meant to describe an object capable of floating on the surface. In other words, the magnitude of the buoyant force is greater than the magnitude of the gravitational force. To the physicist, "Positive Buoyancy" is redundant way of describing the upward buoyant force of an object, and in no way expresses whether an object will sink or float. Why the diving community adopted this silly phrase eludes me, especially, when a term already existed for what they were trying to describe: Buoyant. Another proper term would be to say an object Floats.

Neutral Buoyancy describes a situation where the buoyancy of an object is equally counter-balanced by the weight of an object. For the most part, divers adjust their buoyancy to achieve equilibrium underwater, so perhaps this term is appropriate, although I use the term Neutral instead, as it is shorter. Neutral Weighting would also be acceptable, as would saying an object Hovers.

Negative Buoyancy is just wrong. It is never buoyancy that causes something to sink, it is gravity. Buoyant forces can never be 'downward'. Negative Buoyancy is used to describe an object that Sinks. The term Heavy is more appropriate, and I have sound it readily understood by other students/divers.

So long story short, if you want to impress a diving nerd, don't use the made up terms even the textbooks repeat, use more correct terms like Buoyant, Neutral, and Heavy. Or say an object Floats, Hovers, or Sinks.

Okay, rant over. I hope I educated at least one of you. Begin the ridicule!

Tom

Good point Tom. Well said.

 

[Sinbad the Diver]

I enjoyed the read OP, but as at least someone else has said (couldn't consider reading all the pages of respones) dude....you need another hobby, and I never thought I'd say that to a diver.

I do think though this falls into the basic category of most effective communication. If we define "bouyancy" as whether we go up, down, or stay in the same place it makes it easiest to communicate to the masses that you are either postively, negatively or neutraly.

And btw....the big draw to this post was the title, Physics Rant...I honestly thought you were going to rant about a law of physics....at which point I was going to have to pull out the ol.... "Physics...it's not just a good idea.....It's the law" quote .

 

[eponym]

I thought we *started* with satire...

Now that's a winner.

-Bryan

 

[Pelagic Penguin]

Yep. Love this post.

As a new diver, getting PADI certified, I actually got MORE confused with the positive-neutral-negative terms. Sucks when you know too much.

That's ok. There's this kid..well, a college student my age...I don't know how he even passed his exams for certification. But he did. And it was probably thanks to foiled buoyancy terms, haha.