Physics Rant: The Truth to Buoyancy in Diving

[4sak3n]

No, I meant F=ma.

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

Vice versa for negative buoyancy.

If you have neutral buoyancy, you aren't accelerating at all in the vertical direction. But that doesn't mean you aren't moving up or down.

Aaah, gotcha.

Reminds me of that old high school problem which blows all our minds when we first hear it. How a motorcyclist (or driver or runner or alien space ship or your mother-in-law on her motorised wheel chair) is accelerating* when he/she/it travels round a circular track with the magnitude of their velocity constant.

Also explains why newbs often overcompensate with changes in bouyancy, hitting the power inflator too much or letting too much gas out of their BCD.

*okay, okay ... has a changing acceleration for all you pedantics.

 

[4sak3n]

Even worse is going to the movies with me and my wife. She is a NP, so I pick apart the logical errors, and she picks apart the medical miracles ever so popular on modern TV

Tom

You two must have an absolute ball watching Star Trek episodes.

 

[bobcatdiver]

Damn I'm not 100% sure but I think that I had you as one of my professors in college.

 

[hudson]

buoy·an·cy

1. 1. The tendency or capacity to remain afloat in a liquid or rise in air or gas.
   2. The upward force that a fluid exerts on an object less dense than itself.

The problem resolves itself when you consider the definition in use in diving circles is the first (admittedly poorly worded) one.

 

[Blackwood]

Also explains why newbs often overcompensate with changes in bouyancy, hitting the power inflator too much or letting too much gas out of their BCD.

Yup, and the lack of immediate feedback compounds that problem.

I strive to be neutral at all times excepting when I'm on the surface and at the instants when I start a change in depth.

To the best of my abilities, I ascend neutral (though at the start of my ascent I become momentarily positive by expanding my lungs), descend neutral (opposite), and hover neutral.

 

[b1gcountry]

Buoyancy is a SCALAR quantity, not a VECTOR.

Buoyancy and Gravity are both forces. Every force is a vector, and has magnitude and direction. Even if a vector is always pointed down or up, they still have a direction.

...and Gravity and Buoyancy can indeed "go sideways" at least from the perspective of someone in Hawaii observing someone in New Jersey. If you are sitting on the Moon, looking at the Earth, which way is up?

(Note included Wikipedia references. While not authoritative, they are convenient, and easy to read.)

 

[b1gcountry]

My gawd, you may have destroyed curves in gradeschool, but that statement and link makes it quite clear that peers have nothing to fear from you nowadays.

You would be weightless at the earth's core, given the surrounding mass of the earth pulling you equally in all directions. Gravity affects you less as you descend (though you'd be hard-pressed to measure it).

Roak

You are forgetting distance, which also affects gravitational force. Whatever is above you in the atmosphere no longer contributes its mass to the equation, but your distance from the center point is now reduced, so whatever mass is still "beneath you" will produce a greater force. If most of the mass is still beneath you, and not above you, you get heavier. If the object is a consistent density, then you become lighter.

Look it up, it's interesting.

But yes, everyone agrees that at the Earth's center of gravity, the Earth produces no gravitational pull.

Tom

 

[Charlie99]

...and Gravity and Buoyancy can indeed "go sideways" at least from the perspective of someone in Hawaii observing someone in New Jersey. If you are sitting on the Moon, looking at the Earth, which way is up?

You may insist that gravity is most fully described as a vector force, but I find it useful to approximate it with a simple scalar quantity, or even (gasp!) a constant.

For my normal everyday life, I find it sufficiently accurate to assume that it gravity operates in one direction only ---- down.

This is the heart of the difference in how we look at buoyancy. I treat buoyancy (or more precisely, "net buoyancy") as a scalar quantity, the same as I do gravity. I find this to be a very useful simplification and approximation, and definitely one that is sufficiently accurate for real world use.

Charlie Allen

 

[Charlie99]

buoy·an·cy


1. The tendency or capacity to remain afloat in a liquid or rise in air or gas.

2. The upward force that a fluid exerts on an object less dense than itself.

The problem resolves itself when you consider the definition in use in diving circles is the first (admittedly poorly worded) one.

I see the terminology problem a bit differently.

There is the "buoyant force", which is the #2 definition above. And there is "net buoyancy", which is the difference between the downward pull of gravity and the upward buoyant force on a submerged object.

Sometimes "buoyancy" is used to mean the "buoyant force" (just the upward force a fluid exerts on an object) and sometimes "buoyancy" is used to mean the net difference between the buoyant force and the pull of gravity.

When the "buoyancy" you are referring to is the "net buoyancy", then the term negative buoyancy is quite proper. It simply means that the gravitational pull on an object is greater than the upward "buoyant force".

Charlie Allen

p.s. B1gcountry, you are free to use vector addition to combine gravity and buoyant force to calculate the net force on the object. You will get the same answer I get using simple algebraic addition.

 

[b1gcountry]

For my normal everyday life, I find it sufficiently accurate to assume that it gravity operates in one direction only ---- down.

This is the heart of the difference in how we look at buoyancy. I treat buoyancy (or more precisely, "net buoyancy") as a scalar quantity, the same as I do gravity. I find this to be a very useful simplification and approximation, and definitely one that is sufficiently accurate for real world use.

Charlie Allen

I agree. No need to use a diamond hammer when a sledge will do all you need a lot quicker.

As I said to start with this was mostly a rant. Everyone has something they get sensitive to. This is one of mine. Part of the problem is that there isn't a good set of terms for what we are describing. As I said Neutral Buoyancy is a fair use, but Positive Buoyancy and Negative Buoyancy are technically incorrect. Are my terms perfect? No. Does this matter what word people use?

As long as they understand what they are really describing, probably not. My concern is that the terms in use may actually be a hindrance to some people. I've certainly come across folks who just didn't understand it. I've explained it to quite a few people.

I know I'll never 'convert' people into using my terms, but if I can explain things to them a little better, I'll be happy.

Tom

PS here's a short story:

I got home from work and unlocked my girl's cage. I put her collar on, and she ran barefoot through the park, drooling as she went.

If I said "Dog" instead of "Girl", would I have been more easily understood? Point being: semantics don't matter, except when they do.