Misconceptions and Fallacies

[Walter]

That is just absurd. The air is shot in at high pressure because you're pushing it into a pressure vessel. Oxygen and Nitrogen do not have sufficiently different weights such as to stratify in the tank. Fill it slow. Fill it fast. Roll the tanks. It makes NO difference.

I cannot understand why people try to make more of this than it is. Nitrox is AIR with a different ratio of Oxygen and Nitrogen. Air will not stratify in a tank (or anywhere else, for that matter). Nitrox won't either. Why would it?

-Charles

First, you say my statement is absurd, then you proceed to agree with me. Either we are both wrong or we are both correct.

 

[miketsp]

Got to say this has been kind of an entertaining thread. For a recreational activity/sport that has a strong element of science (understanding of gas laws, bubble theory in the bloodstreams, etc.) there seems to be a littany of scientific misperceptions in physics (like some of the weight versus mass stuff), thermodynamics (water in a wetsuit keeps one warm, my favorite), etc. It's fun that there are so many intellectual discussions in SB on our sport, providing lot's of education for all - you sure don't see that on a tennis forum!

I think part of the problem is concepts.
The OP was clear that when he talked about being neutrally buoyant this must include the diver plus his equipment and as someone else pointed out if you put a spring loaded weighing scale on the bottom and brought the diver into contact, it would read zero.
Other posters however tried to then separate the diver from his BC saying the BC exerts an upward force and the diver's weight exerts a downward force, so there was still weight present. This is a change of concept and not what the OP asked.

As for the poster sitting on his chair, of course there is weight.
If you put a weighing scale between him and the chair it will register the weight of the poster. If you put it between the chair and the floor it will register the weight of the poster plus his chair.

 

[Blackwood]

I think part of the problem is concepts.
The OP was clear that when he talked about being neutrally buoyant this must include the diver plus his equipment and as someone else pointed out if you put a spring loaded weighing scale on the bottom and brought the diver into contact, it would read zero.
Other posters however tried to then separate the diver from his BC saying the BC exerts an upward force and the diver's weight exerts a downward force, so there was still weight present. This is a change of concept and not what the OP asked.

I said absolutely nothing to separate a diver from his equipment.

If there is mass present, there is weight present. You say you are a physicist?

As for the poster sitting on his chair, of course there is weight.
If you put a weighing scale between him and the chair it will register the weight of the poster. If you put it between the chair and the floor it will register the weight of the poster plus his chair.

Why is there weight when a chair is opposing the gravitation of my mass, but there is not weight when displaced water is opposing the gravitation of my mass?

That's ridiculous.

Neutral buoyancy implies the following:

SUM(Fy)=0,

where Fy are vertical forces. Assuming that the only forces that matter are the gravitational force and the buoyant force:

buoyant - weight = 0

That's what it means. You weigh what you weigh: local acceleration of gravity times your mass.

 

[Blackwood]

Got to say this has been kind of an entertaining thread. For a recreational activity/sport that has a strong element of science (understanding of gas laws, bubble theory in the bloodstreams, etc.) there seems to be a littany of scientific misperceptions in physics (like some of the weight versus mass stuff), thermodynamics (water in a wetsuit keeps one warm, my favorite), etc. It's fun that there are so many intellectual discussions in SB on our sport, providing lot's of education for all - you sure don't see that on a tennis forum!

What, no talk of why fuzz makes tennis balls go faster?

 

[miketsp]

I said absolutely nothing to separate a diver from his equipment.

If there is mass present, there is weight present. You say you are a physicist?



Why is there weight when a chair is opposing the gravitation of my mass, but there is not weight when displaced water is opposing the gravitation of my mass?

That's ridiculous.

Neutral buoyancy implies the following:

SUM(Fy)=0,

where Fy are vertical forces. Assuming that the only forces that matter are the gravitational force and the buoyant force:

buoyant - weight = 0

That's what it means. You weigh what you weigh: local acceleration of gravity times your mass.

You seem to be a little paranoid my friend and you're starting to repeat yourself.

I wasn't answering your post and the poster that tried to separate the BC from the diver in order to prove that the diver wasn't weightless was Plot.

Also at no time did I claim to be a physicist, that was another poster.

And as for your phrase that "If there is mass present, there is weight present."
That's already been commented on by myself and others and I'm not going to waste my time getting into a loop of endless repetition.

 

[Blackwood]

You seem to be a little paranoid my friend and you're starting to repeat yourself.

I wasn't answering your post and the poster that tried to separate the BC from the diver in order to prove that the diver wasn't weightless was Plot.

Also at no time did I claim to be a physicist, that was another poster.

And as for your phrase that "If there is mass present, there is weight present."
That's already been commented on by myself and others and I'm not going to waste my time getting into a loop of endless repetition.

Paranoid? Naw.

I'll repeat myself as much as necessary. The comments I saw neither refute nor contradict my statement.

The rest? Fair enough.

 

[Temple of Doom]

People, why are you arguing a point which you have absolutely nothing you can back it up with? I see people 'contradicting' posts like the ones that Blackwood has been making, by throwing out non-sequitors and logical fallacies. There hasn't been one iota of truth/accuracy to the "floating=no weight" argument (do you know why, it's because it's entirely incorrect).

Also, I agree that the argument has turned away from the OP, but discussions have a natural progression. This discussion has turned to talking about weight. There is value in correcting people who make incorrect statements, so please let's curtail attitude that people "should" stop talking about it. It's as valuable as any other posts.

--

Now, as per weightlessness...

If you are in our universe, and you have mass, then you have weight.

Now, the following assumes that you're in our universe, and that you are not pure energy (can we agree on that?), so:

You *always* have weight.

Period. Full stop. End of story.

Floating and being in freefall can make you feel like you are not being pulled down to the Earth, but you are, and you still have weight.

You always have weight. It changes depending on how close you are to the Earth (and other objects), but matters not what you are doing at that distance (whether you're under water or on a chair).

Please don't be confused that a bathroom scale measures weight. It measures the force that you and the earth are pushing on each other when you are standing still (which is quite close to your weight). If you bounce up and down a bit on the scale without actually jumping, it will say your weight is fluctuating by a couple hundred pounds. Do you think your weight is actually fluctuating by a couple hundred pounds? Of course not.

If you could design a scale that would measure the net force that you and the ocean are pushing on each other when you're diving, you'd see that it would measure your weight (to umpteen significant figures), even though you were floating.

If you ask every astronaut/cosmonaut/tikonaut that has ever been in space if they have *ever* actully been weightless and not in freefall, they would *all* tell you "no".

Now we may be able to put this to rest. I'd think that if the opposite argument would like to respond, they would please cite a source corroborating their viewpoint.

Craig

PS: Sorry if the tone comes off as snotty. It is a bit. I was frustrated at seeing people like Blackwood speak accurately and have everybody else pooh-pooh his arguments with nonsense.

 

[roakey]

They're not, but often confused.

Roak

 

[roakey]

It's not. A BP/W is an excellent recreational setup!

Roak

 

[TomP]

I realize this thread is now a little stale but I was just reminded of this misconception.

My kit is sufficiently negative so i don't wear any additional weights. I've been told by many people on many occasions that even though it is unnecessary, i should wear additional weight so i'd have something to ditch in the event of an emergency.