Holding your breath at depth ???

[Oldenboldiver]

So, it's the Rate of O2 consumption that varies rather than the Amount? Is my interpretation of that position correct?

Uncle Pug: I have no problem with sarcasm, it's just that when dealing with strangers or responding in a sensible discussion I don't use it because I don't know how they'll react and it lends nothing to the point being raised. On other diving forums that I frequent it is rarely used for that very reason. However, now that I understand the way that you use it I'll know to ignore it in future.

Thank you all for your thoughts, very interesting.

 

[Uncle Pug]

Oldenboldiver once bubbled...
now that I understand the way that you use it

...there was no sarcasm. Merely an attempt to explain it simply and intuitively for you.

 

[IndigoBlue]

If this student is not a freediver, then why is he/she asking?

Freediving is about breathholding, while scuba is about breathing deeply and slowly.

They are opposites.

Interesting question for a freediving class.

 

[Charlie99]

Oldenboldiver once bubbled...
So, it's the Rate of O2 consumption that varies rather than the Amount? Is my interpretation of that position correct?

rate x time = amount.

Yes, it's the rate that varies, not the amount.

 

[the_dumper]

IndigoBlue once bubbled...
If this student is not a freediver, then why is he/she asking?

Freediving is about breathholding, while scuba is about breathing deeply and slowly.

They are opposites.

Interesting question for a freediving class.

I was explaining to the class why you should never hold your breath when using scuba, and discussing the relationships between pressure, volume and density e.g on a dive to 30m the air breathed would be four times as dense, and what would happen to a flexible container and inverted jar taken to the surface etc.

As I said on my original post:
"Ok, for the purpose of this question we are going to ignore the holding your breathe underwater rule , because otherwise the thread is just going to get full of "you shouldn't be holding your breath underwater anyway" replies."

 

[DA Aquamaster]

It's been a long time since 6th grade science class but:

Force X Distance = Work

Or depending on the situation and how "distance" is defined:

Force X Time = Work

You can use more force over a shorter distance (or time) or less force over longer distance (or time) and achieve the same amount of work.

The thing to remember that in real systems, to do it faster may require even more force than the physical formula would suggest due to additional factors such as drag (which increases as the square of the velocity) or due to the inability for the power source to as efficently generate the force required. In most cases this is excess heat that is wasted and yet still requires a chemical reaction to produce.

Running is less efficient that walking, requires more force that produces more heat so ultimately more energy is required over the shorter period of time to accomplish the same net amount of work. Less efficient = more calories required.

Lifting is a little more complicated. If you lift a bowling ball up to a table slowly with a winch that holds the strain when you are not cranking, the work is essentially the same as if you lifted quickly with a winch with a faster gear ratio. More force is required over the shorter time but the work is the same assuming you are not losing efficiency to gain the needed extra force.

However, if you have to hold the weight of the ball against the accelleration of gravity throughout the lift, it is essentially the same as lifting the ball that much farther - the distance it would have fallen in the same period. So the most efficient non asssited lift would be the fastest rate you can manage with the force available, assuming you are not expending a huge effort to accellerate the ball at multiple G forces.

In terms of O2 and CO2 you could potentially hold your breathe longer under water at depth as there are both more O2 molecules in the fixed volume of your lungs due tot he increased pressure and due to the higher pressure you would still have adequate PO2 for the O2 to pass into your lungs at 02 percentages where this would not be able to occur at the surface (the mechanisim behind shallow water blackout when a free diver ascends to shallower water with little remaining O2 in his lungs.)

The problem is that you can only derive benefit from this by accepting higher CO2 levels as you cannot eliminate the CO2 without breathing. The bad news with this is that elevated CO2 levels also leave you at risk for an O2 hit. So trying to use markedly slower breathing to take advantage of the extra O2 in Nitrox can be a real problem and is one of the reasons that conservative PO2's of 1.2 to 1.6 are the norm (and why 1.6 is not widely regarded as "conservative".)

 

[adder70]

DA Aquamaster once bubbled...
It's been a long time since 6th grade science class but:

Force X Distance = Work

Or depending on the situation and how "distance" is defined:

Force X Time = Work



Running is less efficient that walking, requires more force that produces more heat so ultimately more energy is required over the shorter period of time to accomplish the same net amount of work. Less efficient = more calories required.

However, if you have to hold the weight of the ball against the accelleration of gravity throughout the lift, it is essentially the same as lifting the ball that much farther - the distance it would have fallen in the same period.

Force x Distance = Work - Yes

Force x Time = Work - NO

It is

Power x Time = Work

Thus a higher rate of energy used (Power) in a shorter time is the same overall work, or energy used.

Running is the most efficient method of covering distances at the given speeds. Throughout your life, you cover distance at different speeds and naturally tend to use the most efficient method possible for each speed. For faster speeds, the ligaments and tendons in the legs and feet act as springs to recover some of the energy of falling for each stride and launch a person forward (fallen arches limit/eliminate this effect, causing problems - hence the issues with FA and the military). For covering certain distances with a given amount of food, one might actually be better to move faster to avoid the food being used simply to maintain the base metabolism for an extended period.

Actually, Force x Distance still holds, and the work required will be the same regardless of speed, with the caveat that different efficiencies will affect the outcome. Holding a ball using muscles is quite different than supporting it with machinery. Simply maintaining a bowling ball against gravity will use energy as muscles use energy to stay still. This situation produces no external work, so the efficiency is essentially zero. Thus, the required work is still the same yet the work input is markedly higher with a slow lift using biological equipment for support.

That is a quickly written and surely somewhat disjointed explanation, but might clear up some issues concerning the physics and biology of the aforementioned situations.

 

[Don Burke]

cyklon_300 once bubbled... it shows that an individual (170 lbs) walking (2 mph) uses 102 cal. to go 1 mile.

Running (10 mph) shows an expenditure of ~120 cal. to cover the same distance.

The cal/hr for these two activities is obviously quite different, but for an equivalent distance the caloric requirement is pretty similar.

Surprised me...

That's closer than I expected, but still about a 15% difference.