Shift of CoG and CoB - different amplitude

[Rogerdd]

Empirically when we bend the knees at 90 degrees (modified frog kick position) and extend our arms in front, it tends to correct the upright position head up/legs down to something more horizontal. And I experience it myself, no doubt about that.

Theoritically, when we shorten the effective length of our legs and lengthen that of our arms, we shift the center of gravity towards the head. And that is often just the explanation given as to why we can correct the tilt (upward trim=COB closer to the head than COG; no more tilt=no more moment=COG and COB coincide).

However when we shift the COG this way by moving our legs/arms, it should also move the COB, and in the same direction. So the only possible explanation for the more horizontal position is that COG moves further towards the head than the COB does, but that part is not obvious to me.

The COB is the centroid of the polygon delimited by our body, which includes gear/tank/etc.
Given our ‘body’ doesn’t have a uniform density, I understand the impact of moving legs/arm will make the COB and COG move with different amplitude.

The question is why COG moves further right (head is towards the right) than the COB does, and not the opposite?

 

[Rogerdd]

No physicist here

 

[inquis]

The average density of legs/arms is greater than the density of water.

 

[happy-diver]

Yeah fighting your suit and holding your arms out front hands clasped unlike Superman sounds like a great idea
Have I told you about the divers that do not think, water is difficultly torturous enough to push their fins against

These divers actually decide to wear heavy rubber fins for some fantastically unfathomably unmanagable reason

 

[Duke Dive Medicine]

Empirically when we bend the knees at 90 degrees (modified frog kick position) and extend our arms in front, it tends to correct the upright position head up/legs down to something more horizontal. And I experience it myself, no doubt about that.

Theoritically, when we shorten the effective length of our legs and lengthen that of our arms, we shift the center of gravity towards the head. And that is often just the explanation given as to why we can correct the tilt (upward trim=COB closer to the head than COG; no more tilt=no more moment=COG and COB coincide).

However when we shift the COG this way by moving our legs/arms, it should also move the COB, and in the same direction. So the only possible explanation for the more horizontal position is that COG moves further towards the head than the COB does, but that part is not obvious to me.

The COB is the centroid of the polygon delimited by our body, which includes gear/tank/etc.
Given our ‘body’ doesn’t have a uniform density, I understand the impact of moving legs/arm will make the COB and COG move with different amplitude.

The question is why COG moves further right (head is towards the right) than the COB does, and not the opposite?

No physicist here either, but from an old salvage diver's perspective, the center of buoyancy is based on displacement of water, which is dependent on volume of a particular area of the body rather than mass. If there's a difference in the magnitude of the shifts. the explanation might lie there.

Best regards,
DDM

 

[crofrog]

However when we shift the COG this way by moving our legs/arms, it should also move the COB, and in the same direction. So the only possible explanation for the more horizontal position is that COG moves further towards the head than the COB does, but that part is not obvious to me.

Generally speaking, the COB moves opposite as you flatten out. The gas in the wing migrates from near your head torwards your butt. With a drysuit you have more gas moving around, eg, down to your feet, but you can balance it put by keeping some around your hands or shoulders.