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Is it possible to counter-act g-force for a jet-pilot, by him putting on a scuba-diving suit and filling the cockpit with water? On earth we are constantly pulled down, or accelerated with one g. In this situation, if we put the jet-pilot in a pool, he would neither sink nor float.

If we could increase Earth's gravity to say 9 g's, the pilot in the pool would float even more.

Is this correct? Thanks...

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4 Answers 4

If you fill the cockpit with water, the pilot will feel a buoyant force. Humans have about the same density as water, so ignoring the scuba suit, the pilot will feel a buoyant force about equal to his own weight.

The plane's maneuvers don't change this result much. By the equivalence principle, when the plane accelerates, the water in the cockpit and the pilot both get heavier. The buoyancy changes in the same way the pilot's weight does. If he feels weightless before takeoff, he'll continue feeling weightless during maneuvers. If he's supported by buoyancy before takeoff, he'll continue to be supported by buoyancy during maneuvers. (This assumes that the jerk, or time derivative of acceleration, is low enough that the water can remain in hydrostatic equilibrium during the maneuver.)

Although basic buoyancy works the same, there will be a pressure change during accelerations. If the plane is accelerating up at 9g, he'll feel pressure as if he's under water ten times as deep as he really is dense as the water really is. As Bruce pointed out, this is a big pressure gradient, and the pilot will still realize that the force beneath him pushing up is stronger than the force above pushing down. He will essentially feel heavy.

Ignoring compression, changing Earth's gravity won't change whether something sinks or floats. Something more dense than water sinks. Something less dense floats. If we multiply gravity by some constant, an object immersed in water has the net force on it also multiplied by that constant. So if you have something that floats and has a net force on it of $1N$ upward under normal gravity, if you double the gravity, it will experience a net upward force of $2N$ and have double the acceleration (ignoring drag).

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In addition to the buoyant effect, you'd also reduce the effects of acceleration on the pilot by adding a whole bunch of extra mass to the plane (all the water to fill the cockpit), which would cut into its acceleration... –  Chad Orzel Jan 3 '11 at 13:25
I would clarify (as a final point) that the method won't work as a way to counteract the acceleration force, and the pilot will feel g-forces just as if there was no water. –  Malabarba Jan 3 '11 at 13:29
@Chad Good point, thanks! –  Mark Eichenlaub Jan 3 '11 at 13:51
@Bruce I would disagree that he "feels g-forces just as if there was no water." There is a net force on him just as if there were no water, but what that net force feels like is different than if there were no water. –  Mark Eichenlaub Jan 3 '11 at 13:57
I suppose the jerk --and also a bit the impracticality of flying in a diver suit in a waterfilled cockpit-- was what necked the engineer who ever proposed it. –  Raskolnikov Jan 3 '11 at 14:54

Your logic is mostly precise, but it won't counteract g-forces.

(I'll assume the jet flyes straight up, for simplicity, but it doesn't make any difference.) Like you said, if the pilot was originally in equilibrium inside the water and we accelerate the jet at 9g, the buoyancy would get 10 times as strong (9+1). However, the pilot will still feel a force accelerating him. The only difference is that, instead of being pushed by the jet seat, he will be pushed up by the water itself.

So he will still feel the g-force, only a little differently. Instead of the force being concentrated on the area of contact between him and the seat, it will be distributed through the area of contact between him and the water (approximately half of it, to be more precise). But the net force will be the same.

The final effect will be that he'll float inside the water. The water is accelerating up with the jet, and he'll accelerate up as well (because of all the extra buoyancy).
If he were submersed in a heavier fluid to start with, the effect would be the same, except he would float partially above water.
If he were submersed in a lighter fluid to start with, then he would be able to touch his jet seat, and the upward acceleration would come partially from the seat and partially from the liquid.

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@Bruce I agree with your physics, but saying "it won't work" is equivalent to telling someone who wants to go swimming to take the load off that that won't work, either, since the net force on them is the same. –  Mark Eichenlaub Jan 3 '11 at 13:55
@Mark I'll be more specific –  Malabarba Jan 3 '11 at 14:01
@freeside no. Diving depth odes not affect buoyancy the way you're thinking. Visit en.wikipedia.org/wiki/Buoyancy#Forces_and_equilibrium and see that $B=\rho V g$. Note that water depth does not enter this equation. The effect that the pilot feels is the same as multiplying $g$ in that equation by 10. Simply diving deeper would not reproduce that effect. –  Malabarba Jan 3 '11 at 14:37
If the pilot were uniformly the same density as the water he would feel no effect (except changes in the external pressure as the product of his depth times the effective force of gravity changes). Of course real humans have density variation, especially air filled lungs. If we filled the lungs with a liquid of near water density we could greatly reduce this. We do have a fluid that was developed for artificial blood, and a human immersed in it with lungs full of it won't drown. But, I think it is heavier than water. –  Omega Centauri Jan 3 '11 at 15:37
@Omega: No, he would feel the effect. Just look at it from a reference frame in the ground. The pilot is accelerating upwards with the jet, thus he is suffering an (pretty strong) upwards force. If he is suffering that very real force (it's not a gravitational force), then he is feeling it. Since he is only in contact with the water, then this force must have been exerted by the water. –  Malabarba Jan 3 '11 at 17:14

I came upon this discussion while musing about the effects of spinoculation on cells in culture (e.g., see http://www.ncbi.nlm.nih.gov/pubmed/21795326). Forgive me, I'm a virologist and not a physicist, though I did teach 8th grade physical sciences for a while and was often challenged to go far beyond the text and understand things more deeply than the most curious and intelligent student in the class. It seems with respect to the pilot that both Connor and Eichenlaub may be correct in their own way, depending upon what one means by "feeling." We tend to "feel" acceleration via our inner ear which, being encased in a bony homeostatic case, is not subjected to the equilibrating pressures of the surrounding water. On the other hand, the pilot's posterior would feel relatively fine, and he would presumably not suffer blackout from acceleration because (as with a pilot's pressure suit), blood would not be depleted from his brain. So, assuming the pilot was breathing (necessarily pressurized) air during a long acceleration, would he (she) get the bends when acceleration ceased? Sounds like an "ask NASA" question. It all might be relevant to spinoculation, in which cells do survive but change.

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If you suspended a jellyfish in a bucket of water and accelerated the bucket upwards at say 100g, the jellyfish will remain stationary relative to the water. IE the jellyfish will not experience any acceleration, it is weightless as it normally is. All of the additional forces due to acceleration are imparted on the buckets bottom and sides.

So by this rational the human body should be able to withstand very high g forces when submerged in water, as long as all air cavities were filled with fluid of equal density.

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protected by Qmechanic Dec 3 '13 at 22:09

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