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FlyHandler t1_j0zycjq wrote

If there were small fluctuations then yes, the fluid would slow the humans movement compared to if they were in an air-filled vessel. If the G-forces were over a prolonged time, the the humans would at some point depending on the size of the tank reach the external wall of the tank and then the liquid wouldn't help any more.

It's like if you were in a car crash and in stead of airbags you had filled the car with water. It wouldn't help much because there is not much water around you. But if you had several meters of water between yourself and a fixed boundary then you the water would slow you down before you hit anything other than water.

In outer space there isn't any turbulence since it is a vacuum, so I can't see much use there though. Also a fluid filled container would be very heavy which is a big problem to launch up into space.


labroid t1_j10xabm wrote

I respectfully disagree. If the tank were upright on the ground, and a person got in, they would float at a certain depth, as their buoyant force (which comes from water pressure which depends on gravity) matches the weight of the person (which is also dependent on gravity). When you accelerate, the person's weight goes up, and the water pressure (and bouyant force) go up exactly the same amount, so they would stay at the same depth. The problem is the water pressure gradient at high Gs would kill them.

Edit: Spelling


Game_Minds t1_j10yw5w wrote

I think this, they would actually float 'up' in the direction of acceleration if there was a difference in density between the person and the water, because as the water experiences differential pressure the body might be able to maintain internal pressures. However, when the lowest pressure at any point in the tank is above a lethal amount it would still kill you. So at mega high accelerations you aren't safe even in a very large but not infinite tank

Edit: I think this is right. The acceleration of hitting the ground only kills you because it causes forces to act across your body unevenly very rapidly, g forces from acceleration work the same way. So if you don't implode from the pressure, you're still experiencing an acceleration difference across different parts of your body inhomogeneously because the actual thing keeping you intact in the water is your skin. You would still die of g forces suspended in a liquid


labroid t1_j114f3e wrote

The person displaces their weight in water - that's the "on Earth" definition. The more general definition is they displace their mass. Their weight is m * a and the water they displace is rho * V * a (rho is density, V is volume, and a is acceleration). Where you float is therefore

m * a = rho * V * a

So you cancel the a on both sides and get

m = rho * V

(for fun note that rho * V is the mass of the water). So the general form of buoyancy is that you float where you displace your mass in water. Note this is completely independent on acceleration (g's). So the person will stay in the same position, unless compressed by the extra pressure of the pool, at which point they will "sink" (move in the direction of the acceleration vector). Of course getting compressed is the "then you die" part of the problem...


Game_Minds t1_j120vu3 wrote

The fluids in a person's body have different densities at a given pressure

It would be like putting you in a centrifuge, some of your parts like your bones would have different buoyancy than others and would experience pressure to separate out

It would be significantly less pressure than other forces acting on you and it might be too small to overcome the forces your body exerts


FlyHandler t1_j163n8y wrote

I think we need to make a small experiment to figure this out. I don't think it matters wether it's in space ore here on Earth.

Take e.g. a small plastic container and fill it with water so it's about the same density as water. Put it into a bucket of water. Rapidly push the bucket sideways to see if the bottle inside moves with the water or if it crashe sagainst the bucket wall.

Any volunteers?

If the container doesn't crash into the wall, the water provides no damping so the human would not benefit anything from being submerged.

If the container does crash into the wall the water would slow the human down but the human would hit the tank wall at some point if the acceleration lasts long enough.


labroid t1_j16938b wrote

Sideways isn't the same thing. Newton's first law says the water will slosh out and the object will try to stay still relative to 'the universe' as the glass moves. If you move the glass up, however, the buoyant forces will increase. If the object if floating, it will continue to do so.

So do your experiment accelerating upwards. Of course it will be a mess when you stop :-)


cheeze_whiz_shampoo OP t1_j0zzy9t wrote

So people would be forced 'back' against the wall of the tank but just at a slower rate compared to an air filled environment?


mfb- t1_j101hsw wrote

You can use a liquid that matches the average human density. Water is pretty close already.

Where would you need a very large acceleration? 1 g is no problem, 2 g is likely acceptable for a long time, 3-5 g is okay for the time needed to reach Earth orbit.


FlyHandler t1_j109tbu wrote

Yup, basically. And when they are pressed against the wall then the person will be subject to the same G force as the tanks movement.


Jagid3 t1_j112cnj wrote

But the force is not relevant. Unless their body shatters at some point, they are virtually the same density as the rest of the water.

By your logic, no part of the spacecraft could survive any part of the journey, because it's touching itself and the engine.