# I Inertia vs Fluid Dynamics

#### XDreamdwellerX

I've searched online over multiple sites and can find no answer to this. I know the basics on acceleration, mass and few others. To try and paint the right picture, take a person and put him in a hollow object--a tube shape--and accelerate them. They are subject to inertia. Now take that person, and while inside the hollow tube, put him inside another container and fill it completely with water. Then accelerate the tube again while he's submerged in the container (and just to be clear, there is absolutely no air inside the container with the man, just him and water).

So my question is this: Will that person feel any inertia from the acceleration? Based on what I know, which I know isn't much, I would think he/she would be unaffected in that environment.

#### LURCH

Can’t give you a direct answer, it let’s examine the scenario a step at a time, and see what we get. Not sure I get the exact setup you’re describing, so let me know if I seem like I’m coming out of left field.

As the tube accelerates, the container within it accelerates, right? And as the container accelerates, so does the person inside it. But the useful question is, “why?” Can you figure out why the person accelerates, I mean the direct mechanism that makes the person accelerate when the container does? If so, that might help reveal the answer you are seeking.

#### XDreamdwellerX

Can’t give you a direct answer, it let’s examine the scenario a step at a time, and see what we get. Not sure I get the exact setup you’re describing, so let me know if I seem like I’m coming out of left field.

As the tube accelerates, the container within it accelerates, right? And as the container accelerates, so does the person inside it. But the useful question is, “why?” Can you figure out why the person accelerates, I mean the direct mechanism that makes the person accelerate when the container does? If so, that might help reveal the answer you are seeking.
I see why I had trouble with this question on Google :) The whole basis for my question is, believe it or not, for space travel. So to make it better...suppose you are on a spaceship that is completely capable of accelerating from 0 to say, something crazy like, 10,000 mph in a second. Any human would be crushed by the inertia exerted on their bodies. However, if that person(s) on the craft, is in a completely liquid environment when the ship accelerates, they should be able to survive that kind of acceleration....and my guess for this is because the outside force is not able to "move" the water to any other area within the container.

For anyone else who reads this, yes yes, I know it's a movie. However, the movie Mission to Mars, I think makes my point. (SPOILER AHEAD) in case you haven't seen the movie. At the end Gary Sinise, is in a craft about to be launched from Mars into space (at speeds way beyond our capabilities). But before this happens, he is placed into a tube and is completely filled with some kind of water. I say "some kind of water" because in the movie, he's able to breathe it. But forgetting the breathing part of it, I found the reason the movie did this is because the human body is supposedly unaffected by the crushing inertia from accelerating so fast.

Does any of what I've described sound correct?

#### PeroK

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I see why I had trouble with this question on Google :) The whole basis for my question is, believe it or not, for space travel. So to make it better...suppose you are on a spaceship that is completely capable of accelerating from 0 to say, something crazy like, 10,000 mph in a second. Any human would be crushed by the inertia exerted on their bodies. However, if that person(s) on the craft, is in a completely liquid environment when the ship accelerates, they should be able to survive that kind of acceleration....and my guess for this is because the outside force is not able to "move" the water to any other area within the container.

For anyone else who reads this, yes yes, I know it's a movie. However, the movie Mission to Mars, I think makes my point. (SPOILER AHEAD) in case you haven't seen the movie. At the end Gary Sinise, is in a craft about to be launched from Mars into space (at speeds way beyond our capabilities). But before this happens, he is placed into a tube and is completely filled with some kind of water. I say "some kind of water" because in the movie, he's able to breathe it. But forgetting the breathing part of it, I found the reason the movie did this is because the human body is supposedly unaffected by the crushing inertia from accelerating so fast.

Does any of what I've described sound correct?
Except in the movies, proper acceleration is proper acceleration. It doesn't matter how you are accelerated you will feel the required force.

#### Grinkle

Gold Member
Does any of what I've described sound correct?
Not sure what you are picturing. If a persons lungs are filled with an in-compressible fluid, then I speculate the chest cavity will be relatively more crush proof than if it is filled with a compressible fluid like air. That doesn't mean a person won't feel acceleration - and that acceleration will feel just like more and more gravity, whether one is inside a water tank or not.

Give thought to post #2 by @LURCH - that is the key to developing the right instincts for your question.

#### jbriggs444

I've searched online over multiple sites and can find no answer to this. I know the basics on acceleration, mass and few others. To try and paint the right picture, take a person and put him in a hollow object--a tube shape--and accelerate them. They are subject to inertia. Now take that person, and while inside the hollow tube, put him inside another container and fill it completely with water. Then accelerate the tube again while he's submerged in the container (and just to be clear, there is absolutely no air inside the container with the man, just him and water).
The setup seems needlessly complicated. The tube contributes nothing to the problem. Ignore it. There is a fluid-filled container and a man inside. The question is whether the man feels acceleration when the fluid-filled container is accelerated.

Alternately, the question is whether a neutrally buoyant body feels stress from the fluid it is floating in.

If a fluid-filled container is accelerated, a pressure gradient will immediately form. The fluid and every similarly dense object within will accelerate at the same rate as the container. This is buoyancy at work. One can see the effect experimentally with air balloons and helium balloons in an automobile.

A large part of the body's feeling of acceleration comes from the muscles which are used to retain a particular posture in the face of that acceleration. If you hold your arms outstretched, it takes effort to do so in the face of gravity. You can feel that effort. If you jump into the pool and float, neutrally buoyant, you can hold your arms outstretched without effort. There is nothing to feel.

If we idealize the situation to a uniformly dense and incompressible man floating in an equally dense incompressible fluid then no, there would be nothing to feel.

However, if we keep things realistic... At 10,000 mph per second (about 500 g's) you would likely have a problem with your high-density bones pushing through your flimsy low-density flesh. Yes, there would be something to feel.

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#### Grinkle

Gold Member
If a fluid-filled container is accelerated, a pressure gradient will immediately form.
Acceleration creates a pressure gradient that is equivalent to gravitational tidal force, is that right? That is how I am thinking of this situation.

If we idealize the situation to a uniformly dense and incompressible man floating in an equally dense incompressible fluid then no, there would be nothing to feel.
If a tube of water is lowered into a gravity well, the water won't compress - as long as the material comprising the tube walls does not break, a tube of water can withstand a whole lot of tidal force without showing any spaghettification strain. Filling a human's lungs with fluid helps - but just immersing a human in a tube of water if their body cavities are still filled with air - does that help the body withstand acceleration? I don't think so, but maybe I am missing something. I think you are saying the same thing - to help, you have to make the human the same density as the fluid.

#### A.T.

It doesn't matter how you are accelerated you will feel the required force.
You don't feel forces directly. You sense the deformations in your body caused by non-uniformly applied forces, while more uniformly forces cause less deformation. So your sensation and potential damage to your body depend very much on how you are accelerated.

#### XDreamdwellerX

Ah ok. So ultimately I'll try and end with this:

If anyone has ever seen the movie Event Horizon, you'll know why I'm asking this. In the movie, this theory is used. In order for the crew to survive the acceleration forces (they say approx. 30g's) they place them in capsules filled with water and a hose to breathe.

I know it's a movie. I'm just curious if this is something that could be useful in some way to us.

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