# B Acceleration and gravity

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1. Sep 11, 2017

### Jane

I'd like to ask about a hypothetical situation. A human, wearing a jetpack, for example, accelerates forward with 200,000 m/s^2 and therefore, experiencing extreme g-force of roughly 20,408 g. If there will be a hypothetical device that will be able to redirect the gravity force (g) to elswhere, so this energy won't kill a human, will his motion continue?

2. Sep 11, 2017

### sophiecentaur

If the human is being accelerated at that rate, Newton's Second Law of Motion tells you that there has to be a force on her from the jet pack (F = Ma). The Force goes with the acceleration and there would have to be a localised force on one side of the body, causing differential forces over her. The only time the differential forces would be zero would be in a Uniform gravitational field of 20408g. Where are you going to get hold of one of those?

3. Sep 11, 2017

### Jane

Maybe I didn't formulate it correctly, sorry.

I'll try to rephrase. A vessel that travels with the same acceleration, and a human inside. If said human will have the same acceleration, independent of extermal acceleration, would it mean that a human will have zero gravity? If so, would he still moving with the vessel, even if he's experiencing zero gravity?

It's hard to explain properly, what I want to know.

4. Sep 11, 2017

### Staff: Mentor

Hi @Jane, is the human essential to your question or could we replace him with something less squishy, like an electron?

5. Sep 11, 2017

### Jane

Sorry, but human is essential, because I wanted to know, if it's theoretically possible to reduce g force, so a human will be able to withstand 20,000 g, without dying.

6. Sep 11, 2017

### Janus

Staff Emeritus
If you are accelerating the Human by a jet pack or if he is in an accelerating space ship driven by a rocket. The acceleration he is undergoing is transferred to him from whatever is providing the accelerating force via whatever part of his body that is in contact with it. For him, it will be as if he is on the surface of a planet with an extremely high gravity. He will be squashed to a pulp. The only way that he could be accelerated without feeling it is if whatever force is accelerating him acts on all points of his body equally. The only thing we know of that does this is gravity, which get s back to sophiecentaur's reference to requiring a uniform gravity field, which is something we are not capable of generating.

There are some ways of lessening the effects of acceleration on the body. You can with stand more g force when prone vs. standing up, for example. Floating in a neutral buoyancy tank could also help, but even this has limits. Neither of these methods can allow you to withstand the type of acceleration you are talking about.

7. Sep 11, 2017

### sophiecentaur

I think you missed my point in my last post. In order to accelerate the human inside the vessel, the vessel will Press against the human - same as the car seat presses against you when it is accelerating. You can't avoid that except when you and the car are in free fall - dropped from a helicopter. Then you both are subject to g and there is no difference so you feel no force from the car on your body. Once you hit the ground, the car is subjected to a massive (negative) acceleration and the bottom of the car will exert a massive force on You.

8. Sep 11, 2017

### jbriggs444

So all we need is a 20,000 g gravitational field that is uniform (to within a few g's) across the diameter of a human body. So that means uniform to within about one part in ten thousand over a distance of about 2 meters.

Gravity goes as inverse square. So if we imagine a gravitational point source, we're talking about 2 meters being one part in twenty-thousand of the distance to the gravitating point. That's 40,000 meters = 40 km distant.

So just conjure up a handy planetoid with a 40 km radius or less and a gravitational field strength of 20,000 g at 40 km. Sounds like a black hole or maybe a big chunk of neutronium. Now all you have to do is to accelerate that planetoid at 20,000 g's without tearing it apart and without affecting the human.

There are some significant engineering challenges in setting this up

9. Sep 11, 2017

### Jane

So, if there was a way to make some sort of anti-gravity field around a human, and then put this field with the human on the spaceship -- no acceleration that ship has will affect said human, right? Because he's in the enti-gravity field? That way, he can travel without any g force affecting him in a bad way.

10. Sep 11, 2017

### jbriggs444

An "anti-gravity field" is the same thing as "magical pixie dust". There is no such thing.

11. Sep 11, 2017

### sophiecentaur

Sorry @Jane but the man's right (according to any form of legit Physics you care to think of).
PS did you notice that PF did it's very best during all those posts to ignore even the possibility that you actually were implying anti-gravity all along?

12. Sep 11, 2017

### Staff: Mentor

So a human undergoing 20000 g acceleration from a jet pack is very dead and crushed into a messy jelly residue, probably with very clearly defined layers of sediment separating out different materials with very slight differences in density.

13. Sep 11, 2017

### sophiecentaur

What you could call a 'raspberry jam job'.

14. Sep 11, 2017

### A.T.

Some study exposed frogs in breathable fluid to very high g's without observed damage. Don't remember the numbers, link should be in some old thread on this here on PF. Also note that it's not just about the acceleration, but also about how fast the acceleration changes (jerk).

15. Sep 11, 2017

### sophiecentaur

It would be necessary to fill all the air spaces in the body (sinuses / middle ear etc), I think. It would be the ultimate G Suit. I guess, if this were part of a bigger exercise - to get a person into very high speed space flight, for instance - it could be worth a lot of preparation of the passenger's body.
But, as a possibly arbitrary 'large number', I think the OP has over done it for required mechanical strength of any vehicle and contents (even the electronics) as well as for the passenger.
But Dale has a point about separation of internal components. You can centrifuge blood with only a modest g force for a minute or two. I am not planning to be a volunteer.

16. Sep 11, 2017

### jbriggs444

For what it's worth, Wikipedia suggests the utility of such a technique is limited.

https://en.wikipedia.org/wiki/Liquid_breathing#Space_travel:

"Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15 to 20 G."

17. Sep 11, 2017

### Staff: Mentor

Yes, that is exactly what I was thinking of. Blood centrifuges are in the range of a few thousand to a few tens of thousands of g. So in the range that the OP is talking about plasma is separated from cells, even when the cells are completely suspended in the fluid. So I don't think that simply filling in the air cavities will prevent squishing.

18. Sep 11, 2017

### A.T.

Here some data for mice:

...when their lungs are emptied from air, the maximum acceleration reaches 3800 Gx for more than 15 minutes without any physical impairment.

19. Sep 11, 2017

### jbriggs444

Nice find. Far larger numbers than I would have imagined.

I do see that this is specifically not "liquid ventilation" but is instead "extracorporeal circulation":

20. Sep 11, 2017

### Staff: Mentor

I agree, but remember scaling laws. Mass scales as the cube of size, but strength only scales as the square. So I wouldn't expect this to scale to human size well

21. Sep 12, 2017

### Jane

Thank you all a lot for answers and efforts to help me, but I still have some questions left. Sorry if it's too much, though.

If it's not possible for a human to withstand so much g, how do they live in the international space station? It travels with around 7 km/s. So, acceleration wouldn't be that high, yes, but it'd still be uncomfortable. And how about Apollo 10 ultimate speed record? 11.08 km/s with people inside. It's speed, not acceleration, but it was high as well. Shouldn't it be extremely uncomfortable for them?
Also do I understand it right, that for tolerating g forces, a human needs to wear a special suit, which does... what? Redirects the blood flow? Or air in the lungs? Or both?

22. Sep 12, 2017

### ModusPwnd

As you said, speed is not acceleration. Constant speed with no acceleration is very much like standing still. When you're in a car you feel it accelerate up to speed, but when cruising you feel still. Ever been on an airplane? When cruising at constant speed you dont feel any g forces. Nothing is pushing on you at constant speed.

23. Sep 12, 2017

### Jane

So, for example, if we have a hypotetical ship that can reach the Moon, and if we don't want to experience g force, we should do the following? (correct me if I'm wrong)
The top speed is 5,000,000 m/s. The distance between Earth and Moon is approximately 384,400,000 meters.
If we want to reach the Moon fast, we just accelerate to 5,000,000 m/s for a second and reach the moon in 76.88 seconds, but the pilot will experience around 500,000 g, which pretty much will kill her.
Now, if we will accelerate gradually, and reach the max speed of 5,000,000 m/s in 500,000 seconds, it will take much more time to reach the Moon, but all the way there the pilot will experience only 1g.

24. Sep 12, 2017

### Jane

Also, you said when a person in the car and it accelerates, they can feel g forces, but when the car cruising, they don't. So, I need to know, when these g forces diminish? As in, a car accelerates to the speed of 100 kph in 5 seconds, and then cruising on the same speed. When will g force diminish?

25. Sep 12, 2017

### A.T.

They experience 0g there. Have you never seen videos from the ISS?

Speed is not acceleration. We don't feel speed.

When the car stops accelerating the horizontal g-force will go to zero. The vertical will still be 1g on level ground.