# B Acceleration and gravity

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1. 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?

2. 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.

3. 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.

4. 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?

5. 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.

6. Sep 12, 2017

### Jane

Thank you so much, you answered my question quite well.

7. Sep 12, 2017

### jbriggs444

5 million meters per second is way too fast for the slow journey. If you continuously accelerate at one g then you can solve $d=\frac{1}{2}at^2$ for time and discover that it will take you about two and a half hours to reach the moon. Then you can use $v=at$ to calculate that your impact velocity on the moon would be about 90 km/sec.

A more realistic journey would spend half the time accelerating outward and half slowing down on the approach. That would take about three and a half hours total with a peak velocity of a little over 60 km/sec.

Still, 120 km/sec total delta V is well more than we have at our disposal. The actual trips to the moon involved peak velocities that were a small fraction of that. For efficiency, the burns are kept brief, using just enough fuel to make use of Hohmann transfer orbits.

8. Sep 12, 2017

### sophiecentaur

Extreme acceleration seems to be not much use in any circumstances, in fact. Your point about a Lunar mission is well made. If the plan is for a very long journey then the acceleration time (at both ends) is a small fraction of the total journey time so what would be the cost / benefit situation of subjecting passengers to hideous conditions in order to shorten a journey of many years by a few days / hours. And there's the Energy budget to consider.
Of course, if there were some system for Regenerative Braking . . . . . . . . (only joking')

9. Sep 12, 2017

### Jane

When you told about that uniform, I thought about a hypotetical situation. So, a human accelerates with the same 20,408g. It moves her with increasing speed, but the same force affects her body. A hypotetical costume diminishes the force that affects her body. That 20,408 g that would've destroy her otherwise.
Would she still move in space, or will just stand dead on her tracks?

10. Sep 12, 2017

### sophiecentaur

We have been pointing out the lethal consequences to bodily liquids and tissues at that level of acceleration - with or without 'Anti-g' measures. How can you then suggest the traveller would be in a position to move about? She would be DEAD.

11. Sep 12, 2017

### Jane

Okay, got it, sorry.

12. Sep 12, 2017

### Staff: Mentor

The acceleration on the ISS is approximately 0 g. The ISS is not firing its engines like a big jet pack.

Speed, by itself, has no adverse effect.

13. Sep 12, 2017

### Staff: Mentor

All costumes produced to date obey the laws of physics, one of which is F=ma. If the costume reduces net force then acceleration must also reduce (assuming that the wearer refuses to be cut into small pieces with reduced mass). There simply is no way to reduce net force and keep high acceleration.

14. Sep 12, 2017

### jbriggs444

Newton's second law says F=ma. There is no magical pixie dust exception for fancy costumes. Your acceleration is determined by the force on your body.

The best one can do is to try to spread that force out evenly so that the associated stresses are not too extreme (g suits, recumbent position, water bed, liquid breathing, etc). But 20,000 g's is too extreme to mitigate that way.

Edit: Dithered too long and @Dale beat me to it.