# How to define a level of danger of acceleration?

## Recommended Posts

How to define a level of danger of acceleration in gravitational field and without gravitational field?

##### Share on other sites

What dangers are you referring to?

##### Share on other sites

Dangers to human body.

##### Share on other sites

Dangers to human body.

Dangerous is rapid slowing down.

Edited by Sensei
##### Share on other sites

What the human body can withstand depends on a combination of the amount of acceleration, its direction and its duration.

##### Share on other sites

You could look at the G-forces pilots can cope with (in flight suits) before blacking-out.

##### Share on other sites

Most modern, highly maneuverable, military jets are stressed for +9g / -3g.

Pilots tend to go gray around their visual field ( low blood flow to brain ) at less than this. Probably about +7g sustained, before passing out.

I seem to recall several crashes of the F-20 Tigershark due to this.

##### Share on other sites

Dangerous is jerk-rate of change of acceleration.

##### Share on other sites

How to define a level of danger of acceleration in gravitational field and without gravitational field?

Gravitational acceleration should not be a problem unless tidal forces become significant, unless of course something lies in your path...like, say, the Earth...

##### Share on other sites

Dangerous is rapid slowing down.

Turn an armchair then

danger of positive acceleration = danger of negative acceleration

What the human body can withstand depends on a combination of the amount of acceleration, its direction and its duration.

Well. What about math?

##### Share on other sites

Well. What about math?

Generally not as dangerous.

I don't understand the question. What math do you expect with the original question being incredibly vague? The answer depends on the direction and duration. As MigL posted, pilots (wearing g-suits) can withstand 7-9 positive g's but only about 3 negative g's, and that's for a short duration. I doubt anyone has studied long-term additional g effects, since that's hard to simulate. We do have experience with what happens long-term in micro-g environments, since we continue to send astronauts into space.

##### Share on other sites

Well. What about math?

What math do you expect? What we know about the dangers of acceleration is purely from experiment (and accident). Some references here: http://hypertextbook.com/facts/2004/YuriyRafailov.shtml

I'm sure Google will find many more if you search for something like how many gs can human survive

##### Share on other sites

Well. What about math?

Force = mass * acceleration

acceleration = ( new velocity - old velocity ) / time it took

(so when new velocity is equal to old velocity, acceleration is 0)

(remember that v is not scalar, but vector)

and

p = m * v

so

F = ( new momentum - old momentum ) / time it took

(so when new momentum is equal to old momentum, force is 0)

If new momentum is very different from old momentum and/or time is very short, force is very strong.

The most exposed to damage part of our body is brain - when body is rapidly decelerating, brain has still the same velocity, and "flies" inside of skull smashing on bone.

Edited by Sensei
##### Share on other sites

Dangerous is rapid slowing down.

So, if I'm hit by a train and, thereby rapidly accelerated to the sped of the engine, I will be OK because I'm not slowing down, but speeding up.

Or are you looking at it from the driver's point of view. He seem me coming towards him really fast, but suddenly (and messily) stop just in front of him.

It really is all a matter of perspective.

##### Share on other sites

So, if I'm hit by a train and, thereby rapidly accelerated to the sped of the engine, I will be OK because I'm not slowing down, but speeding up.

Collision with train I would split to two stages: acceleration of body, and then deceleration during hitting ground. If ground is solid, not liquid, the most of damage to body will come from this stage.

I don't think so OP talked about such extreme (and not uniform) way to accelerate body as is collision.

Rather more natural one as f.e. acceleration of free fall body, or launching rocket with astronauts.

If I understood OP intention correctly, he is asking what uniform acceleration of human body will cause death (if any).

I would rephrase OP question:

"if astronaut in suit would jump out of spaceship 100+ km above Moon, or other planet without atmosphere, would he be still alive prior hitting Moon/planet surface? Or he would die just because of acceleration of planet during jump..?"

Edited by Sensei
##### Share on other sites

"Collision with train I would split to two stages: acceleration of body, and then deceleration during hitting ground. If ground is solid, not liquid, the most of damage to body will come from this stage."

If, and only if, the ground is as rigid as the train.

That's unlikely.

Also, I'd be dead by then so it wouldn't matter.

"Rather more natural one as ...launching rocket with astronauts."

That's an unorthodox use of the word "natural"

"if astronaut in suit would jump out of spaceship 100+ km above Moon, or other planet without atmosphere, would he be still alive prior hitting Moon/planet surface? Or he would die just because of acceleration of planet during jump..?"

Right, but for the wrong reason.

If the planet was made of marshmallow or something, so he fell into it and was cushioned and it took him 100 km to come to a halt, the average acceleration during his "stop" would be the same as his acceleration during the fall.

He would be fine.

What you are ignoring is the very large value of the acceleration that happens when you stop something suddenly

It's not that he's stopping that causes the damage, it's that he does it over a small distance.

That's why the train driver isn't damaged by the train starting or stopping, but someone hit by a train (and so accelerated to the speed of the train very quickly) is smeared across the engine.

##### Share on other sites

One Soyuz crew had to escape the launcher by firing the emergency rocket that pulls the capsule away. In their optimum position, they survived some 20g unharmed. The limited speed difference must have helped as well.

##### Share on other sites

What math do you expect? What we know about the dangers of acceleration is purely from experiment (and accident). Some references here: http://hypertextbook.com/facts/2004/YuriyRafailov.shtml

I'm sure Google will find many more if you search for something like how many gs can human survive

I think change of momentum/energy per measurement unit is dangerous due to that the change isn't simultaneous to different parts of body.There factors can be defined .

##### Share on other sites

Momentum/energy change isn't simultaneous to different parts of body in start of acceleration.Bones accelerate earlier than blood.

##### Share on other sites

I think change of momentum/energy per measurement unit is dangerous due to that the change isn't simultaneous to different parts of body.There factors can be defined .

Hrm, yes. Most of the examples here involve damage due to different parts of the body accelerating differently from other parts. This is typical of human experience, where we are accelerated via an external force that pushes on the edge of the body, but doesn't directly push on the insides. What about if all parts are accelerated directly, like if you could switch on a uniform gravitational field while the body is in free fall (no other forces pushing on part of the body)?

Assuming Born rigid acceleration, where no part of the body is displaced from another due to acceleration, I suspect that there might be no limit to safe acceleration??? Assuming you're accelerated by some method that has no other effect other than acceleration, you would still be able to detect the acceleration, but I'm not sure what it would feel like if no part of you moves relative to any other. There would be temporal distortions.

And if we're not assuming approximately Born rigid acceleration, you could always cause damage at some point just by having different parts tear away from each other, I think even if you had fairly low acceleration and enough time.

##### Share on other sites

Then different action of different forces to different parts of body creates a danger.

## Create an account or sign in to comment

You need to be a member in order to leave a comment

## Create an account

Sign up for a new account in our community. It's easy!

Register a new account