[Physics] High speed does not kill. Does acceleration do it ? or jerk

accelerationjerknewtonian-mechanicsreference framesvelocity

In a recent question the OP asked why high speed will not kill us.
The accepted answer, highly upvoted, stated very first that

Speed doesn't kill us, but acceleration does.

The second answer (also well upvoted) concurs:

The danger comes from acceleration, not velocity.

Is that true?

Two comments by hdhondt and Adam D. Ruppe somewhat debunk it. but they
are only comments. They could not be answers, as the question
"Why doesn't the speed of Earth (moving through space) kill us?" was not
about acceleration.
I wrote this question, and my answer to it (below), before I had read these
comments. Like many users, I do skip a lot of comments. When I finally
saw these comments already made, I wondered whether I should persist
with this question. The point seems important enough to justify it, considering the high popularity of this topic.

Best Answer

Acceleration does not kill us any more than speed. If your head and feet do not move at the same velocity long enough, whatever the cause, you are in trouble. Velocity does not kill us when the whole body has the same velocity.

Similarly, I doubt acceleration kills us when all parts of the body accelerate, but without having to transmit forces. It is said in a comment:

It's not the fall that kills you; it's the sudden stop at the end.

The sudden stop kills you because the deceleration (negative acceleration) that stops you is actually caused by a force transmitted through your body which cannot withstand it. The acceleration throughout the fall, no matter how strong, which applies uniformly to the whole body will not hurt it: you are in free fall.

If the same acceleration were produced by the pull of an engine attached to your feets and pulling your whole body (even without friction), rather than gravity applied uniformly to every atom of your body, your body could well be torn to pieces.

I am no expert on jerk, but I somehow doubt that it is any more danger, despite contrary statements in this accepted answer and this comment

The human body uses bones and muscles to maintain its integrity while transmitting forces. The problem of jerk is that it changes the values of forces, thus requiring muscles to adapt constantly.

But free fall satellite motion does have jerk, since the direction of gravity is constanly changing, and its magnitude depends on distance. This is generally true of non uniform gravity field.

I think, a good way of understanding what can hurt us is to model the human body as two masses, head and feet, joined with a spring. If the distance between the masses changes by more than, say, 5%, the human model is considered dead. Now, if you add a strong structure, some kind of G-suit, that forcibly preserve the distance between head and feet, thus carrying all forces that need to be transmitted, then the human model is pretty safe.

Note that submitting the head and feet to different acceleration can have undesirable effects if the difference is important. But if the body is strong enough, it can sustain small differences which it compensate with internal cohesion forces. So one might say that speed can be more dangerous than acceleration, when it is an issue of uniformity across the body.

To place these issues on the level of personal experience: we do not feel speed, but we do not feel acceleration either, or jerk. What we do experience is forces propagating through our body, when our body accelerate because it is submitted to forces applied only to some parts of it, rather than uniformly. We experience the tension of the muscles that preserve our body structure against these forces. And we perceive jerk as a need to adapt muscle tension.