Acceleration / velocity & safety

[brendang]

Hi all

I'm writing a paper on the relation of safety to acceleration and velocity. The actual topic of the essay is that only acceleration is related to safety, however I disagree, as unless in a vacuum, the forces of friction of external gases and fluids would affect the human body at high speed...

Am I right in this assumption or is it flawed? Can the two's relation to safety only be correctly compared in a vacuum (however impractical) or enclosed space where the only force is gravitational?

BG

 

[ozymandias]

I find it a bit difficult to follow you.
Momentum and kinetic energy are both directly linked to velocity, not acceleration, which measure, e.g., the amount of damage you'd sustain if you were to hit - say - a hard surface.
Care to elaborate more on your statements?

--------
Assaf
http://www.physicallyincorrect.com/"

 

[brendang]

If you hit a hard surface, wouldn't you be accelerating (well, decelerating)?

I need to compare only velocity where acceleration is 0 or the net force is 0. However even when the net force is 0 (as in terminal velocity), wouldn't the human body still sustain injuries? Not on Earth because people can survive terminal velocity, but what if the gravity was 100-fold, and the terminal velocity was much more...wouldn't the friction of air damage say human tissue?

I can't see how I can accurately compare the two when external forces other than gravity are involved...

 

[wysard]

If you hit a hard surface, wouldn't you be accelerating (well, decelerating)?

Yes, you would.

I need to compare only velocity where acceleration is 0 or the net force is 0. However even when the net force is 0 (as in terminal velocity), wouldn't the human body still sustain injuries? Not on Earth because people can survive terminal velocity, but what if the gravity was 100-fold, and the terminal velocity was much more...wouldn't the friction of air damage say human tissue?

Friction, of air, or any other medium for that matter, is a boundary layer problem. While it is apparent to you that the boundary of hitting a wall is clear, perhaps you had not considered that air friction still is a boundary issue. The air is assumed not to be moving, or moving minimally with respect to the falling person. That means that where the two meet, there is a "hitting" of the object with another object. So the damage you see is still the product of acceleration. Specifically the difference between the inside of the body falling and the outside undergoing negative acceleration which causes friction and strain stresses on the falling body. Quite possibly damaging it if the force applied exceeds the tolerances of the body in question.

 

[brendang]

Thank you. That has cleared it up.

 

[brendang]

Actually, it has posed more questions for me now...lol.

Would a human body experience any problems if accelerating in a vacuum? I assume not now, because there is no outside friction, and so there'd be no internal force...?

 

[Integral]

Actually, it has posed more questions for me now...lol.

Would a human body experience any problems if accelerating in a vacuum? I assume not now, because there is no outside friction, and so there'd be no internal force...?

Sure they would have a very hard time breathing.

Your point and what your goal is, is very unclear. If you are talking about safety in every day life why are you dreaming up exotic scenarios? Shouldn't you be condidering the world we live in?