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The third derivative is jerk. That's how fast your acceleration is changing. A "jerk" of 1G per second would be expressed as 32 feet per second cubed.



I was totally confused by the previous explaination, but this makes sense.

I'd never heard the term "jerk" used in that context before. After a search, it appears as if some people do, if fact use it that way. Interesting.

I guess you really do learn something new every day.
quade -
The World's Most Boring Skydiver

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So Jerk = d3x/dt3

No-one seems to have told Halliday and Resnick



Yeh, jerk is a real thing... I thought you were kidding in your first post when you said "learn something new every day." Most textbooks don't mention it, because it gets hard to think that far down the line past position. I've confused myself trying to think about the FOURTH derivative of position. :S I KNOW it exists, but I'm having trouble trying to figure out how it must FEEL. :S

jerk <--- the first Google result I found...

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I was totally confused by the previous explaination, but this makes sense.



Not surprising, I think BV is more eloquent than me. :)
Hehe, something I just found. I apologize for totally hijacking this thread. [:/]

> We are currently investigating movements that go beyond minimum jerk,
> specifically to employ the second time derivative of jerk. I have been
> told that the (possibly) official term for the first time derivative of
> jerk is "snap". This suggests that the second and third time derivatives
> of jerk (i.e. the 5th and 6th derivatives of position) would naturally be
> referred to as "crackle" and "pop". Does anyone know of any references
> that might support such a contention?
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A big, and repeated, problem on forums of any kind is that when the discussion gets technical there is a wide range of perspectives. As one of the two physics professors here, I can say that

1) I have encountered jerk, da/dt, before

2) This is a complicated issue, moreso than anyone has said thus far.

3) It is more an issue of acceleration than jerk, IMHO. Forces are related to accelerations, not to jerks. So saith Newton, and unless we are going to get relativistic, Einstein isn't disagreeing.

4) It isn't possible to give non-physics types a year of physics in a forum. So I often just give up and let the thread deteriorate rather than take on the impossible task. This is also true when I am the one on the forum who doesn't know much, and the real experts can't write a book explaining it to me. It isn't snooty-ness, just practical reality.

ps - I hope nobody takes this posting as hostile, agressive, or anything else unkind.

-- Jeff
My Skydiving History

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> It is more an issue of acceleration than jerk, IMHO. Forces are
> related to accelerations, not to jerks.

In a rigid system, that's true. In a flexible system full of damping, spring constants and suspended weights, jerk is very much an issue - and a human/harness combination is a very flexible system. A man standing in an elevator who is, over the course of 20 seconds, subjected to a gradually increasing but very high lateral acceleration (say, 10G's) will likely survive the experience; a man who experiences the same thing with a very high jerk (say, 10G's within 20 milliseconds, then a constant 10G's) will probably not.

Even with mostly rigid systems it can be an issue. I do vibe testing of electronics assemblies all the time, and often it is the (cyclic) change in acceleration, not the absolute acceleration, that causes resonances and failures in electronic devices - and those are pretty rigid systems with just a little flexibility.

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A big, and repeated, problem on forums of any kind is that when the discussion gets technical there is a wide range of perspectives. As one of the two physics professors here, I can say that

1) I have encountered jerk, da/dt, before

2) This is a complicated issue, moreso than anyone has said thus far.

3) It is more an issue of acceleration than jerk, IMHO. Forces are related to accelerations, not to jerks. So saith Newton, and unless we are going to get relativistic, Einstein isn't disagreeing.

4) It isn't possible to give non-physics types a year of physics in a forum. So I often just give up and let the thread deteriorate rather than take on the impossible task. This is also true when I am the one on the forum who doesn't know much, and the real experts can't write a book explaining it to me. It isn't snooty-ness, just practical reality.

ps - I hope nobody takes this posting as hostile, agressive, or anything else unkind.



Just looked it up in a technical dictionary, and found 2 definitions, one being da/dt, the other dv/dt (=a). (Actually, there was another definition too, "dork or unpleasant person".)

Physics is neither snooty, hostile, aggressive nor unkind, but sometimes it makes you go "Hmmmm".
...

The only sure way to survive a canopy collision is not to have one.

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4) It isn't possible to give non-physics types a year of physics in a forum. So I often just give up and let the thread deteriorate rather than take on the impossible task. This is also true when I am the one on the forum who doesn't know much, and the real experts can't write a book explaining it to me.



Agreed... I just got out of a lengthy argument on a fark.com forum about physics... the only reason I bothered posting was because two people were having a very heated debate and neither of them had a clue what they were talking about. :S

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In a flexible system full of damping, spring constants and suspended weights, jerk is very much an issue - and a human/harness combination is a very flexible system. A man standing in an elevator who is, over the course of 20 seconds, subjected to a gradually increasing but very high lateral acceleration (say, 10G's) will likely survive the experience; a man who experiences the same thing with a very high jerk (say, 10G's within 20 milliseconds, then a constant 10G's) will probably not.



That was my point exactly Bill! What I was trying to explain with my "bouncing" example... B|

And keep in mind that in your said vibe tests of "rigid" bodies, the reason the jerk matters is the reason the quotes are around the word rigid. We all know that nothing's completely rigid, just the things we consider "solid" (one of your electronics components for example, or even a structural steel beam, to a lesser extent) are much more rigid than the guy hanging from the harness. Not that I'm claiming to have any understanding of failure on an atomic level. B|
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n a rigid system, that's true. In a flexible system full of damping, spring constants and suspended weights, jerk is very much an issue - and a human/harness combination is a very flexible system. [...]
Even with mostly rigid systems it can be an issue. I do vibe testing of electronics assemblies all the time, and often it is the (cyclic) change in acceleration, not the absolute acceleration, that causes resonances and failures in electronic devices - and those are pretty rigid systems with just a little flexibility.


Now, i'm puzled. I thought that when you can't suppress the jerk off, the main effect is the back and forth propagation of a shake along the device. It would have a very significant effect on hard material, but not on soft material (like soft flesh).
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Come
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Dear Physics nerd,
When you deploy a parachute you do not experience acceleration. There are 2 things, snatch force and opening shock. Snatch force is when the deploying canopy is accelerated to the same speed as the jumper. Opening shock is the jumper being decelerated by the canopy and the air mass associated with it. This is measured in G's. What makes G's hurt more or less is time, duration of the loading. If you take 12 G's over 2 /3 sec. it is not too bad. If you take 7/8 G's over 0.2 or 0.3 sec. it will knock your dick in the dirt. There is no "jerk" involved unless its the person under the canopy.:S:S
My idea of a fair fight is clubbing baby seals

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G is a measure of acceleration. 1G = 9.81m/s^2 (10 is often used as an approximation) This is the gravitational pull of the earth. So when people refer to 10G or however many, its 10x9.81.

And from what I have heard, sustained G is worse for the body than quick Gs. Thats how people survived stupidly high Gs when they only have them for an instant.

And when you open your cannopy you DO experience acceleration, only its negative (decceleration). Snatch and shock will all be attributed to acceleration. If it snatches, its because you still want to go fast while the canopy is slowing down, so it pushes you into the harness more, cos the forces are greater (F = ma).

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When you deploy a parachute you do not experience acceleration.



Interesting. So are you still going 120 mph when you land? My parachute changes my speed. That's called acceleration.

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Opening shock is the jumper being decelerated by the canopy and the air mass associated with it.



True. But you just said "When you deploy a parachute you do not experience acceleration."

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There is no "jerk" involved unless its the person under the canopy.



Yes, there is. It's been explained several times in this post that jerk is the change of acceleration with time. When you deploy your parachute you accelerate. Unless you have been accelerating like that since your birth, and continue until you die... there is jerk. Your acceleration goes from zero (stable freefall pre-deployment) to a range of values over your deployment (deceleration, depending on your POV), then back to zero when you are again at equilibrium under canopy. The rate of that change is jerk.
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If this really is going to be a problem for you and the doctor says no, how about getting a rigger to create a drogue system for you - just like on a tandem.

This will severely limit what you can do as a skydiver (ie no FF little RW) but you will be able to SKYDIVE:)
The use of a Drogue chute on a solo would significantly slow your freefall so that your subsequent canopy opening would be far softer. It may not exlude hard openings per say, but any hard opening you did have would be much softer and closer to a normal terminal opening.

A drogue chute system and well packed dosile F111 canopy combined may well reduce any risk your pacemaker company assess to an acceptable level.

RIGGERS - do you see any problems with fitting a drogue chute to a solo jumper?

PHYSICS NERDS - I am right arent I in saying a slower FF would = a lighter G force/jerk force (without creating more argument over which is more important) on opening?

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PHYSICS NERDS - I am right arent I in saying a slower FF would = a lighter G force/jerk force (without creating more argument over which is more important) on opening?



Not necessarily.

You could still get slammer openings that would produce just as much G, just the canopy would have to open in half the time (assuming you're going at 1/2 the speed)

But assuming the canopy takes the same time to open, then you'd get half the G if you're going half the speed.

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> It is more an issue of acceleration than jerk, IMHO. Forces are
> related to accelerations, not to jerks.

In a rigid system, that's true. In a flexible system full of damping, spring constants and suspended weights, jerk is very much an issue - and a human/harness combination is a very flexible system. A man standing in an elevator who is, over the course of 20 seconds, subjected to a gradually increasing but very high lateral acceleration (say, 10G's) will likely survive the experience; a man who experiences the same thing with a very high jerk (say, 10G's within 20 milliseconds, then a constant 10G's) will probably not.

Even with mostly rigid systems it can be an issue. I do vibe testing of electronics assemblies all the time, and often it is the (cyclic) change in acceleration, not the absolute acceleration, that causes resonances and failures in electronic devices - and those are pretty rigid systems with just a little flexibility.



If the acceleration is spatially uniform (say, generated by applying a time varying but spatially uniform field) then no forces will be generated between parts of the body regardless of da/dt. What is really causing the type of problem you describe is that the forces are applied in a non-uniform manner to the non-rigid system - df/dx or div(f). Or am I missing something?
...

The only sure way to survive a canopy collision is not to have one.

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If the acceleration is spatially uniform (say, generated by applying a time varying but spatially uniform field) then no forces will be generated between parts of the body regardless of da/dt. What is really causing the type of problem you describe is that the forces are applied in a non-uniform manner to the non-rigid system - df/dx or div(f). Or am I missing something?



That's kind of a good point, and confuses me. Thinking of Bill's elevator example... it is true that all of the forces the elevator exerts on you are concentrated on say, your feet (your weight is caused by a uniform field and is therefore also uniform). So your hypothesis would be that if they were even distributed throughout your body, then the jerk wouldn't matter? Might be true, but I think the only way to solve that problem would be to have every molecule of your body attached to the elevator, hehe, which is obviously impossible. Then you have to consider sub-atomic deflections as a result of the jerk. :S I think your "df/dx" term (same thing you were trying to explain to me eariler with your "hangman" example) is a valid consideration, but I think is affected by (and vice-versa) the da/dt (jerk) term. Without such a large jerk, the force distribution would not vary so much across the guy's neck. It's all related...

In a deployment situation, you have uniform weight forces (from uniform field) pulling down on all molecules of your body, and localized forces pulling up at your harness attachment points. No matter how large or small the jerk is, you still have only intertial forces pulling down and the same upwards forces on your harness points. So I think df/dx would stay the same (force distribution across body). A larger jerk could give you whiplash though (even with the same force distribution df/dx), simply because your neck is a sort of damper in this system and is stressed dynamically much more severely. This is all so fun/confusing to think about... :| Hmm... so my new opinion is that external df/dx (which would just be the forces drawn in the FBD if you do that kind of thing) doesn't matter much, but internal df/dx (i.e. solid mechanics) is what determines if you snap your neck, and results from jerk.
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>If this really is going to be a problem for you and the doctor says no,
> how about getting a rigger to create a drogue system for you - just
> like on a tandem.

A bag lock would still result in a pretty hard reserve opening shock with such a system, and would increase the likelihood of a mal (more complex system.)

>RIGGERS - do you see any problems with fitting a drogue chute to a solo jumper?

Nope; Strong used to offer a student rig with a drouge for tandem-progression transition purposes.

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>What is really causing the type of problem you describe is that the
>forces are applied in a non-uniform manner to the non-rigid system . . .

Well, actually I think the problem is applying a uniform field to a body which is not uniformly restrained. A common injury in impact, for example, is the aorta separating from the heart, because the two structures (aorta and heart) are not equally attached to the skeleton (the 'rigid' part of the system.) A very short high acceleration pulse may cause the heart and aorta to translate, but not over enough distance to cause them to separate. A longer pulse might.

In the elevator example the issue is onset, or jerk. A rapid onset pulse would cause the person to be rapidly accelerated towards the wall (well, in relation to the elevator at least) and then decelerated very, very rapidly when he hit the wall. A slow onset pulse would cause him to fall relatively gently against the wall and stop there; while on the wall he can better deal with the acceleration.

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In addition to the unequal attachments, you also have to consider the material themselves.

An extreme example would be Silly Putty.

It can withstand quite a bit of stretching if the force is applied slowly, yet if the force is applied rapidly, it fractures.

Ketchup in a bottle is similar.

Do a quick google for "thixotropic".

(Actualy bill, I assume you know this . . . just continueing on for the masses.)
quade -
The World's Most Boring Skydiver

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