The111

Yeh, my comment about having to analyze one instant in time was kind of a half-joke... because if you consider acceleration as a function of time (which as you pointed out would be necessary), you are admitting that jerk matters. But I see where you're coming from when you say that it seems unnecessary to name jerk. You're right that the whole situation could be analyzed without explicitly naming jerk, but it would be present in the variation of a(t), and I think that an in-depth anaylsis of situations where flexible bodies are subjected to low and high da/dt at a single constraint point, the results would show what my intuition feels... that the high da/dt situation results in higher stresses in the weak points of the flexible body. That, to me, is why jerk is worthy of a name. Also, because it is something that you can physically "feel" pretty well. Admittedly there are 4th, 5th, and 6th time derivatives of position that have been jokingly called snap, crackle, and pop... this is obviously ridiculous because it is very hard (for me at least) to physically imagine the "real-life" results of one of these things. But jerk is very real to me... I get motion sick in cars when someone else is driving (depending on how good or bad they drive)... and it is not the acceleration at all which makes me nauseous, but the jerk (which is very well-named, since it causes the car to visibly "jerk" around!). I, on the other hand, drive like a jerk... but I think that's from your other definition. One last thing, I think I will rescind my previous statement that there is a direct relationship between jerk and stress. As I stated above, the high da/dt situation results in higher stresses in the weak points of the flexible body, i.e. jerk can result in higher stress. But this is really a result of the complex dynamics across the body and its different particles reacting to each other's motions. In a direct, single-particle situation, jerk will have no affect on force applied to the system, and therefore I can't really say there is a relationship stress=f(jerk). Though I do still think jerk affects stress, it's more of a thing that happens unique to each body's construction (the stresses occurring at different locations than the jerk is applied) and there's not some missing law of physics that considers jerk. You are right about that. :) Hopefully this paragraph made sense... I was thinking about the seatbelt/whiplash example for most of the things I tried to explain... I guess a simple analog to my mistake (when I said stress=f(jerk)) would be saying (incorrectly) that stress=f(velocity), because it was experimentally observed that the faster you drive a car into a wall, the higher stresses you create in the car's structure. While the observation is very true, it doesn't imply a relationship between the two. Velocity on its own creates no stress, it's the specifics of this situation which are allowing stress to be generated, and all of those specifics can be described by a Newtonian system of DiffEQ's. :) www.WingsuitPhotos.com

I'm very edit-happy in case you hadn't noticed. I edited my post about 298 times and was hoping I would finish before you read it. Make sure you read the last part. I'll reply more later, I have to go to bed. I enjoy this. :) www.WingsuitPhotos.com

Agreed. Jerk has nothing to do (in the sense that it does not affect them) with a material's properties, including its sensitivity to strain rate. However, I stand by my position that jerk is directly related to strain rate. So a material which, by definition, is strain-rate sensitive, I would also consider "jerk-sensitive". Honestly, a "jerk-inclusive" mathematical analysis of a real-life situation is probably slightly over my head at the moment. But one reason I've always loved physics is because it's so intuitive (parts of it, at least). I'll use my simple car driving example again. In a car which is being decelerated at a very large magnitude, my torso will be pressed forward against the seatbelt and my head may even be hanging forward with my neck bent, if the "G-forces" are too strong for my neck to hold my head upright. But as long as the deceleration is applied smoothly and then held constant, I don't think my neck would break. But a very rapid onset of deceleration (jerk), while my torso was constrained by the seatbelt, would give me whiplash, or if severe enough, break my neck. You are right that the fundamental rules of force-acceleration dependence (Newton) are not altered by jerk. The surface force exerted on your torso by the seatbelt is determined ONLY by the deceleration and doesn't care about the jerk. And if your body was truly rigid (which nothing is, especially not a body), the whole system would not care about jerk. But intuitively I can see a less than rigid body getting whiplash as I described above. My guess is that a detailed analysis of the situation would need to describe the time-domain motion of every single part of the body. The overall difference in translational response (due to inertia) of the head and torso would be such that it would generate certain forces in the neck, and these forces would be higher in the high-jerk case, to give you whiplash. That's what I suspect... as much as I love discussing this I'm not gonna even attempt that analysis right now... If you want to analyze the situation without considering jerk, and it is a situation where acceleration is dynamically changing, then you will only be able to analyze one moment of the entire situation at a time (a=constant), and I don't think that would be sufficient to see the overall picture. Keep in mind it's not actually the force of the seatbelt on the torso that's causing the guy's neck to break (that seatbelt force isn't dependent on jerk)... it's the tensile force in his neck that his own head and torso are respectively exerting on each other... and this force comes from the dynamic response of each member to the jerking and deceleration of the car. This dynamic response will be different in a situation with or without jerk. I think that's the center point of this whole debate. The actual simple forces transmitted from car to seatbelt to human are independent of jerk, as you and Newton would both agree. :) But the flexible body would respond inertially to its "one-point constraint"... and it would respond different based on different jerks (a phenomenon of motion, which would help describe the subsequent motion of the not-entirely-constrained body). These different motions of the different parts of the body would generate different internal impulse forces from momentum. Just like billvon pointed out with the heart and aorta separating. And since it is impossible to constrain any system entirely, I think jerk will always come into play in determining the interal forces which build up in the system. But fundamentally, you are right... the jerk does not affect the forces transmitted to the constraint points between the "force giver" and the "force receiver". But if the force receiver is flexible and only connected to the giver at one point, the internal forces which arise will be dependent on the subsequent motion of all particles in the system, and inherently, dependent on jerk. www.WingsuitPhotos.com

Maybe you should find better bands. If you only get exposed to bands through the radio (and plan on buying CD's of these bands), you are more often than not setting yourself up with a one-hit wonder. There are a million good bands out there capable of making solid albums, and ironically, most of them aren't associated with the RIAA (or on the radio). For starters, try the new CD by The Mars Volta if you're ready for the most groundbreaking rock and roll since Pink Floyd. I'm 100% serious. These guys are no joke. And you can easily find the whole thing for download on Soulseek, which kills Kazaa, although it's been going through some problems lately... I hope it doesn't go down. www.WingsuitPhotos.com

Hmm, I read the page, when they defined the acronym LQTS they were nice enough to leave QT embedded as an acronym still. WTF is QT? www.WingsuitPhotos.com

Actually, I didn't refer to a relationship between stress and strain rate (I am aware of the relationship). I tried to explain that force rate (jerk) is related to strain rate, and therefore would affect stress... as I said in my first post stating that jerk (not just G force) affects the stress the jumper feels. Even with your "silly putty" type materials where there is a relationship between stress and strain rate, there is still a relationship between stress and strain (like any solid). And even with stress in metals (which as you pointed at is generally looked at as a function of strain), strain rate is also a factor, just not nearly as large of a factor as in more flexible materials. With any material, both of your relationships are true. (1) stress = f(strain) (2) stress = f(strain rate) Both are true! (for anything) However in most cases it may be more pertinent to focus on one relationship. The reason you see certain relationships focused on in texts is because of the normal loading conditions of certain materials, and the relative importance of strain and strain rate for those materials. For instance, metal structures are normally designed to hold constant (or slowly varying, i.e. low-jerk) strains, and strain rate doesn't have too much of an effect on these types of solids, so you generally see equations relating stress to strain for rigid materials. Flexible structures, on the other hand, are normally designed to hold time-variant (quickly varying, i.e. high jerk) strains, which also have a more pronounced effect on these types of materials (as opposed to rigid ones), so you might see equations in a book relating stress to strain rate for flexible materials. Now what (1) and (2) above really say, are what I said a very long time ago, way back when we were still talking about skydiving. (1) The ability of a hard opening to hurt someone is dependent on the acceleration involved (G force). (2) The ability of a hard opening to hurt someone is dependent on the jerk involved (G force rate). Both are true! The ability it has to hurt you, is how much stress it generates in the vital areas of your body. Both strain and strain rate affect stress produced in a material, and both force (acceleration) and force rate (jerk) affect stress produced in a jumper on opening. www.WingsuitPhotos.com

Yeh, I wondered if you were thinking about the sign. www.WingsuitPhotos.com

Actually, strain rate is pretty damn analagous to jerk, I think. Strain is directly related to stress, which is related to force, which is related to acceleration. Rate of change of acceleration is jerk (as we've discussed), and I think rate of change of strain in a solid would be totally affected by the magnitude of the jerk (rate of change of acceleration). Succinctly, the more quickly the acceleration (and force) changes, the more quickly the strain changes. The importance of jerk has to do with the material (as billvon pointed out earlier, electronic components or more "solid" objects), and the importance of strain rate also has to do with the material (as quade pointed out with silly putty and the human body). www.WingsuitPhotos.com

Well, then are you saying that since I am heterosexual and choose not to have children I shouldn't be able to get married. What about all the heterosexuals that CANNOT have children do to medical reason or other physical disabilty. Aren't we taught to just love on another, not just the straight people. Judy I did not say that anyone should/shouldn't be able to get married. I simply pointed out the fundamental difference between gay and straight couples. There are some straight couples who cannot procreate on their own, but there are no gay ones who can. I haven't expressed love or hate toward anyone in this thread. www.WingsuitPhotos.com

Are we talking about advances in technology? I was thinking about that too... However it is my belief/opinion that we will be cheating nature when we get to this point. Call it religious (though I'm not) or whatever you want, I'm not even sure I can explain why, but that's the way I feel. Maybe because I think it's neat for sex to be an act of love which creates the new life and ties him to the lovers, rather than have him be a test tube baby. But then again, maybe I'm just a homophobe, because on the opposite end of the spectrum, I eagerly anticipate the day when I can replace any body part I want and be immortal. Hehe, I really think this will happen one day but probably not in my lifetime. And I'm sure a lot of people would call my eagerness to embrace this "cheating nature", and it probably is, but I can't help it, it sounds neat. Though I spend a lot of time thinking about the ramifications it would have on life. Would anything be as precious if we didn't age and always had second chances? Would even SKYDIVING be cool? The quote in my sig wouldn't even apply if we could always bionically renew ourselves... www.WingsuitPhotos.com

Actually, I stated that my opinion is that our government has way too much involvement in our lives and no couples should receive benefits simply for being a couple. I did, however, throw out a possible idea as to why the social/legal/societal "benefits" which are already offered to heterosexual couples are not offered to homosexual couples. As far as rights and benefits... All citizens of our country [should] have the same rights. That's an opinion. A heterosexual couple has one benefit that a homosexual couple doesn't. They can make a baby. That's a fact. Unless you want to argue whether or not making babies is a benefit or detriment. I'm not quite sure where you're going with the last part of your post. I never advocated less benefits for homosexuals (although by nature they are already short-changed). I actually advocate less "benefits" for everyone (social security, etc). That would mean less taxation. Which would mean more jumping. I think we can all agree that more jumping is a good thing. I hope so. www.WingsuitPhotos.com

Obviously, that's not a great way to start your life, either of those things would be incredibly insensitive to say to a child, but they're both true from an objective point of view. 1. My parents adopted my brother because they could have no more of their own, and weren't looking forward to a third miscarriage. 2. My brother was made available because his mom fucked up in having him (she wasn't ready). It ain't pretty but it's the truth. He's loved though and he loves his home and understands his background. www.WingsuitPhotos.com

Exactly. In short, a gay couple has no way to prevent human extinction. To extend the lifespan of the human race. I apologize if this comment offends anyone, but that's a very basic analysis of the situation, and how a homosexual couple differs from a heterosexual one. I think any two consenting adults should be able to do whatever they want... I think the idea of a legally mandated marriage for any couple is kind of dumb in the first place, and I've already said that if our country was truly capitalistic then NO couples would receive any benefits simply because they are married. The government has WAY too much of a role in our lives. The government’s only job is to fulfill its proper and legitimate functions - protecting individual rights and maximizing individual freedom. That last line was plagiarized from an Objectivist webpage, but it's how I feel. The government should not have a hand in our pockets in a free market economy. www.WingsuitPhotos.com

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. 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. www.WingsuitPhotos.com

I think social security, pension, etc is bullshit in a "capitalistic" country anyway... but that's WAY off-topic for this post. www.WingsuitPhotos.com

I apologize if I offended you, but I made my point from a purely logical stance. I'm sure there are many emotions involved on many different sides in an adoption (I have an adopted brother who I love very much) but from a logical POV, this is my opinion: Any couple that biologically produces a child which they are unable to physically, emotionally, or financially care for, has fucked up (to put it very bluntly, maybe offensively). Unfortunately, their child can suffer the results of their fuckup. Fortunately, adoption saves some of these children and defers them to a better future. Birth does not happen by accident (yes, I am aware of rape, but I'm making a generalization which covers most cases). What is offensive to me, are the vast amount of couples which irresponsibly give birth to children they can't care for. That is a HUGE mistake. I'll say they've fucked up 100 times over, no matter how much it offends them. Adoption is a great thing, and I don't think my views on that can be considered offensive (though my refusal to sugar-coat the reason for adoption may be offensive to some). As I said already, I love my adopted brother, and I will say without hesitation that our family has provided him a better life than his mother (who fucked up) could have. www.WingsuitPhotos.com

I would guess a slower freefall with a staged deployment would yield consistently softer openings, but that's just my guess... www.WingsuitPhotos.com

Good point. But they're still not making the contribution of the child. And yes I understand they can make the contribution of raising the child in a safe, healthy home, but they can't make the biological contribution of the birth. And why am I focusing on the biological necessity of birth? While a gay couple could, in theory, raise a happy healthy child that they adopted from another family who fucked up and couldn't care for that child, they're depending on someone else's fuckup to get that child. A well functioning society (which ours obviously isn't) should not depend on fuckups to promote its function. That's a tragedy, truly. But it's honestly irrelevant. Gay couples may be able to save children from fucked up homes by giving them a better one, but their ability to raise a child depends on someone else's fuckup. Like I said above, a healthy society should not rely on fuckups to function. Without these fuckups, the gay marriages would not be able raise children and create a "family tree." www.WingsuitPhotos.com

Ever heard of adoption? I will elaborate. A couple in a "gay marriage" could never create and maintain a family across generations without depending on heterosexual couples to create babies for them. www.WingsuitPhotos.com

damn! www.WingsuitPhotos.com

I have a feeling I will get flamed for this, but... I'm not trying to turn this argument to the morality of homosexuality, or to bring up the "it's wrong because it's not procreation and therefore not natural" argument which homosexuals hate so much (and I can understand why). But... perhaps the reason married heterosexual couples receive "societal benefits" (legal, economic, etc) is because they generally benefit society by introducing a new human life (overpopulation arguments aside). Granted, I said generally... not all heterosexual couples will produce a child, but the general "American picture" of heterosexual families is mommy, daddy, kids. These kids, besides benefiting society (assuming they are raised half-intelligently) by being those who will carry on our culture and country, also are a burden to their parents. So perhaps the benefits the parents receive are both a reward for their contribution, and an assistance to their burden. A homosexual couple has no way of procreating (again, NOT trying to argue the morality of homosexuality) and therefore will not make the contribution or have the added burden of parenthood. Just an idea... Again, not gay-bashing, but the word family generally carries the context of continuity through generations. I.e, grand-parents, parents, kids, etc. Generation to generation. A couple in a "gay marriage" could never have such a lifestyle. www.WingsuitPhotos.com

Interesting. So are you still going 120 mph when you land? My parachute changes my speed. That's called acceleration. True. But you just said "When you deploy a parachute you do not experience acceleration." 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. www.WingsuitPhotos.com

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. That was my point exactly Bill! What I was trying to explain with my "bouncing" example... 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. www.WingsuitPhotos.com

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. I KNOW it exists, but I'm having trouble trying to figure out how it must FEEL. 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? www.WingsuitPhotos.com

That a(x) curve would be spiky. www.WingsuitPhotos.com