schmit.paul

Oops billvon, I might have repeated a bit from your post, sorry about the redundancy!

John, Your grasp on classical gravity/forces/acceleration is as finely-tuned as many of my fellow peers in the physics community, so my compliments! I'll offer up a few points to perhaps add a little more clarity to the above discussion: First, my interpretation of the stomach-in-your-throat feeling is this--your body is set up internally in such a way that frictional forces between your skin/bones/organs and pressure/viscous forces between your blood and blood vessels counteract gravity 100%...if they didn't, your blood and organs would rush down to your feet while you're simply standing on the ground perfectly still. So without any external acceleration of your body, you have the force of gravity pointing down and frictional/pressure-viscous forces pointing up, and they exactly counteract. Now, imagine you now pull the ground out from under you and you go into a feet-first freefall. Under the influence of gravity, every object accelerates at the same rate (Galileo discovered this by showing balls of lead and far-less-dense wood fell at the same rate when dropped from a tower...of course, a lead ball of the same size would have a higher terminal velocity, but this is irrelevant to our discussion). Thus, without the aid of frictional/viscous forces, your body/organs/blood in initial feet-first accelerated freefall would maintain its internal configuration (every part and piece of your body accelerates at the same rate). Thus the two different systems (standing on the ground and accelerated freefall) have different internal equilibria, and when you transition from one to the other your body internally "reconfigures" itself (in the case of jumping, during your ~5 sec on the hill). The new equilibrium includes increased blood flow to your head (since your blood is no longer inclined to pool at your feet, and the pressure gradient in your vascular system is allowed to equilibrate) and likely a slight upward lift and decreased frictional force between your organs and their surrounding infrastructure (for the same reason as your blood--they're no longer being held in place via friction and pressure to fight gravity's tendency to pull them down toward your feet, so naturally there's a recoil effect that points toward your head). Once you reach terminal velocity, this effect ceases and you are once again in an inertial reference frame, as if you were standing on the ground, and your internal system settles back into its original state. The reason I emphasized feet-first freefall is because this is the only time I've ever had the stomach-in-throat experience--skydiving, amusement parks, whatever. I obviously don't get this feeling when I accelerate forwards or backwards in a car/airplane/etc, and accelerating upwards headfirst (identical to diving downward out of a plane headfirst) also doesn't give the same feeling (though this situation has its own unique effect, namely an increased effective gravity that momentarily pulls blood away from your head and increases the pressure of your organs in the direction of your feet). One last thing I'll mention is that to some extent, your forward movement when jumping out of a plane is not as relevant to the mitigation of the stomach-in-throat feeling as one might think. The equations describing the motion of a body falling through free-space demonstrate an interesting feature--they are dimensionally decoupled. In other words, your horizontal and vertical movements are completely independent...thus a ball released from a moving plane and another ball released from a stationary balloon at the same altitude will hit the ground at the same time, in spite of the differences in horizontal motion between the two balls--the vertical motion is entirely independent. Now, going from a ball to a body makes things more complicated, because reorienting your body produces new forces that couple horizontal and vertical motion (lift and drag), but neglecting this I'd argue that jumping feet first out of a balloon and jumping feet first out of a plane will produce the same initial effect on your body vertically (assuming you could maintain a vertical body orientation in spite of the shifting relative wind on the hill), and you should experience the stomach-in-throat feeling in both instances. As a relative newcomer to skydiving, however, I've learned first-hand that the wind loves to reorient your body almost immediately as soon as you leave the plane, and jumping out of a plane feet-first doesn't always mean you maintain that orientation on the hill. And John, you're exactly right about Einstein's theory of gravitation. Interestingly, objects tend to move on geodesics (shortest path between two points along a curved surface), which in vacuum without any massive bodies is just a straight line in space, but with mass added winds up forming a curved path through space due to the effect of mass on the curvature of space-time. Hope that wasn't too ridiculously long-winded, sorry if it was Paul [edited for better wording] "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Haha, I like your reply billvon! Royd, I can't quite wrap my head around what is causing your angst...it seems your reasons are more political than scientific/epistemological. I'll readily acknowledge that politicians make life for us scientists very difficult--with little to no scientific background, they still control nearly our entire research budget, forcing (many, but not all of) us to compete fiercely for limited funding and tailor our fundamental directives to try to cater to the cultural hot topics of today (currently climate change and energy top the list, and I happen to be working in a field that could have major repercussions in both arenas). In this respect I am on your side; in a perfect world free of useless wars, incendiary bigotry, and rampant corporatism, scientists could explore in any direction they so desired without recourse to risk and reward, leading to more momentous discoveries and a more honest discourse between scientists and nonscientists. However, beyond that your diatribe lost me completely. The religious notion of "guilt," at least as far as the Judeo-Christian doctrine is concerned, is that you are born imperfect and are destined to suffer eternally because of it unless you spend the entirety of your mortal life paying penance by adhering to some archaic, incomplete moral doctrine (and let's not forget giving up a significant fraction of your income to the church). While the details concerning climate change and other environmental matters are certainly not 100% determined (and no good scientist will ever say they are, no matter how seemingly concrete the theory seems to be), we can offer hoards of empirical evidence to support our claims, and we put our reasons and doubts out there publicly for the world, and in particular the rest of the scientific community, to see. There are no conspiracies or deceptive veils, no preying on the desperate, downtrodden, and uneducated, and dishonesty is treated with harsh consequences when this self-regulating community discovers someone practicing dubious science. And as far as taxes are concerned, the total federal scientific research budget across all agencies is roughly 30 billion dollars (http://en.wikipedia.org/wiki/National_Science_Foundation), approximately the cost of 5 Nimitz-class aircraft carriers, or less than 3 months in Iraq, or less than 6% of the total US Dept. of Defense annual budget, or about 3% of our annual Social Security expenditure. If you're pissed about your taxes, don't blame science. Don't get me started on the pope!!! "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Thanks a lot, makes sense! "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Hi guys, quick question. I've got a pretty solid track after doing a bunch of coach dives with some good instructors at Eloy, but I've never gotten consistent advice about the best way to make minor to moderate heading changes while in a track while staying stable. Some have said that lowering one arm lifts that side of your body, causing more air to deflect toward that side, turning you the other direction. It also seems like dropping a shoulder could accomplish the same thing. I'm sure in theory you could use your legs to make adjustments, but it seems like that sort of maneuver would be inherently more prone to causing instabilities. What do you guys prefer? "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Lorena Bobbitt, anyone? "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Amen. Though a friend of mine has sworn to me there's an adult video production company out there that many well-known bands have done soundtrack music for (including Queens of the Stone Age) that produces porn with such hilariously entertaining story lines that the pornographic aspect winds up a secondary perk. Still, not sure it'd be worth 40 bucks, or whatever the hell they're charging for adult DVDs nowadays, but I doubt you'll find any of that stuff on your garden variety dirty website. Burning Angel is the name of the company, I think. "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Thanks guys for the follow ups. I wasn't aware most manufacturers don't do direct sales...I guess the couple of websites I went to where I happened to be looking for it had "order online" links, but perhaps those links would have forwarded you to the geographically nearest dealer with online ordering options... I'd consider buying parts and pieces used, except that then you don't qualify for the demo program. Considering I intend to get my money's worth out of the program before I place an order ($75/day rental fee times as many days I can get in during the allotted period of time = at least $1000 in waived rental fees), it may be just as frugal to buy everything new. I suppose I could put together a new rig without an AAD and buy a used AAD separately and have a rigger install it, but that's about as much as I could finesse the system. "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Hi guys, Considering starting up the demo program at Square3 at CK in Jersey (for anyone unfamiliar, you put a deposit down toward new gear and can jump rental gear for free for up to 6 months, so there's no rush to put in a new gear order). I am a little curious what their mark-up is for new gear. I'm sure a lot of you, especially more experienced jumpers who have gone through a number of rigs and know what they like, order new pieces of gear straight from the manufacturer. So my question is, purchasing new through a third-party company like Square has GOT to add some overhead to the cost of new gear, but how much typically? It seems like most canopy/container manufacturers are hesitant to put factory-order prices in clear view on their websites, so it's hard for someone green like me to assess the situation. Nevertheless, I think the benefit of having plenty of time to try out lots of demo gear for free before purchasing a specific rig will outweigh whatever cost savings come from piecing together a rig purchasing from individual manufacturers, but this is something worth knowing down the road.... "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

power tools, violent sports, fishing, jock itch...and all of the overblown personal anecdotes geared at show-boating our "manliness" in front of the rest of our male compatriots (and of course all of the women who would undoubtedly be trolling around the "men only" board looking for an acceptably alpha skydiving suitor ) "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

#3's a real interesting one. Not only does it passively endorse slavery (not surprising though, considering the temporal origin of the Old Testament and the powerful, aristocratic people who put it together), but also says nothing about your wife getting to rest...now, I know adult women weren't necessarily starting fortune 500 companies back in the day, but either the OT trivializes their domestic duties as something less than work and not to be recognized on the Sabbath (I think Oprah and her legion of loyal followers would beg to differ), or perhaps the wife falls under the category of "female slave." Either way, something is lacking there. We can do this all day, the OT is a very complex, interesting, but ultimately somewhat antiquated book of morality--certainly a worthy source for a basic moral code, but only if viewed through a foggy lens. I don't mean to disrespect your belief system or Catholicism specifically--I mean to point out the inherent absurdity in all organized religion if taken too seriously. But that's not to say I don't recognize the value religion has as a way to instill basic moral values and a sense of community...but most people take it much farther than that, some are even willing to die for it, without a shred of proof that their belief system is any more valid than that belonging to the person they are fighting. Shalom. (I'm not Jewish, but that word encompasses a lot of good sentiments in a nice, succinct manner) "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

It's very true, I'm actually on a DoD fellowship right now that covers all my tuition plus more than enough money to live on debt-free (ok, I'm not rushing to gold-plate my car, but I will be able to buy myself a new rig soon enough). So you're right, being a paid student while the economy sorts itself out is kinda nice! Good for you for doing summer research at a big lab, Los Alamos is def reputable, and that should go a long way as far as letters of rec go to help you get into a good school. But enjoy yourself right now after 4 years in the "trenches"...hell, if I were taking time off right now I'd probably get certified as a rigger and live on the DZ! "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Absolutely. Just grounded myself this weekend getting over a fever, don't like to go out and take any more risks than what comes with the turf already. Actually skydiving has seemed to be much easier on my shoulder than many of the other activities i participate in, so it's been one of the only physical activities (besides weight lifting) that I've been able to do without much worry. But rest assured, I won't be back up in the air until my strength and range of motion is back to normal. Interestingly enough, in the meantime I've found I have the flexibility in my right arm to reach back and throw while maintaining stability, while putting my right hand over my head to stabilize while throwing with the left would probably not work so well (labral tears put the shoulder at its most vulnerable position with the arm raised completely above the head). So I've lucked out in a way, and haven't had any problems. "Ignorance more frequently begets confidence than knowledge." ---Charles Darwin

Ha! Very cool