I was reading an article about rollercoasters, and it talked about “g-forces” (which I understand) and “negative g-forces” (which I don’t understand.) What is a “negative g-force?”
it is the g-force, but in the opposite direction.
If the g-force is your weight with earth’s gravity, 2g’s is the equivalent weight if the earth’s gravity were twice as strong. If you weigh 150 lbs now, you’d weigh 300 on this planet. You feel it as being pulled “down” or into your seat (in the negative direction).
A positive g results in “pulling” you out of your seat, or being pulled “up” (the positive direction).
You feel negative g forces when you are on the inside of a turn, loop, bank and feel positive g forces when you are on the outside (including going over the top of a hill).
Forcefully,
Spritle
I think you have that backwards, Spritle. “Negative G” is the feeling that you are floating and “positive G” is the feeling that you are heavier than you are.
Since gravity pulls toward the centre of a mass, positive is in the direction of the centre; so in your example, the 150 lb. person who “weighs” 300 is experiencing two positive gees. If the same person were to fly in an airplane that is on a parabolic trajectory (on the downward side) and the speed and angle are just right, the 150 lb. person would “weigh” zero. If the airplane steepened its dive and the person was stuck to the overhead, then he would be experiencing negative g.
Oops. Johnny L.A. is correct. I must have been standing on my head or something. The onlything worse than incorrect information is misinformation. Sorry.
I still don’t get it. If I’m floating, does it matter how many negative G’s I’m experiencing? Once I’m floating, I’m floating, or will excess negative G’s force me up into space?
I don’t think there is really such a thing as “negative g”. It’s just short-hand. Take the airplane example: If the airplane is falling faster than you are, then your weight is negative relative to the deck of the aircraft (assuming it’s upright while diving) – that is, you’re stuck against the overhead. If you and the airplane are falling at the same speed, then you are weightless relative to the deck of the aircraft – you’re floating within it. If you are falling faster than the airplane, then you are stuck to the deck (or more likely, the seat).
Even in space, gravity pulls the astronauts toward the earth. The reason they are “weightless” is that they are flying along at about 17,500 miles per hour. They are still falling toward the planet, but at their speed, the surface of the planet (being curved) is “falling” away from them at the same rate they are falling toward it. Viola! Relative weightlessness!
But you’re not “floating” unless you’re only pulling one negative g in a one g environment like, well, the world around you. In this scenario the one g and one negative g cancel each other out.
Imagine it this way - no outside forces acting on you to confuse things. Pulling zero g would be floating (like in interplanetary space). One g puts you on the floor with your regular weight, 3 g’s puts you there with three times your weight.
One negative g sends you toward the ceiling with your full weight. 3 negative g’s send you there with three times your full weight. Get the picture?
Negative g’s aren’t necessarily a “floating” experience, they’re a “being hurled at the ceiling” experience.
Fighter pilots pull negative g’s when the push forward on the stick and make the plane fall out from under them. Around 4 1/2 or 5 negative g’s, they experience “red out,” which is an effect of all of their blood being forced into their heads by the negative g-forces. After a few seconds of red out, they are rendered unconscious and will quite likely hit the ground at high speed and die.
Positive gees are anything that points down, which may even be less than normal weight. Negative gees point up. Zero gees is floating.
All of this is from the frame of reference of the person experiencing the gees, which is probably non-inertial and may be inverted. Thus, if you’re hanging from your ankles on the playground equipment, then you’re feeling -1 gees, and if you’re in the textbook free-falling elevator, then you’re feeling zero gees, regardless of orientation. You’re probably feeling positive one gee right now.
***Popular Science,***eh? 
So, what are the effects of a redout? I’d never heard of that before, but the article mentions that too many negative g’s will cause one. It seems to me that these would be more dangerous than a blackout, in that you could burst a blood vessel in your brain. Is this the case?
Y’know, other than loss of consciousness, I’m not sure what the effects of a redout are…but an aneurism would seem likely after a while, wouldn’t it? Hmm…maybe later I’ll check the trusty ol’ internet for more info - if nobody beats me to it…
I don’t know nothin’ bout y’all’s highfalutin’ science, but I do know that the Blue Streak at Cedar Point, while looking very wussy, delivers negative g forces, and it feels very very cool. More than floating, less than being catapulted at the sky. From which I extrapolate that it is between 1 and 2 negative g’s.
Cedar Point is where we’re headed for vacation at the end of the summer, so now I have something to look forward to.
And yes, Smoke, I was reading Popular Science. Interesting stuff; perhaps a trip to Japan for a ride on the other rollercoaster will happen in the future.
KVS: if you’re going to Cedar Point, you’ve got a lot more to look forward to than Blue Streak. You’ll experience the MOST negative Gs on Magnum’s bunny hop return run to the station, and you’ll experience about 6 full seconds of zero G action on Millennium Force. I didn’t touch my seat the whole way over the third hill. I grew up very close to CP, and I’ve been there about 35 times…fantastic park, and a great place to be for a roller coaster enthusiast such as myself.
Jman
G-LOC, or Gravitational Loss Of Consciousness, is a problem with pilots in high performance aircraft, which can outmanuever the human body. In a high bank turn “upwards” the blood flows to the legs (hence G-Suits, which constrict the lower body) and the driver can pass out from lack of oxygen. Pushing the stick forward (“down”) gives negative G’s, wherein the body is pressing upward against the shoulder staps, and blood rushes to the head. THIS may be the “red-out” mentioned. I am not a pilot, I just killed them on active duty. But Zero G is floating. Everything is from the frame of reference of the body, based upon the orientaion of the body. (i.e. someone hanging upside down in a plane with the stick pushed forward would experience positive G’s)
Man, Uncle Bill, I’m not usually one them sig-hunters, but this one’s too great to let die. 
If you don’t want it, can I have it? Please?
Continuing a bit on the hijack, I feel obliged to mention that no discussion of Cedar Point is complete without the Gemini. Best coaster in the park, and the lines are never more than 15 minutes. Blue Streak isn’t too bad, but Magnum is just plain boring: Two hills and it’s over.
Let me try to describe what “pulling g’s” means. It is better to start with Load Factor, an aeronautical term, and that’s what we’re really talking about. Load Factor is a dimensionless quantity. Take the sum of all forces other than gravity acting on something, and divide by the weight of the “thing” you’re talking about. The sign is determined by whether the other forces are in opposition to gravity (positive) or in the same direction (negative). In most cases, you’re only interested in forces in the same direction (more or less) as gravity.
Now, why to people call this “g’s”, especially when it is really dimensionless? Well, it sounds really cool - guys like Chuck Yeager say “there I was, pulling 8 g’s and my caution/warning panel lit up like a Christmas tree!”
The g’s is just a conversational way of relating the forces that you have internal to the object to what they would be in a “normal” one g gravitational field. That’s it. No big mystery.
As I sit in this chair, my weight is 160 pounds. The chair holds me up with 160 pounds of force. I’m at a load factor of 1.
In an airplane in a 60 degree bank turn, I’d still weigh 160 pounds, but the seat of the airplane would have to push on my butt with 320 pounds of force (2 times) to get me to move through the turn. Anything I might be holding in my hand would “feel” twice as heavy.
In and airplane that’s in steady inverted flight, I’d be in a load factor of -1, that is, my 160 pounds would be hanging on the seatbelts. In upright flight, in a pushover, the seat might be pushing on me with only 80 pounds (1/2) or zero pounds (“zero gee”) or even pulling on the straps (negative gees!).
And, Johnny LA, if your helio has a teetering rotor, you don’t want to go negative.
I fly a fully-articulated rotor now, but I used to fly R-22s (great little heli, BTW). I took the mandated addition training (SFAR 73) after some airplane pilots killed themselves in R-22s. Here’s a Robinson safety notice:
Actually, my favourite safety notice is SN-24, which is an excellent discussion called LOW RPM ROTOR STALL CAN BE FATAL:
Bolding mine. Ya gotta love an official document (the safety notices are in the Safety Tips section of the POH) that uses the word “doomed”.
“g” is also an “aeronautical” term, which is why my POH describes such limits as “+3/-1.5g” or “+6/-3” when discussing just how much stress you can put on the plane before you have worry about pieces falling off. Some aerobatic planes also have “g-meters” It’s not just because it “sounds cool”, pilots do actually talk in “g’s”.
Not quite. I’m in one of those moods so I’m going to get fussy. In a 60 degree bank in level flight you actually DO weigh 320 pounds. “Weight” is the force acting upon your body. Your mass, that is, the amount of stuff making up your body, does remain at 160 pounds (pounds being a measure of either mass or weight, depending on context). Anything in your hand would actually BE twice as heavy, although, again, the mass remains the same. In addition, your arm holding the object also weighs twice as much.
If you are in a descending turn while in a 60 degree bank you will experience less than 2 g’s. If you are climbing in a 60 degree bank you will experience more than 2 g’s.
If you have a weakness in a blood vessel red out could, theorectially, cause a blow-out in the tube. This is more likely if you already have high blood pressure or blood vessel problems. Most folks think of it happening in the brain (which is indeed truly bad) but you can also hemorrhage into the interior of your eyeball, causing blindness at least in theory - I don’t know if it’s actually happened or not.