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Aristotle (384–322 BCE) -

To understand the physics behind g-forces, we must first ask ourselves why do objects fall to the ground? Greek philosopher Aristotle was among the first to investigate gravitational theory by putting forth the idea that objects moved toward their "natural place."

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Galileo Galilei (1564–1642) -

Two thousand years later, Italian astronomer, physicist, and engineer Galileo Galilei concluded that all objects tend to accelerate equally in free fall.

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Isaac Newton (1642–1726) -

In 1687, English mathematician, physicist, and astronomer Isaac Newton formulated the laws of motion and universal gravitation, more commonly called the law of gravity.

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Albert Einstein (1879–1955) -

The next step in our understanding of gravity comes from German-born theoretical physicist Albert Einstein. His general theory of relativity, developed between 1907 and 1915, describes the relationship between matter and motion.

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Paul Stapp (1910–1999) -

In simple terms, the definition of g-force is a force acting on a body as a result of acceleration or gravity. In the 1940s, US Air Force flight surgeon and physician John Stapp built models for testing the effects of g-force on the human body. He became a pioneer in studying the effects of acceleration and deceleration forces on humans.

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Rocket-propelled research sled -

Stapp himself risked his life to test the effect of acceleration on the human body. These six photos from a June 1954 experiment show Stapp enduring the effects of acceleration and deceleration of riding in a rocket-propelled research sled.

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Development of the ejector seat -

Stapp's research would ultimately lead to the development of ejector seats. Pictured is an ejector seat being tested using a high speed rocket sled at Edwards Air Force Base in California in 1959.

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High-G training -

The Space Race between the two Cold War adversaries, the Soviet Union and the United States, accelerated during the 1950s. Space flight training involved what's known as High-G training and included centrifuge, Anti-G Straining Maneuvers (AGSM), and acceleration physiology. Pictured is a Soviet cosmonaut in a centrifuge during his training at Zvyozdny Gorodok (Star City) in the early 1960s.

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Human centrifuge -

G-forces are a measurement of the type of acceleration that causes weight, like the kind you feel when you're pressed into your seat during a roller-coaster loop. But that sensation of weight is magnified many times over in a human centrifuge, when it's used as a dynamic flight simulator. Pictured is a US military test subject undergoing g-force testing (level 2 out of 10) while seated and wearing padded eye protection. G-forces have distorted his face into a grin.

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Centrifugal forces -

Aviators and astronauts undergo human centrifuge. They are subject to high levels of acceleration ('G') to ascertain their requisite G Tolerance and G Readiness. The centrifuge simulates a realistic G on demand and a sustained high G flight environment without the need of a live aircraft. Pictured is a RAF pilot in England undergoing training in 1955.

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Space program -

Project Mercury astronaut Donald K. Slayton (1924–1993) emerges from a human centrifuge gondola during the training phase at the Aviation Medical Acceleration laboratory in Johnsville, Pennsylvania, in 1960. The training program was aimed at providing the astronauts with acceleration experience in preparation for manned space flight.

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Anti-G flying suit, MK4A -

This rubber anti-gravity suit dates back to 1950 and was worn by pilots during test flights. The suit is an overall designed to cover the legs and abdomen, consisting of two layers of rubber with water between. When laced tightly to the body, it prevents blood rushing to the head. This stops the pilot from blacking out when experiencing extreme g-force. The first gravity suit, the Franks Flying Suit Mk II, was invented by Canadian scientist Wilbur Franks, and was first used during the Second World War.

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Human centrifuges today -

Modern human centrifuges are high-tech pieces of equipment, like this Centrifuge TsF-7 at the Yuri Gagarin Cosmonaut Training Centre in Zvyozdny Gorodok (Star City). High-G training today still includes centrifuge, Anti-G Straining Maneuvers (AGSM), and acceleration physiology, but advances in knowledge and technology have contributed to extending pilots' G tolerance in both magnitude and duration.

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Human tolerance -

Human tolerances to g-forces depend on several factors, including the magnitude of the gravitational force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body. Vertical g-forces, denoted as positive because they force blood towards the feet and away from the head, often induce grayout, where the vision loses hue, but is easily reversible on leveling out (pictured, simulated stages of a grayout).

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Tunnel vision -

Another effect of positive vertical g-force is tunnel vision, where peripheral vision is progressively lost. If vertical g-forces are maintained, blackout will take place. If not ultimately reduced, death may occur.

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Negative G-force -

Resistance to negative or downward g-force is much lower and more problematic because it drives blood to the head. This can cause what's known as redout, a painful expansion of blood vessels in the head that is potentially very dangerous. Usually a redout will only ever be experienced by pilots.

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Reno Air Races crash -

A tragic example of pilot blackout took place in 2011 at the Reno Air Races when James K. "Jimmy" Leeward's aircraft suddenly pitched up, rolled inverted, then nosedived to the ground. Leeward was killed, along with 10 spectators. A mechanical failure had sent his plane hurtling higher into the sky to the point where he experienced a staggering 17 g, which quickly incapacitated him and likely rendered him unconscious.

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Orders of magnitude -

G-forces act on us in all sorts of different ways. While they are capable of providing fun, g-forces are also a formidable foe to the human body, capable of taking us out within a few seconds if we underestimate them. So, what g-forces can we tolerate, and how fast and furious can we go before they kill us? The gravitational force of standing on Earth at sea-level, for example, is 1 g.

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Saturn V rocket -

The Saturn V rocket that took the Apollo astronauts into space pulled an uncomfortable 3 g just after launch.

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Dragsters -

Top-fuel dragsters can accelerate from zero to 160 km/h (99 mph) in 0.86 seconds. This typically yields 5.4 g.

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Aerobatics airplane -

Outstanding physical fitness and flight suits typically enable Red Bull Air Race pilots to maintain consciousness and endure an astonishing 10 g.

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Roller-coaster -

Roller-coasters are notorious for inducing swooping, sickening sensations. That's because the average ride generates a roughly, albeit brief, 5 g.

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Inverted -

Incidentally, a classic example of negative g-force is in a fully inverted roller coaster that's accelerating (changing velocity) toward the ground. Passengers are hurtling toward the deck faster than gravity would accelerate them, and are thus pinned upside down in their seats. Fortunately, all roller coasters have to be designed so people don’t black out.

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"Wall of Death" -

The "Wall of Death" carnival sideshows that first appeared in the early 1900s saw stunt motorcycle riders traveling at speed along the vertical wall of barrel-shaped wooden arenas, their machines held in place by centrifugal forces.

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G-force stunts -

Automobile are also employed in "Wall of Death" stunts, working on the same principle. Pictured are daredevil performers at the Janmashtami festival in Rajkot, in the Indian state of Gujarat.

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Accelerating the Bugatti Veyron -

Staying with vehicles, accelerating in a Bugatti Veyron from 0 to 100 km/h (62 mph) in just 2.4 seconds will generate a whopping 1.55 g. The Bugatti Veyron Super Sport is the world's fastest production car, with a top speed of 415 km/h (257 mph).

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Formula One car turning -

But that's nothing compared to a Formula One car, which can generate a bone-crushing 6-6.5 g at high speed on tight corners alone.

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Formula One car braking -

And when you have to break suddenly at full speed in a Formula One car, drivers can still experience an equally numbing 6.3 g.

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Luge -

High-performance sport extends to activities like the luge. Exponents of this exciting, adrenaline-fueled competition regularly expose themselves to up to 5.2 g.

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Aerobatics glider -

The leisurely sport of gliding is sometimes anything but. Pilots maneuvering specialized aerobatics gliders can experience a positive 7 g and a more dangerous negative 5 g during competition.

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Sneezing -

A more down-to-earth statistic is the incredible 2.9 g generated by an uninhibited sneeze. No wonder you should always cover your mouth!

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Back slapping -

And did you know that greeting someone with a hearty slap on the back is akin to hitting them with an amazing 4.1 g?

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Mantis shrimp, nature's big hitter -

The natural world, too, can astound with its ability to generate incredible g-forces. Surprisingly, it's the diminutive mantis shrimp that is one of the bigger hitters. The acceleration of its claw during a strike can produce a killer 10.4 g blow.

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The Sun -

But it's when you leave this world that things really start to heat up. The gravitational acceleration at the surface of the Sun, for example, reaches 28 g. But a coronal mass ejection from the solar corona generates a frightening 480 g!

Sources: (ThoughtCo) (National Aviation) (ETC Aircrew Training Systems) (Medical Daily) (Because Learning) (How It Works) (National Transportation Safety Board) (Space Answers) (PBS) (Nature Publishing Group) (The Astrophysical Journal)

See also: Dazzling facts about the Sun

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