g-force

This article is about a measure of acceleration. For G force (disambiguation), see G-force (disambiguation).

g-force (also gee-force, gee-loading) is a non-SI vector measure of acceleration, where 1 g (pronounced [dʒiː]) is defined to be an acceleration of the same magnitude as the nominal acceleration due to gravity on Earth at sea level – an acceleration equal to 9.80665 m/s², or approximately 32.174 ft/s².

Sometimes g-force also refers to the force resulting from an acceleration: (in that case, the unit is pounds-force or Newtons). Because of the confusion between this term being a force or an acceleration, it is often better to call the unit of acceleration simply g rather than g-force. Thus g-force is considered by some to be a misnomer and is not an accepted technical term.

Unlike simple acceleration, g-force is a measure of the magnitude of the acceleration relative to the local gravitational acceleration vector, rather than being compared to an inertial reference frame.

The symbol g is properly written in lowercase and italic, to distinguish it from the symbol G, the gravitational constant, which is always written in uppercase; and from g, the abbreviation for gram, which is not italicized.

Explanation

The acceleration a body internally "experiences" is the apparent weight per unit mass.

It is found by vector addition of the opposite of the actual acceleration (in the sense of rate of change of velocity) and a vector of the local gravity (about 1 g downward for the Earth's surface). For example, being accelerated upward on Earth with an acceleration of 1 g doubles the experienced weight to a g-force of 2 g. Conversely, falling gives an experienced weight of 0 g. A person standing at the top of a mountain experiences a g-force slightly below 1g due to greater distance from the centre of the Earth.

The term microgravity μg-force indicates a very low g-force, such as might occur for an object in contact with the walls of a space station in low earth orbit due to tidal forces. An object free floating in a space station would be (theoretically) experiencing zero-gravity due to the lack of external forces; neglecting air effects.

It is a normalized force vector since dividing the resultant force vector applied to a body by the body's weight (magnitude at sea level) cancels the mass, resulting in a "fractional-g" -magnitude vector, e.g. a person sitting on a chair at sea level is experiencing "1g," due to his weight.

Negative g-forces are often used on roller-coasters. Moments of zero g or negative g's are known as air time. The human body can withstand less negative g's than positive.

Convenient definition

G-forces are the forces riders feel while riding a roller coaster. These are caused by changes in the speed and direction of that train and rider. They are measured two ways: vertically and laterally. Positive G forces are felt on the car's path up the hills, where they feel as if they weigh more than they do. This is reversed on the car's descent where negative G forces occur, causing the riders to feel weightless.

Usage of the unit

• The g is used in aerospace fields, where it is a convenient magnitude when discussing the loads on aircraft and spacecraft (and their pilots or passengers). For instance, most civilian aircraft are capable of sustaining up to 4.33 g (42.5 m/s², 139 ft/s²), which is considered a safe value. Military aircraft and their pilots can experience up to 7 g with pressure suit.
• The g is used in automotive engineering, mainly in relation to cornering forces and collision analysis.
• The g is used in expressing the amount of acceleration/shock force a device or component part of a device can withstand, for example mechanical wrist-watches that can withstand 7g, aerospace rated relays that can withstand 50 g, GPS IMUs units for military howitzer shells that can withstand 15,500 g.
• g-forces are an important part of roller coasters and other themepark rides. They are often displayed in ride statistics.
• g-force is often used to describe a relatively long term acceleration: A short term acceleration is usually called a shock and is also measured in gs.

Human tolerance to g-force

Human tolerances depend on the magnitude of g-force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body.

The human body is flexible and deformable, particularly the softer tissues. A hard slap on the face may impose hundreds of g-s locally but not produce any real damage : a constant 15 g-s for a minute, however, may be deadly. When vibration is experienced, relatively low peak g levels can be severely damaging if they are at the resonant frequency of organs and connective tissues.

To some degree, g-tolerance can be trainable; and there is also considerable variation in innate ability between individuals. Further some illnesses reduce g-tolerance, particularly cardiovascular problems.

Vertical axis g-force

Aircraft in particular exert g-forces on the axis aligned with the spine. This causes significant variation in blood pressure along the length of the subjects body and this limits the maximum g-forces that can be tolerated.

One often hears the term being applied to the limits that the human body can withstand without losing consciousness, sometimes referred to as "blacking out", or g-loc (loc stands for loss of consciousness). A typical person can handle about 5 g (50m/s²) before this occurs, but through the combination of special g-suits and efforts to strain muscles—both of which act to force blood back into the brain—modern pilots can typically handle 9 g (90 m/s²) sustained (for a period of time) or more. Resistance to "negative" or upward gees, which drive blood to the head, is much less. This limit is typically in the -2 to -3 g (-20 m/s² to -30 m/s²) range. The vision goes red and is also referred to as a red out. This is probably due to capillaries in the eyes bursting under the increased blood pressure. Humans can survive about 20 to 40 g instantaneously (for a very short period of time). Any exposure to around 100 g or more, even if momentary, is likely to be lethal.

Horizontal axis g-force

The human body is considerably more able to survive g-forces that are perpendicular to the spine. In general when the acceleration pushes the body backwards (colloquially known as 'eyeballs in') a much higher tolerance is shown than when acceleration is pushing the body forwards ('eyeballs out') since blood vessels in the retina appear more sensitive to that direction.

Early experiments showed that untrained humans were able to tolerate 17 g eyeballs-in (compared to 12 g eyeballs-out) for several minutes without loss of consciousness or apparent long-term harm.

Human g-force experience

• Amusement park rides such as roller coasters typically do not expose the occupants to much more than about 3 g . Some notable exceptions are Oblivion in England, Speed at Oakwood Theme Park in Wales, and Titan in Texas, which all have a maximum of 4.5 g, and SheiKra at Tampa which pulls 4 g. The record for the most g forces on a roller coaster belongs to Mindbender at Galaxyland Amusement Park, Edmonton, Alberta, Canada, at 5.2 g. The highest g on a thrill ride can be experienced on Detonator at Thorpe Park, which reaches 5.5 g at the end of the drop by firing riders downwards pneumatically.
• A sky-diver in a stable free-fall experiences his full weight of 1 g after reaching terminal velocity.
• A scuba diver or swimmer experiences his full weight of 1 g, but buoyancy largely cancels the weight of his body. However, density differences do create forces. The lungs are significantly buoyant.
• Astronauts in Earth orbit experience 0 g, or 'weightlessness'. They are still strongly attracted by the Earth's gravity. The value of gravity acceleration at the level of a 600 km. (372 mile) high orbit is about 83% of the sea level gravity acceleration. However as they are in 'constant free fall' they don't 'feel' any acceleration.
• Passengers on planes on a parabolic trajectory experience 0 g (as in the Vomit Comet)
• Aerobatic and fighter pilots may sometimes experience a greyout between 6 and 9 g. This is not a total loss of consciousness but is characterized by temporary loss of colour vision, tunnel vision, or an inability to interpret verbal commands. They also experience a 'redout' at negative g. These effects are mostly caused by blood pressure differences between the heart and the brain.
• Pilots in the Red Bull Air Race commonly exceed 10 g for seconds during turns, occasionally surpassing 12 g.
• Formula One drivers usually experience 5 g while braking, 2 g while accelerating, and 4 g while cornering. Every Formula One car has an ADR (Accident Data Recovery) device installed, which recordes speed and g-force. According to the FIA Robert Kubica of BMW Sauber experienced 75 g during his 2007 Montreal GP crash.

Everyday g-forces

• 3.5 g during a cough.
• 2.9 g during a sneeze.

Strongest g-forces survived by humans

Voluntarily: Colonel John Stapp in 1954 sustained 46.2 g in a rocket sled, while conducting research on the effects of human deceleration. See Martin Voshell (2004), 'High Acceleration and the Human Body'.

Involuntarily: Formula One racing car driver David Purley survived an estimated 179.8 g in 1977 when he decelerated from 173 km·h⁻¹ (108 mph) to 0 in a distance of 66 cm (26 inches) after his throttle got stuck wide open and he hit a wall.

See also

• Earth's gravity
• Shock (mechanics)
• Cushioning

External links

• Acceleration The Physics Hypertextbook
• Wired article about enduring a human centrifuge at the NASA Ames Research Center
• eXtreme High Altitude Calculator - value of g at any altitude
• Video of Pilot g-force training