G-force

The []g-force or gravitational force equivalent is a mass-specific force, expressed in units of standard gravity.
It is used for sustained accelerations that cause a perception of weight. For example, an object at rest on Earth's surface is subject to 1 g, equaling the conventional value of gravitational acceleration on Earth, about.
More transient acceleration, accompanied with significant jerk, is called shock.
When the g-force is produced by the surface of one object being pushed by the surface of another object, the reaction force to this push produces an equal and opposite force for every unit of each object's mass. The types of forces involved are transmitted through objects by interior mechanical stresses. Gravitational acceleration is one cause of an object's acceleration in relation to free fall.
The g-force experienced by an object is due to the vector sum of all gravitational and non-gravitational forces acting on an object's freedom to move. In practice, as noted, these are surface-contact forces between objects. Such forces cause stresses and strains on objects, since they must be transmitted from an object surface. Because of these strains, large g-forces may be destructive.
For example, a force of 1 g on an object sitting on the Earth's surface is caused by the mechanical force exerted in the upward direction by the ground, keeping the object from going into free fall. The upward contact force from the ground ensures that an object at rest on the Earth's surface is accelerating relative to the free-fall condition.. Stress inside the object is ensured from the fact that the ground contact forces are transmitted only from the point of contact with the ground.
Objects allowed to free-fall in an inertial trajectory, under the influence of gravitation only, feel no g-force a condition known as weightlessness. Being in free fall in an inertial trajectory is colloquially called "zero-g", which is short for "zero g-force". Zero g-force conditions would occur inside an elevator falling freely toward the Earth's center, or inside a spacecraft in Earth orbit. These are examples of coordinate acceleration without a sensation of weight.
In the absence of gravitational fields, or in directions at right angles to them, proper and coordinate accelerations are the same, and any coordinate acceleration must be produced by a corresponding g-force acceleration. An example of this is a rocket in free space: when the engines produce simple changes in velocity, those changes cause g-forces on the rocket and the passengers.

Unit and measurement

The unit of measure of acceleration in the International System of Units is m/s². However, to distinguish acceleration relative to free fall from simple acceleration, the unit g is often used. One g is the force per unit mass due to gravity at the Earth's surface and is the standard gravity, defined as metres per second squared, or equivalently newtons of force per kilogram of mass. The unit definition does not vary with location—the g-force when standing on the Moon is almost exactly that on Earth.
The unit g is not one of the SI units, which uses "g" for gram. Also, "g" should not be confused with "G", which is the standard symbol for the gravitational constant. This notation is commonly used in aviation, especially in aerobatic or combat military aviation, to describe the increased forces that must be overcome by pilots in order to remain conscious and not g-LOC.
Measurement of g-force is typically achieved using an accelerometer. In certain cases, g-forces may be measured using suitably calibrated scales.

Acceleration and forces

The term g-"force" is technically incorrect as it is a measure of acceleration, not force. While acceleration is a vector quantity, g-force accelerations are often expressed as a scalar, based on the vector magnitude, with positive g-forces pointing downward, and negative g-forces pointing upward. Thus, a g-force is a vector of acceleration. It is an acceleration that must be produced by a mechanical force, and cannot be produced by simple gravitation. Objects acted upon only by gravitation experience no g-force, and are weightless.
g-forces, when multiplied by a mass upon which they act, are associated with a certain type of mechanical force in the correct sense of the term "force", and this force produces compressive stress and tensile stress. Such forces result in the operational sensation of weight, but the equation carries a sign change due to the definition of positive weight in the direction downward, so the direction of weight-force is opposite to the direction of g-force acceleration:
The reason for the minus sign is that the actual force on an object produced by a g-force is in the opposite direction to the sign of the g-force, since in physics, weight is not the force that produces the acceleration, but rather the equal-and-opposite reaction force to it. If the direction upward is taken as positive then positive g-force produces a force/weight on any mass, that acts downward. In the same way, a negative-g force is an acceleration vector downward, and this acceleration downward produces a weight-force in a direction upward.
If a g-force is vertically upward and is applied by the ground or applied by the floor of an elevator to a standing person, most of the body experiences compressive stress which at any height, if multiplied by the area, is the related mechanical force, which is the product of the g-force and the supported mass. At the same time, the arms themselves experience a tensile stress, which at any height, if multiplied by the area, is again the related mechanical force, which is the product of the g-force and the mass hanging below the point of mechanical support. The mechanical resistive force spreads from points of contact with the floor or supporting structure, and gradually decreases toward zero at the unsupported ends. With compressive force counted as negative tensile force, the rate of change of the tensile force in the direction of the g-force, per unit mass, is equal to the g-force plus the non-gravitational external forces on the slice, if any.
For a given g-force the stresses are the same, regardless of whether this g-force is caused by mechanical resistance to gravity, or by a coordinate-acceleration caused by a mechanical force, or by a combination of these. Hence, for people all mechanical forces feels exactly the same whether they cause coordinate acceleration or not. For objects likewise, the question of whether they can withstand the mechanical g-force without damage is the same for any type of g-force. For example, upward acceleration on Earth feels the same as being stationary on a celestial body with a higher surface gravity. Gravitation acting alone does not produce any g-force; g-force is only produced from mechanical pushes and pulls. For a free body such g-forces only arise as the "inertial" path that is the natural effect of gravitation, or the natural effect of the inertia of mass, is modified. Such modification may only arise from influences other than gravitation.
Examples of important situations involving g-forces include:

The g-force acting on a stationary object resting on the Earth's surface is 1 g and results from the resisting reaction of the Earth's surface bearing upwards equal to an acceleration of 1 g, and is equal and opposite to gravity. The number 1 is approximate, depending on location.
The g-force acting on an object in any weightless environment such as free-fall in a vacuum is 0 g.
The g-force acting on an object under acceleration can be much greater than 1 g, for example, the dragster pictured at top right can exert a horizontal g-force of 5.3 when accelerating.
The g-force acting on an object under acceleration may be downwards, for example when cresting a sharp hill on a roller coaster.
If there are no other external forces than gravity, the g-force in a rocket is the thrust per unit mass. Its magnitude is equal to the thrust-to-weight ratio times g, and to the consumption of delta-v per unit time.
In the case of a shock, e.g., a collision, the g-force can be very large during a short time.

A classic example of negative g-force is in a fully inverted roller coaster which is accelerating toward the ground. In this case, the roller coaster riders are accelerated toward the ground faster than gravity would accelerate them, and are thus pinned upside down in their seats. In this case, the mechanical force exerted by the seat causes the g-force by altering the path of the passenger downward in a way that differs from gravitational acceleration. The difference in downward motion, now faster than gravity would provide, is caused by the push of the seat, and it results in a g-force toward the ground.
All "coordinate accelerations", are described by Newton's laws of motion as follows:
The second law of motion, the law of acceleration, states that meaning that a force F acting on a body is equal to the mass m of the body times its acceleration a.
The third law of motion, the law of reciprocal actions, states that all forces occur in pairs, and these two forces are equal in magnitude and opposite in direction. Newton's third law of motion means that not only does gravity behave as a force acting downwards on, say, a rock held in your hand but also that the rock exerts a force on the Earth, equal in magnitude and opposite in direction.
In an airplane, the pilot's seat can be thought of as the hand holding the rock, the pilot as the rock. When flying straight and level at 1 g, the pilot is acted upon by the force of gravity. His weight is. In accordance with Newton's third law, the plane and the seat underneath the pilot provides an equal and opposite force pushing upwards with a force of 725 N. This mechanical force provides the 1.0 g upward proper acceleration on the pilot, even though this velocity in the upward direction does not change.
If the pilot were suddenly to pull back on the stick and make his plane accelerate upwards at 9.8 m/s², the total g‑force on his body is 2 g, half of which comes from the seat pushing the pilot to resist gravity, and half from the seat pushing the pilot to cause his upward acceleration—a change in velocity which also is a proper acceleration because it also differs from a free fall trajectory. Considered in the frame of reference of the plane his body is now generating a force of downwards into his seat and the seat is simultaneously pushing upwards with an equal force of 1450 N.
Unopposed acceleration due to mechanical forces, and consequentially g-force, is experienced whenever anyone rides in a vehicle because it always causes a proper acceleration, and also always a coordinate acceleration. Whenever the vehicle changes either direction or speed, the occupants feel lateral or longitudinal forces produced by the mechanical push of their seats.
The expression means that for every second that elapses, velocity changes metres per second. This rate of change in velocity can also be denoted as per second, or For example: An acceleration of 1 g equates to a rate of change in velocity of approximately for each second that elapses. Therefore, if an automobile is capable of braking at 1 g and is traveling at 35 km/h, it can brake to a standstill in one second and the driver will experience a deceleration of 1 g. The automobile traveling at three times this speed,, can brake to a standstill in three seconds.
In the case of an increase in speed from 0 to v with constant acceleration within a distance of s this acceleration is v²/.
Preparing an object for g-tolerance is called g-hardening. This may apply to, e.g., instruments in a projectile shot by a gun.