G-Force The g-force of an object is its acceleration relative to free fall. On earth this is 1g, or 9.8 meters per second squared (m/s^2) or equivalently 9.80665 newtons of force per kilogram of mass (N/kg). Astronauts experience unusually high and low g-forces. G-force can also be seen on rollercoasters. When the coaster goes down the drop, you are pushed back into your seat because of g-force. Negative g-forces is when gravity is pushing you downwards causing you to feel weightless. (Going down a rollercoaster) Positive g-forces is when gravity is pushing against you causing you to feel heavier. (Going up a rollercoaster) The g-force or gravitational force equivalent is a mass-specific force (force per unit mass), expressed in units of standard gravity (symbol g or g0, not to be confused with "g", the symbol for grams). 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 9.8 m/s2. 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. (Free fall is the path that the object would follow when falling freely toward the Earth's center). 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 (in vacuum), or (to good approximation) inside a spacecraft in Earth orbit. These are examples of coordinate acceleration (a change in velocity) 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 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 (a downward force) is 725 newtons (163 lbf). 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 (this is similar to the situation of a person standing on the ground, where the ground provides this force and this g-force). 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 1,450 N (330 lbf) 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 (in the absence of gravity) also always a coordinate acceleration (where velocity changes). Whenever the vehicle changes either direction or speed, the occupants feel lateral (side to side) or longitudinal (forward and backwards) forces produced by the mechanical push of their seats. The expression "1 g = 9.80665 m/s2" means that for every second that elapses, velocity changes 9.80665 metres per second (35.30394 km/h). This rate of change in velocity can also be denoted as 9.80665 (metres per second) per second, or 9.80665 m/s2. For example: An acceleration of 1 g equates to a rate of change in velocity of approximately 35 km/h (22 mph) 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, 105 km/h (65 mph), 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/(2s). Preparing an object for g-tolerance (not getting damaged when subjected to a high g-force) is called g-hardening. This may apply to, e.g., instruments in a projectile shot by a gun.