G-force, or gravitational force equivalent, measures the acceleration or deceleration experienced by an object relative to Earth’s standard gravity. When a body is at rest, it experiences one G, which provides our normal sense of weight. Experiencing G-forces means an outside mechanical force is acting on the body, causing a change in speed or direction that alters our perceived weight. This shift in apparent weight is the root of the intense physical sensations associated with high acceleration.
What Exactly is a G-Force?
The G-force unit is based on the acceleration due to gravity on Earth, where one G is approximately 9.8 meters per second squared. G-forces are not about speed alone, but rather the rapid change in velocity, which is acceleration or deceleration. For instance, a car traveling at a constant 100 miles per hour exerts no extra G-force on its occupants. However, slamming the brakes or taking a sharp turn creates a sudden, measurable G-force.
The sensation of a G-force comes from inertia, which is the resistance of your body’s mass to a change in motion. When a vehicle accelerates, the seat pushes on your body to change its direction, and your body pushes back, making you feel heavier or lighter. In simple terms, a force of four Gs means every part of your body weighs four times its normal amount in the direction of the force.
The Feeling of Being Pressed Down (Positive G-Forces)
The most common and most studied G-force is positive Gz, where the force acts from the head toward the feet. This is typically felt during an upward launch or a tight turn in an aircraft. The sensation is characterized by feeling impossibly heavy, as if the force of gravity has suddenly multiplied. A person weighing 150 pounds would feel an apparent weight of 750 pounds under five Gs.
The pressure exerted by positive G-forces makes the limbs feel like lead, and even simple movements become nearly impossible. Breathing becomes labored because the increased apparent weight of the chest cavity and diaphragm resists the expansion necessary for inhalation. The soft tissues of the face and cheeks are noticeably pulled downward, causing the skin to sag toward the chin.
The most dangerous effect of sustained +Gz is on the circulatory system, as the force pulls blood down toward the lower extremities and abdomen. The heart struggles to pump blood upward to the brain against the overwhelming force, leading to a significant drop in cerebral blood pressure. As blood drains from the head, visual effects begin, progressing from “greyout” (loss of color and peripheral vision) to “blackout” (complete temporary loss of sight). These visual symptoms warn that the brain is being starved of oxygen, leading to G-force induced Loss Of Consciousness (G-LOC).
The Feeling of Being Lifted (Negative and Lateral G-Forces)
Negative G-forces, or -Gz, act in the opposite direction, from the feet toward the head, creating the uncomfortable sensation of being lifted out of a seat or experiencing weightlessness. This occurs when a vehicle crests a hill or when an aircraft performs a downward maneuver, causing the seat belt to press down hard. The body’s organs also feel like they are floating upward inside the torso.
Unlike positive G-forces that drain blood from the head, negative G-forces cause blood to rush and pool in the head and face. This sudden influx causes painful pressure in the sinuses and eyes, and the face swells noticeably. The high pressure can lead to “redout,” a visual phenomenon where the entire field of vision turns reddish due to the engorgement of capillaries in the eyelids and retina.
Lateral G-forces, known as Gx or Gy, push the body either from front to back or side to side, and they are common in high-speed turns or collisions. These forces do not create the same circulatory issues as vertical G-forces because the blood is not being pulled along the long axis of the body. Instead, lateral forces cause significant skeletal and muscular stress, often resulting in whiplash or muscle strain as the body is violently shoved sideways against its restraints.
How Much G-Force Can the Body Tolerate?
Human tolerance to G-forces is highly dependent on the magnitude, duration, and direction of the force. For an untrained individual, a sustained positive G-force of four to six Gs is typically the limit before consciousness is lost due to blood pooling.
Highly trained pilots utilize specialized G-suits that compress the lower body to restrict blood pooling. These suits artificially increase pressure, assisting the heart’s effort to keep blood in the upper torso and head, allowing pilots to endure sustained forces of up to nine Gs for several seconds. Body positioning also plays a significant role. G-forces acting from the chest to the back (Gx), such as during a rocket launch, can be tolerated at much higher levels, often exceeding ten Gs for a brief period, because the blood column is shorter.
Momentary or transient G-forces, such as those experienced during a sudden impact or a crash, can be much higher without causing death, though severe injury is likely. Pioneering research demonstrated that the human body could survive a momentary deceleration exceeding 46 Gs. However, such forces measure the body’s structural limits, not its long-term physiological tolerance.