G-force, or gravitational force equivalent, is a measure of acceleration relative to Earth’s gravity, often associated with rapid changes in motion. When this acceleration acts in a direction that opposes the typical downward pull of gravity, it creates a phenomenon known as negative G. This sensation involves feeling lighter than normal, or even lifted, and is distinct from the feeling of being pushed into a seat which is associated with positive G.
Defining Negative G
Negative G refers to an acceleration that pushes a body upwards, away from a surface, creating a sensation of decreased weight or being lifted, in contrast to positive G which pushes the body downwards, increasing the feeling of weight. The “G” in G-force stands for the acceleration due to Earth’s gravity, which is approximately 9.8 meters per second squared (m/s²) or 32 feet per second squared (ft/s²). When experiencing -1G, an individual feels weightless, similar to being in freefall, and if the force increases to -2G, the sensation is one of being lifted with twice one’s normal weight. This occurs because the acceleration is directed opposite to the normal force of gravity, effectively reducing or even reversing the perceived gravitational pull. Negative G is essentially an upward acceleration or downward deceleration that causes this sensation.
The Human Experience of Negative G
Experiencing negative G can lead to a variety of physical sensations, including a feeling of weightlessness or a “stomach lurching” sensation, as internal organs briefly shift upwards within the body. If the negative G-force is strong enough, it can cause an individual to feel actively pushed up out of their seat, highlighting the need for restraints in vehicles capable of such maneuvers. A specific physiological effect of sustained negative G is “redout,” where vision takes on a reddish tint because the upward acceleration forces blood to rush towards the head, increasing pressure in the capillaries of the eyes. Unlike positive G, which can lead to blood pooling in the lower body and potential “blackout,” negative G causes blood to pool in the head, potentially leading to discomfort, swelling of the face, and even burst blood vessels in the eyes or brain. This makes prolonged or extreme negative G generally more unpleasant and potentially more dangerous than positive G.
Common Encounters with Negative G
Negative G is encountered in various everyday situations, often without conscious recognition. A common example is the crest of a roller coaster hill, where the vehicle accelerates downwards faster than gravity, causing riders to feel lifted from their seats, and the sensation of weightlessness or “airtime” on a roller coaster is a direct result of experiencing negative G. Driving over a sharp dip in the road can also produce a brief sensation of negative G, making occupants feel lighter, and similarly, the initial descent of an elevator can create a momentary feeling of reduced weight. In more extreme contexts, aerobatic flying maneuvers, such as inverted flight or outside loops, involve pilots intentionally experiencing significant negative G-forces. These experiences, from mild to intense, illustrate how negative G affects the human body in various scenarios.