G-force, short for gravitational force, is a measurement of acceleration relative to Earth’s gravity. When an object accelerates, it experiences a force that is often described in terms of Gs. One G represents the acceleration due to gravity on Earth’s surface. Negative g-force occurs when the acceleration is in the opposite direction of what is typically considered “down,” or when there is a reduction in perceived weight, leading to a sensation of being lifted or becoming lighter.
Understanding G-Force Direction
The experience of g-forces varies significantly depending on their direction. Positive g-forces occur when acceleration pushes a person into their seat, increasing their perceived weight. This sensation is common during rapid upward acceleration, such as in a rocket launch, or during sharp turns in a vehicle where the force presses you sideways. In these instances, the body feels heavier as blood is pushed towards the lower extremities, away from the head.
Conversely, negative g-forces create a sensation of being lifted out of a seat or feeling significantly lighter, even weightless. This occurs when acceleration is directed downward relative to the body’s orientation, effectively pulling a person upwards. The body experiences a reduction in its apparent weight, as if gravity’s pull has lessened or reversed.
Where Negative G-Forces Occur
Negative g-forces are commonly experienced in various dynamic situations. One familiar example is the crest of a roller coaster hill. As the coaster plunges downward, riders briefly feel a lifting sensation, as if they are being pulled from their seats. This feeling arises because the coaster’s downward acceleration creates a net upward force on the rider.
Another instance occurs in aerobatic aircraft during specific maneuvers. Pilots performing an “outside loop” or a “pushover” maneuver, where the aircraft pitches downward sharply, can experience negative g-forces. Even everyday scenarios, such as a car going quickly over a sharp dip in the road, can elicit a brief, noticeable sensation of negative g-force.
How Negative G-Forces Impact the Body
Experiencing negative g-forces significantly impacts the human body, primarily due to the redistribution of blood. When subjected to negative Gs, blood tends to rush towards the head, neck, and upper torso. This increased blood volume and pressure in the upper body can cause facial congestion, throbbing pains in the head, and a sensation of organs shifting within the abdomen and chest. The eyes may feel as though they are protruding, and a gritty feeling can occur under the eyelids.
A distinct phenomenon known as “redout” can occur, where vision appears to take on a reddish tint, often caused by the lower eyelid engorged with blood. Prolonged or extreme exposure can lead to discomfort, lightheadedness, and even temporary cardiac arrest at around -5 Gs. The body’s tolerance for negative g-forces is considerably lower than for positive Gs, with symptoms appearing between -2 G and -3 G. Excessive intracranial pressure from sustained negative Gs can lead to more severe outcomes, such as permanent retinal damage or a hemorrhagic stroke.