G-force is an attractive force caused by the acceleration due to gravity on one object by another object. Positive g-force increases the apparent weight of a body while negative g-force decreases the apparent weight of a body.
On the Earth's surface, the acceleration due to gravity on a body at rest is expressed as 32.174 feet per second-squared, or 1 G. At the Earth's surface, 1 G allows humans to perform normal activities and bodily functions without the difficulty imposed by greater g-forces.
Twice the acceleration due to gravity or 2 G's, raises the apparent weight of the human body by a factor of two. This g-force can be experienced at many different points on a roller coaster ride. Sustaining higher than normal G's can have serious effects on normal body functioning. Fighter pilots can experience approximately 9 G's in violent maneuvers, but without proper training and the use of anti-G suits, blood flow to the eyes and brain can become restricted to the point that blackout occurs. The human body can sustain higher positive G's than negative G's.
Negative G's reduce the apparent weight of the human body. This is typically experienced in free-fall or in negative G-inducing maneuvers by aircraft and roller coasters. The human body cannot sustain high negative G's, as the blood can pool in the head and lead to a phenomenon called redout.
The value of the g-force varies on other planetary surfaces according to the mass of the planetary body. For example, Jupiter is the most massive planet in the solar system and has a larger g-force value than that of Earth. The force of gravity on Jupiter is 2.5 times that experienced on the Earth's surface, which would make a human weigh 2.5 times as much as on Earth, so that tasks such as walking and lifting objects would be more difficult to perform.