However, too much G force can be detrimental. Once the G force (in a horizontal direction) reaches about 8 Gs your blood will have become too heavy for your heart to pump around your body. As your brain starves of oxygen your vision is effected, in that you will only be able to see in black and white. If your body is still be exposed to a G force this high your peripheral vision will start to deteriorate until it seems that you are looking down the middle of a drinking straw. After this, you will pass out. At 20Gs your will start to bleed and at 24 Gs the human body can start to tear apart internally. At 40 Gs death is instantaneous. So it is important to regulate the amount of G force that would be experienced by an amusement park patron so their ride is thrilling, not deadly.
A body can experience negative Gs on an amusement park ride also. This happens when a body accelerates toward the earth at a rate equal or more than that of gravity. When a person travels downward on a roller coaster at 9.8m/s there will be no normal force from the roller coaster car acting on them - they are in a zero G environment. To experience negative 1G, the roller coaster needs to go over a hill. On climbing the hill the roller coaster need s to accelerate at a rate of 9.8m/s then when the roller coaster goes over the peak of the hill it will change direction and the human body will experience negative 1G. This is because of inertia - all objects "want- to remain travelling at the same rate and in the same direction. When the roller coaster goes over the hill the body will "want- to keep travelling upward. It would take a force equal to how many negative Gs (1) multiplied by the weight of your body to hold you down. A 70kg person would need a harness exerting a force equal to 70kg to keep them in their seat were their body experiencing negative 1G. Instead of your bodily weight being drawn down toward the Earth it will be drawn out in the same direction that your body was originally travelling in.