Inertia

The extraordinary sensations that you feel in a roller coaster are caused by a force called INERTIA. At each turn, rise or fall, the train changes it's speed and you will feel the g force.

The inertia acts on all your body parts, in conclusion, on your internal organs too, which have fragile connections. When you are falling, your stomach and your intestines move up and press the base of your lungs. When you go up, all your internal organs are being pushed in the opposed way.

When you reach the base of a slope in a big roller coaster, the inertia trebles your weight!

At the top of a roller coaster's slope you will feel negative inertia. It acts on the opposed way of gravity, being able to cancel your weight and make you feel like you are floating. In the first and the last carriage you can feel a bigger negative inertia, as those go faster.
At the bottom of the slope you will feel positive inertia. This multiplies the force of gravity and this is why you feel like you are pressed hard in your chair. The biggest positive inertia can be felt in the carriages from the middle of the roller coaster, as those ones have the biggest speed traveling through the "valley".

A Jet Fighter pilot can tolerate up to 9 g's at sudden turns. This huge force pushes the blood towards the legs, and the pilot might faint. This is why pilots wear pressurized costumes and strain their legs' muscles.

In 1954, the american scientist John Paul Strapp voluntary subdued himself to an astonishing 46.2 g's. He was strapped to a sledge powered by rocket engines that went on train tracks. He stopped from 1017 km/h to zero in 1.25 seconds (equivalent to hitting a wall at 190 km/h). He survived, but the blood vessels in his eyes broke, causing him temporary blindness.