The physics in roller coasters are simple, roller coasters are powered by gravity. As the train of cars on a coaster is pulled--purposely slowly--by a cable to the top of the first hill, potential energy is introduced into the system. Once the train descends the hill, potential energy is converted into kinetic energy. That's why even the most awe-inspiring coasters usually start with the biggest drop--the train has to build up enough energy for the entire ride.

But speed isn't the only factor that makes or breaks a ride on a coaster. Positive gravitational forces, or g-forces, press you into the seat when you're at the bottom of a dip, and so-called negative g's create the sense of weightlessness, or "air time," as you're lifted against the safety bar over the peak of a hill.