G-forces are used for explaining the relative effects of centripetal acceleration that a rider feels while on a roller coaster. Consequently, the greater the centripetal acceleration, the greater the G-forces felt by the passengers. A force of 1 G is the usual force of the Earth’s gravitational pull that a person feels when they are at rest on the Earth’s surface; in other words, it can be described as a person’s normal weight. When a person feels weightless, as in free fall or in space, they are experiencing 0 G’s. When the roller coaster train is going down a hill, the passengers usually undergo somewhere between 0 and 1 G. However, if the top of the hill is curved more narrowly than a parabola, the passengers will experience negative G’s as they rise above the seat and get pushed down by the lap bar. This is because gravity and the passengers’ inertia would have them fall in a parabolic arc. G-forces greater than 1 can be felt at the bottom of hills as the train changes direction. In this case the train is pushing up on the passengers with more than the force of gravity because it is changing their direction of movement from down to up. G-forces that are felt when changing direction horizontally are called lateral G’s. Lateral G’s can be converted into normal G-forces by banking turns.