<P> At the top of the loop, the force of the car's acceleration pushes the passenger off the seat toward the center of the loop, while inertia pushes the passenger back into the seat . Gravity and acceleration forces push the passenger in opposite directions with nearly equal force, creating a sensation of weightlessness . </P> <P> At the bottom of the loop, gravity and the change in direction of the passenger's inertia from a downward vertical direction to one that is horizontal push the passenger into the seat, causing the passenger to once again feel very heavy . Most roller coasters require passengers to wear a safety harness, but the forces exerted by most loop - the - loop coasters would keep passengers from falling out . </P> <P> G - forces (gravitational forces) create the so - called "butterfly" sensation felt as a car goes down a gradient . An acceleration of 1 standard gravity (9.8 m / s) is the usual force of Earth's gravitational pull exerted on a person while standing still . The measurement of a person's normal weight incorporates this gravitational acceleration . When a person feels weightless at the top of a loop or while going down a hill, they are in free fall . However, if the top of a hill is curved more narrowly than a parabola, riders will experience negative Gs and be lifted out of their seats, experiencing the so - called "butterfly" sensation . </P> <P> A wooden coaster has a track consisting of thin laminates of wood stacked together, with a flat steel rail fixed to the top laminate . Steel coasters use tubular steel, I beam or box section running rails . The supporting structure of both types may be steel or wood . Traditionally, steel coasters employed inversions to thrill riders, whereas wooden coasters relied on steep drops and sharp changes in direction to deliver their thrills . However, recent advances in coaster technology has seen the rise of hybrid steel coasters with wooden structures, an example being The New Texas Giant at Six Flags Over Texas and, wooden coasters that feature inversions, an example being Outlaw Run at Silver Dollar City . </P>

When do riders free-fall on a roller coaster
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