What the rider feels has a lot to do with the physics of the coaster. A rider can feel the change in speed that the coaster goes through as it goes up and down hills as energy is transferred between kinetic and potential. Also, riders can feel when the brakes are applied. Another thing that riders may feel are g-forces. g-forces are responsible for the feelings of weightlessness that may be experienced at the top of a hill, and for the heavy feelings felt at the bottom of hills. A g-force equal to one, or 1 g is equivalent to the force of gravity at sea level. If the acceleration of the coaster is equal to gravity (9.81 m/s^2), another g has been added and the equivalent force on the rider will be 2 gs. Weightlessness is caused by negative g-forces, as the ride has accelerated in the opposite direction of gravity. The heavy feeling is caused by larger, positive g-forces. g-forces are what cause people to get sick on roller coasters, as the extreme forces may upset a person's stomach. Too many g-forces are bad for humans, both positive and negative.
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