Gravity is the driving force
of a roller coaster. From the moment
the roller coaster train passes the peak of the lift hill, it is the
acceleration due to gravity that brings it back to the beginning. When the train is released from the top of
the lift hill, gravity pulls it down.
The train begins slowly, then picks up speed as it approaches the bottom
of the hill. As it begins to climb the
next hill, the speed decreases. This is
because of the acceleration due to gravity, which occurs at 9.80m/s^{2}
straight down toward the center of the Earth.

The initial hill, or the
lift hill, is the tallest in the entire ride.
As the train is pulled to the top, it is gaining potential, or stored
energy. The higher the lift, the greater
the amount of potential energy gained by the train. This is shown by the equation for potential energy:

U_{g
}= mgh

Where U_{g} is
potential energy, m is mass in kilograms, g is acceleration due to gravity, and
h is the distance above the ground in meters.
Because mass and gravity are constant for the train, if the height of
the train above the ground is increased, the potential energy must also
increase. This means that the potential
energy for the roller coaster system is greatest at the highest point on the
track: the top of the lift hill.

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