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The answer is the following:
Rapid energy transfer between Potential and Kinetic energy makes roller coasters thrilling.
Potential Energy is the energy related to the vertical position of the roller coaster cart. When it is at the highest point, the Potential Energy is at a maximum.
Kinetic Energy is the energy related to the speed of the roller coaster cart. When the cart is at the highest point, the kinetic energy is at a minimum, but as it begins to go down, the Potential Energy is transformed into Kinetic Energy, making the cart to increase its speed and making it a thrilling experience.