If you made a roller coaster out of tubes and a marble represented the cars where does the marble get the initial energy it uses to go through the tubes?
You ate food.Your body took energy from the food and stored it in your muscles.You transferred the energy from your muscles to the marble, when you LIFTED the marble against gravityand placed it on top of the roller coaster. Up there, the energy it had was potential. As it fell to lower heights,that potential energy became kinetic energy.