mass of the object and the speed of the object. As the cars lose speed, they also lose kinetic energy, but that does not stop the whole thing, inertia is what keeps the cars moving. While the cars might slow down when they approach a new hill, it is inertia which moves it forward. Once cars go through loops, turns and smaller hills, the only forces that act upon the cars are the force of gravity, the normal force and dissipative forces such as air resistance.
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