Final answer:
At the top of the second hill, the roller coaster has both potential and kinetic energy, with gravitational potential energy being due to its height and kinetic energy being present due to its motion.
Step-by-step explanation:
At the top of the second hill of a roller coaster, the car possesses both potential energy and kinetic energy. The potential energy is due to the car's elevated position above Earth's surface, which is gravitational potential energy. Even though the roller coaster is moving slowly at this point, it still has some kinetic energy, as kinetic energy is the energy of motion. Due to the law of conservation of energy, the total mechanical energy of the roller coaster remains constant throughout the ride if no energy is lost to friction. Thus, as the coaster climbs the second hill, some of the kinetic energy it gained while descending the first hill is converted back into potential energy, causing it to slow down.