According to the law of conservation of energy, energy is not gained or lost but simply converted from one form to another. When a car is at the top of the roller coaster it is moving slowly but has a high level of potential energy. As it accelerates under the force of gravity the potential energy is converted to kinetic energy. This kinetic energy is then turned back into potential energy as the car loses speed and gains height. The amount of potential energy is less than it had at the previous high spot because some of the energy has been turned into heat.