As energy changes from one form to another (from light energy to chemical energy to kinetic energy to heat energy, for example), it is never created nor destroyed. This principle is called the Law of Conservation of Energy. Often it is hard to see where the energy of motion (kinetic energy) goes to, if it is not destroyed. Usually kinetic energy is converted to heat energy at points where two things move relative to one another.
Eventually, the wind-up or balloon car will stop rolling. What happens to its energy? When the wind-up or balloon car stops, it has no kinetic energy any more, which seems to suggest that its energy was lost – but the Law of Conservation of Energy says that can’t happen. What actually happens is very subtle. The energy of the wind-up is not destroyed, but it is changed from kinetic energy to a form that cannot be used, mainly heat energy. The heat from friction warms everything around, but so little that we usually don’t notice it.