Perpetual motion
A perpetual motion machine of the third kind is one that can eliminate effects like friction and other effects that use energy. Because of the inertia of the mass, it can therefore maintain motion forever. This violates the third law of thermodynamics. Although it is impossible to make such a machine, because the loss of energy cannot be completely eliminated in a mechanical system, it is still possible to reduce the loss a great deal.  Even if such a machine could be built, it could only be used to store energy forever, and not to produce energy. A frictionless flywheel, for example, would eventually slow down and stop if its kinetic energy were tapped for useful work, and we would get no more energy out than the amount that was initially put in to spin up the flywheel.