The term perpetual motion, taken literally, refers to movement that goes on forever. This is possible in the current theoretical understanding of physics as in Newton's First Law of Motion. However, perpetual motion usually refers to a device or system that delivers more energy than is put into it. Such a device or system would be in violation of the law of Conservation of Energy, which states that energy can never be created nor destroyed, and is therefore deemed impossible by mainstream physics. The most conventional type of perpetual motion machine is a mechanical system which (supposedly) sustains motion whilst inevitably losing energy to friction and air resistance.