“motion that continues indefinitely without any external source of energy; impossible in practice because of friction.”

Perpetual motion describes “motion that continues indefinitely without any external source of energy; impossible in practice because of friction.” It can also be described as “the motion of a hypothetical machine which, once activated, would run forever unless subject to an external force or to wear”.There is a scientific consensus that perpetual motion in an isolated system would violate the first and/or second law of thermodynamics.