Many questioners confuse perpetual motion with perpetual motion machines. Here is an attempt to separate the two, and give some answers about each:
Perpetual Motion, in the absolute sense can be seen as an extension of
Newton's 1st Law (The Law of Inertia): "Unless acted upon, a body at rest stays at rest, and a body in motion stays in motion," as well as the
1st Law of
Thermodynamics: "Unless acted upon, the total energy of a system is constant." These Laws suggest that a body traveling through space would move at constant velocity in perpetuity until acted u
pon by some outside agent. Unfortunately, space isn't quite a void, and there is no true frictionless motion, so all bodies are "acted upon" constantly.
Separate from the theoretical idea of perpetual motion is the mechanical fallacy of the Perpetual Motion Machine. The inaccessibility of perpetual motion is further confounded by the 2nd Law of Thermody
namics: "No system operating in a cycle can be absolutely efficient." This hasn't stopped several people from attempting to devise machines that either run forever (violating this 2nd Law), or produce energy without consuming themselves as they run (
also violating the 1st Law).
(Check out Don Simanek's Museum of
Unworkable Devices for some of history's more notorious perpetual