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Originally Posted by Canmoore
but doing this would require an external energy source, so right there, it is not a true perpetual motion device.
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No, it requires an initial input of energy, which any perpetual motion device would need. Even the most optimistic of daydreamer inventors assumed that you would have to pull a lever or something to get their invention to start. The difference is that a perpetual motion device can run on that initial energy input forever, rather than requiring continuous energy input to provide continuous motion. All it requires is that all sources of resistance are removed. According to Newton, objects in motion tend to stay in motion. It requires an external force to stop them. In the real world, there is always an external force, generally friction, that causes the machine to do work, thereby expending energy, thereby dissipating the energy of the system and causing it to eventually stop.
Space probes are probably the closest we have gotten to perpetual motion yet.
Voyager will probably keep on going 'till it runs into a star, a very large external force. If not, it will eventually stop due to gravitational drag, or the infinitesimal drag of the near vacuum of space will slow it to a stop. Or, if it's lucky, it'll keep on going until the Big Whimper, or whatever happens at the end of the universe.