Perpetuo Mobile

This is a response to an E-mail question from a student in a non-major physics course.
Q. Who first started with experiments for perpetual motion? Do you know any scientific law that can prove that this idea is absolutely impossible?
Perpetual motion machines are forbidden by the first two laws of thermodynamics.

There are two types:

  • the type which generates energy from nothing (forbidden by the First Law) and
  • the type which, once charged with energy, uses the same energy forever without dissipating any of it (forbidden by the Second Law).

So-called "working" perpetual motion machines, such as certain magnetic tops, minimize energy dissipation -- typically friction -- in some way. But none of them do it perfectly and they will all run down eventually; notice that these magnetic tops require batteries. Claims that one can extract energy from such machines are totally bogus and I will state categorically that no one has ever done so.

Theoretically one could minimize friction perfectly, but at the cost of not allowing one to extract energy from the machine (since all interactions generate "dissipative forces" which sap energy according to the Second Law).

For example, an object traveling through a perfectly empty universe is in "perpetual motion" but relative to what?
Check out Richard's Perpetual Motion Page.

Copyright © 1999, 2000 by Daniel J. Berger. This work may be copied without limit if its use is to be for non-profit educational purposes. Such copies may be by any method, present or future. The author requests only that this statement accompany all such copies. All rights to publication for profit are retained by the author.

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