I know this is a bad question to most serious Physics but I have a question about what is considered “Perpetual motion.” The Foucault pendulum in the UN consists of sphere that passes directly over a raised metal ring at the centre that contains an electromagnet, which induces a current in the copper inside the ball. This supplies the necessary energy to overcome friction and air resistance and keeps it swinging uniformly. Now the swing of the pendulum is induced but the 36h 45m clockwise shift generated bay the earths rotation is perpetual or as long as the earth rotates. Is this assumption correct?

Does a generator that works on tidal movements not fall in to the same assumption?

  • 1
    $\begingroup$ It's not a bad question to ask what perpetual motion means in physics. If you were asking how to build a perpetual motion machine (or even "look at my perpetual motion machine!"), that's what would make people start complaining. $\endgroup$
    – David Z
    Commented Jun 19, 2011 at 7:14
  • $\begingroup$ For the classification of a perpetual motion machine of the $n$'th kind, see this wikipedia page: en.wikipedia.org/wiki/Perpetual_motion#Classification $\endgroup$
    – Qmechanic
    Commented Jun 19, 2011 at 8:08
  • $\begingroup$ The real intent of my question was to verify that force added by nature does not make it "perpetual motion". I get a kick from watching the Youtubes on "free energy" sometimes is not evident where the extra energy is coming from. $\endgroup$
    – Fortunato
    Commented Jun 19, 2011 at 19:31

4 Answers 4


No, neither of the systems you've brought up is an example of what physicists mean when they say "perpetual motion."

The catch is that in physics, perpetual motion doesn't mean what you might literally interpret it to mean - that is, it doesn't just refer to something that is able to keep moving forever. The laws of physics have no problem with the idea that something can be in motion perpetually; for example, a rock moving at constant velocity through an otherwise empty universe. What a physicist means by perpetual motion is a system that is able to maintain its original state of motion, without being subject to a driving force, in the presence of dissipative forces like friction or bremsstrahlung radiation. A driving force adds energy to the system, whereas dissipative forces take energy away, so if there are dissipative forces but no driving forces, the system's energy has to decrease over time, and thus it can't keep moving in its original pattern. Therefore this particular meaning of perpetual motion is impossible.

In reality, all physical systems are subject to dissipative forces, so it is impossible to construct a machine that maintains its original motion without an external source of energy.

The Wikipedia article on perpetual motion does a very nice job of explaining this in more detail. I suggest you take a look at it.

  • $\begingroup$ The way I'd put it is that what's forbidden is not perpetual motion but perpetual motion machines. The Voyager spacecraft, having achieved escape velocity will remain in perpetual motion (relative to the Solar System). That's no problem. But it's not a machine -- in the sense that it can't perpetually supply useful work to anything. $\endgroup$
    – Ted Bunn
    Commented Jun 19, 2011 at 14:10

The term "perpetual motion" signifies a machine that can keep moving indefinitely without any additional energy inputs, without losing kinetic energy. The Foucault pendulum you describe has additional energy inputs (from the electromagnet), to overcome friction and air resistance, as you say, so it's not a perpetual-motion machine.

Perpetual-motion machines cannot exist, according to the laws of thermodynamics.

By extension, the phrase "perpetual motion machine" now refers to any machine that claims to break / bypass the law of entropy, whether or not it actually includes motion.


From the webster.com dictionary , perpetual:

a : continuing forever : everlasting

In our universe as far as we have learned, nothing continues forever. So the definition of perpetual motion is a relative definition in time.

For a human lifetime the earth will be continuously turning on its axis, as well as with the planets around the sun. An atom will have its electrons in continuous "motion" around the nucleus,( until the expansion of the universe destroys everything,) etc

In that sense, a generator absorbing the energy of the tides is taking advantage of this, limited in definition, perpetual motion. In detail the energy turned into electricity is taken infinitesimally out of the angular momentum of the earth, so the perpetual motion of the earth is delayed by a tiny bit.

The perpetual motion machines on the other hand that crank inventors try to demonstrate, would get more energy out than the one coming in, thus breaking the law of conservation of energy.


From Wikipedia: "Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether they might operate indefinitely or not."

The key is not moving indefinitely, but DOING WORK. People didn't build perpetual motion machines to observe inertial movement, but try to power factories, transportation, etc without any energy source.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.