[Physics] What do we consider “Perpetual Motion”


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?

Best Answer

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.

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