Feynman: Reversible machines, no perpetual motion?

  • #1

Main Question or Discussion Point

Greetings,

I have begun reading the Feynman Lectures to repeat the most important ideas from my undergraduate studies and to improve my intuitive understanding of physics.

In one of the first chapters, the one about the conservation of energy, he demonstrates that the conservation of energy is equivalent to the statement that there can be no perpetual motion.
Even an ideal (=reversible) machine can not drive an external motion perpetually under the assumption of conserved energy (and vice versa, under the assumption of no perpetual motion, energy is conserved in that reversible machine).

What I am asking myself is: Isn't a reversible machine in itself not a possibe perpetual motion?
He defined perpetual motion with respect to an external object that moves because its driven by the machine.
But what about the machine itself? It can go from A to B, but also from B to A without the need of energy since it is reversible. In principle, if reversible machines existed one should be able to design one that moves perpetually like A-->B-->A-->B-->A....

What am I missing?

Regards
 

Answers and Replies

  • #2
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Greetings,

I have begun reading the Feynman Lectures to repeat the most important ideas from my undergraduate studies and to improve my intuitive understanding of physics.

In one of the first chapters, the one about the conservation of energy, he demonstrates that the conservation of energy is equivalent to the statement that there can be no perpetual motion.
Even an ideal (=reversible) machine can not drive an external motion perpetually under the assumption of conserved energy (and vice versa, under the assumption of no perpetual motion, energy is conserved in that reversible machine).

What I am asking myself is: Isn't a reversible machine in itself not a possibe perpetual motion?
He defined perpetual motion with respect to an external object that moves because its driven by the machine.
But what about the machine itself? It can go from A to B, but also from B to A without the need of energy since it is reversible. In principle, if reversible machines existed one should be able to design one that moves perpetually like A-->B-->A-->B-->A....

What am I missing?

Regards
Sure, but that isn’t what defines a perpetual motion *machine*. A perpetual motion machine needs to do work, not just move.
 
  • #3
CWatters
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+1

Otherwise the motion of planets would count as pm.
 
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