Perpetual motion machine - Intuitive meaning

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Discussion Overview

The discussion revolves around the concept of perpetual motion machines, particularly in the context of Feynman's explanation of a weight-lifting machine. Participants explore the implications of conservation of energy, the definitions of perpetual motion, and the conditions under which such machines are theorized to operate.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants seek clarification on Feynman's statement regarding the net result of lifting a weight and how it relates to the definition of a perpetual motion machine.
  • Others argue that a perpetual motion machine is defined as one that can return to its original state while having done useful work, without any energy loss.
  • There is a discussion about the classification of perpetual motion machines into three types, with references to how they supposedly generate energy or do work.
  • Some participants express skepticism about the existence of such machines, emphasizing that they contradict the law of conservation of energy.
  • A few participants mention examples of systems that utilize regenerative braking, questioning if they can be considered perpetual motion machines.
  • One participant suggests that Feynman's wording may be misleading or self-contradictory, leading to confusion about the concept.

Areas of Agreement / Disagreement

Participants generally agree that perpetual motion machines, as defined by Feynman and the laws of physics, cannot exist. However, there are multiple competing views regarding the interpretation of Feynman's explanation and the implications of energy conservation in practical examples.

Contextual Notes

Some participants highlight the importance of definitions and assumptions in discussing perpetual motion machines, noting that misunderstandings can arise from ambiguous wording or interpretations of energy states.

  • #31
CWatters said:
Aleoa.. Can I check you understand that you understand something...

In the following I will assume there is no friction.

If the net torque on the balance is zero this means the balance is not accelerating. That does not mean the balance is stationary.

If the balance starts off stationary then it will remain stationary.

If the balance starts off rotating at a constant angular velocity it will continue rotating at a constant velocity. Even though it keeps rotating this does not mean it is a perpetual motion machine.

It's possible to make a machine that keeps rotating forever (eg two stars in orbit around each other) but this is not what scientists mean by a perpetual motion machine. To be a perpetual motion machine it must also be capable of doing some work even if that's small. Otherwise it is useless.

If i apply the formula of the moment , what i note is that a balanced lever is balanced in every possible angle of the lever with the ground. Is this true in the ideal case ?
 
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  • #32
Aleoa said:
If i apply the formula of the moment , what i note is that a balanced lever is balanced in every possible angle of the lever with the ground. Is this true in the ideal case ?
Yes.
 
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  • #33
CWatters said:
Yes.

And so, an ideal lever is reversible because, employing the same amount of energy used to change the state of the system i can go back to the initial state ?
 
  • #34
No. If your system takes X units of energy to move in one direction and another X units to move it back then it will have consumed a total of 2X units of energy. So it's not reversible.

In a reversible system when it returns to the starting position/condition it will have consumed zero units of energy.

In the case for an ideal balance lever system it takes energy to get it moving (you give it KE) but you get that energy back when you stop it in the new position. So rotating it one way consumes zero energy and rotating it back again also consume zero energy. So it arrives back where it started having consumed zero energy. So an ideal balanced lever is a reversible system but not for the reason you gave.
 
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  • #35
CWatters said:
...by its regenerative braking system.

It couldn't be in the same state anyway because of the brakes would generate heat because.
 
  • #36
bland said:
It couldn't be in the same state anyway because of the brakes would generate heat because.
I said "regenerative braking system" for a reason. Perhaps I should have said "ideal regenerative braking system".

Anyway this is irrelevant to the point I was making. That gravity is conservative.
 

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