Perpetual motion and Newton's first law

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Discussion Overview

The discussion revolves around the concept of perpetual motion in relation to Newton's first law of motion and the laws of thermodynamics. Participants explore the theoretical versus practical implications of perpetual motion, questioning whether it can exist in an idealized scenario without external forces or friction.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions whether the impossibility of perpetual motion is only a practical limitation or if it holds true theoretically as well, referencing Newton's first law.
  • Another participant clarifies that discussions on perpetual motion machines (PMMs) are not permitted on the forum, suggesting that the topic is contentious and often misunderstood.
  • A different viewpoint suggests that while a ball could theoretically move indefinitely in a vacuum, any attempt to extract energy from it would lead to a loss of motion.
  • Another participant argues that the non-existence of PMMs is rooted in the laws of thermodynamics rather than Newton's laws, explaining that the first law relates to energy conservation and the second law addresses energy loss due to friction or resistance.
  • This participant distinguishes between two types of PMMs: those that draw power (first kind) and those that do not (second kind), noting that the former is a theoretical issue while the latter is practical.

Areas of Agreement / Disagreement

Participants express differing views on the nature of perpetual motion, with some emphasizing theoretical aspects while others focus on practical limitations. There is no consensus on the implications of Newton's laws versus thermodynamic principles regarding perpetual motion.

Contextual Notes

Participants highlight the distinction between theoretical scenarios (like an ideal vacuum) and practical realities (like energy loss due to friction), indicating that assumptions about ideal conditions may not hold in real-world applications.

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This might be a very stupid question. But let me still ask. :)

Perpetual motion is not possible. But by Newton's first law, a body remains in its state of motion unless an external force acts on it.

Now, I wonder if the statement "perpetual motion is not possible" is true only in a practical context, or even theoretically? I remember reading one of the initial chapters on Feynman lectures, that to me, sounded like perpetual motion is not possible even in theory.

So assuming an ideal world of no friction, I roll a ball with some force. And then stop the force. It should keep moving at the same velocity, right? Isn't that perpetual motion?

I know I have some notion that is completely wrong. But can't figure out what. I appreciate any help in clarifying the same.

Thanks!
 
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To clarify moving forever and perpetual motion machines (AKA free energy) are two different things. If you had a hypothetically infinite vacuum tunnel and pushed a ball down it it would travel indefinately. The moment you try to get any energy out of it though you will slow it down and eventually stop it.
 
The "non-existence" of perpetual motion machines is not a consequence of Newton's laws but of the laws of thermodynamics.

The non-existence of "perpetual motion machines of the first kind", from which one can draw power, is a consequence of the first law of thermodynamics, the conservation of energy. The machine has only a finite amount of energy to begin with. If you are drawing energy out of it, eventually, it will have no more energy.

The non-existence of "perpetual motion machines of the second kind", from which one does NOT draw power, is a consequence of the second law of thermodynamics, the fact that there always exist friction or resistance so that even if we do not draw power, there is still a loss of energy resulting in the machiner eventually running out of energy.

You can consider the first to be a "theoretical" matter and the second "practical".
 

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