Perpetuum Mobile and Gravitation

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Discussion Overview

The discussion revolves around the law of conservation of energy in the context of tidal power generation, specifically examining the energy transfer between the Earth and the Moon. Participants explore the implications of tidal forces, gravitational potential energy, and the nature of orbits, questioning whether energy loss in one body corresponds to energy gain in another.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants question whether the energy of the tides and the loss of kinetic energy of the Moon are equal, suggesting that understanding this relationship is crucial for explaining tidal power.
  • There is a discussion about how classical theory states that in the absence of friction, a circular orbit can last indefinitely, with momentum direction changes not requiring energy expenditure.
  • One participant argues that changing the direction of momentum does require energy, prompting others to clarify that in a circular orbit, the force applied is perpendicular to the direction of motion, resulting in no work done and no energy expended.
  • Another point raised is that the change in gravitational potential energy due to tidal effects may be a drain on net tidal power rather than a contribution, challenging the initial framing of the energy source.
  • Participants discuss the implications of black holes in this context, questioning whether energy can be extracted from a system involving a black hole without violating known principles of black hole physics.

Areas of Agreement / Disagreement

Participants express differing views on the application of conservation of energy in tidal interactions, with some asserting its validity while others raise questions about the complexities involved. The discussion remains unresolved with multiple competing perspectives on the relationship between tidal energy and gravitational effects.

Contextual Notes

There are limitations in the assumptions made about energy transfer, the definitions of gravitational potential energy, and the effects of tidal forces on the Moon's orbit. The discussion highlights the need for careful consideration of these factors without reaching definitive conclusions.

  • #31
shlosmem said:
humans with their muscles or with their machines
Human made things can certainly change the momentum of objects, without spending energy. A stick connecting two masses rotating in space changes their momentum.
 
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  • #32
shlosmem said:
I’m asking about the process when we apply a force over an object but no energy is spent.

Consider a heavy object resting on a table. The table is applying an upwards force to the object (otherwise the object would move downwards because of gravity) but spends no energy doing so.

The table can support the weight forever without getting tired.
 
  • #33
Ok, Thank you all.
I really appreciate it.
 
  • #34
shlosmem said:
So you are basically saying I can open any screw without spending any energy? this is good news.

No, we are not saying that.

To turn a screw, you will apply tangential forces in opposite directions on opposite sides. As the screw turns, the point of application of these forces moves, so the distance in ##W=Fd## is non-zero. The force is obviously also non-zero, so there's work being done.

It's a different story if the screw doesn't turn. If it's so stuck that the force you're applying doesn't move it, then it's just like the table supporting a weight that I mentioned above - you can lean on the end of your wrench all day without doing any useful work. You will get tired because human muscles burn energy just sitting there, but if you were to hang a weight off the end of the wrench it could sit there forever, twisting the bolt but not moving it.
 

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