Question on Orbital Motion and Conservation

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Discussion Overview

The discussion revolves around the concepts of orbital motion, gravitational forces, and the conservation of energy in the context of the moon's orbit around the Earth. Participants explore the implications of these concepts on energy expenditure and the nature of gravitational forces.

Discussion Character

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

Main Points Raised

  • One participant suggests that the force required to deviate the moon from its straight-line path implies a significant energy expenditure over billions of years, raising questions about the source of this energy.
  • Another participant counters that the exertion of gravitational force does not require energy, as gravity acts on the moon without needing an external energy source.
  • A further contribution notes that an object in orbit maintains a condition of constant potential energy, with energy exchanges occurring due to the elliptical nature of the orbit, affecting kinetic and potential energy.
  • One participant elaborates on the concept of work, explaining that energy is only expended when a force moves an object in the direction of that force, and in the case of the moon, gravitational force acts at a right angle to its motion in a circular orbit.
  • Another participant introduces the curved-spacetime perspective, stating that the moon moves in a straight line within curved spacetime created by Earth's mass, and mentions the loss of angular momentum from Earth to the moon due to their mutual attraction.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between gravitational force and energy expenditure, with no consensus reached on the implications of these concepts for the moon's orbital motion.

Contextual Notes

Participants discuss various interpretations of gravitational forces, energy, and orbital mechanics, with some assumptions about the nature of orbits and energy exchanges remaining unresolved.

BigMacnFries
To make the moon deviate from it's straight line path requires a force and I assume this force requires energy. Considering the mass of the moon and the billions of years it has been orbiting the Earth this seems like a tremendous amount of energy expended. With regard to the conservation of energy law where does this energy come from?
 
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The exertion of a force does not require energy. The Earth simply pulls on the moon because that's what gravity does. That in itself does not require any energy source.
 
Further, for an object to remain at the same distance (roughly) from earth, it remains in a condition of constant potential energy. In actuality, the orbit is slightly elliptical, leading to an exchange of potential and kinetic energy during every orbit.
 
Galileo said:
The exertion of a force does not require energy. The Earth simply pulls on the moon because that's what gravity does. That in itself does not require any energy source.

if i may be a little more anal in this explanation, energy (or "work") is expended only when the force moves an object along the same direction of the force. this is what the dot-product is about in defining work or energy. you can break up the movement vector into two componets: one that is aligned with the force (or in the opposite direction) and one component that is at a right angle with the force. the componet of motion that is aligned with the force will have some change of energy associated with it. the component that is at 90o with the force will have no change of energy associated with it. if the moon was orbiting in a circular orbit (it isn't, but let's say it is), then the force of gravity with the Earth and the motion of the moon are always at a right angle.
 
If you look at it in the curved-spacetime approach to gravity, the moon is moving in a straight line. The mass of Earth provides the curvature. IIRC, the Earth actually loses a tiny amount of angular momentum to the moon, due to mutual attraction (tides and whatnot). It's out of my area, though, so I'd appreciate some additional input myself.
 

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