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Question on Orbital Motion and Conservation

  1. Oct 1, 2005 #1
    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?
  2. jcsd
  3. Oct 2, 2005 #2


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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.
  4. Oct 2, 2005 #3


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    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.
  5. Oct 2, 2005 #4


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    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.
  6. Oct 2, 2005 #5


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    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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