Question on Orbital Motion and Conservation

[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?

 

[Galileo]

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.

 

[russ_watters]

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.

 

[rbj]

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 90^o 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.

 

[Danger]

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.