Why orbit?

[hawksmere]

Sorry if this is an elementary level question but why exactly does the earth (to use one example) rotate (orbit) around the sun. I am aware of the 'straight' line the earth travels in and the bending so spacetime which creates the orbit, or is that incorrect? Someone told me it has more to do with the natural interactions between to most dominant atomic particles that have generated enough mass to act as an orbit. I don't agree with the example given but kind of understand it.

 

So does the sun actually create the curving of space time (assuming this is more on the lines of being the correct explanation) and earth happens to fall into a geodesic in its third dimension. Or am I completely incorrect?

 

 

[Realitycheck]

The orbit is defined as the net result of two opposing phenomena, gravity, which pulls the earth towards the sun, and centrifugal force, which pulls the earth outward as it orbits.

[hawksmere]

but does that force (gravity) fluctuate, as in would fluctuations not mean the orbit is staggerred and not smooth? So the earth is pulling away from the sun but the gravity keeping it in orbit? So the fact that the sun bends spacetime and creates a geodesic field have nothing to do with it or are they interlinked?

[Realitycheck]

There are irregularities in the orbit, but is not major and not real noticeable. Plus, inertia has a smoothing effect on the orbit. No, the sun's effect on spacetime is kind of independent of the orbit of the earth. If the sun were in between you and an object you were observing or near that plane, it could have a very minute effect on what you were seeing.

Edited September 15, 2011 by Realitycheck

[hawksmere]

Technically the planet is moving in a straight line through curved spacetime, and experiences no centrifugal acceleration as there is no friction in space. So the earth is constanly falling but has been captured by the sun's gravity. The earth orbits at a=vsq/R we need to establish the gravitational constant and the mass of the sun. So the closer you get to the sun the quicker the orbit is as G M = Rv squared. Therefore the Sun is actually curving spacetime and the rate of the orbit is calculated by Newtons first and second laws of motion. I understand the gravitational forces that are active but as a result spacetime is curved. Surely.

[swansont]

You can view this in terms of GR but it's not necessary — Newtonian physics covers it, and they are two views of the same thing. The possibility of closed orbits is a consequence of an inverse-square force.

[michel123456]

Isn't the question "why orbit and not spiral falling into the Sun?"

[granpa]

en.wikipedia.org/wiki/Newton's_theorem_of_revolving_orbits Edited September 16, 2011 by granpa

[michel123456]

en.wikipedia.org/wiki/Newton's_theorem_of_revolving_orbits

 

This? (from grandpa's link)

Closed orbits and inverse-cube central forces

Two types of central forces—those that increase linearly with distance, F = Cr, such as Hooke's law, and inverse-square forces, F = C/r2, such as Newton's law of universal gravitation and Coulomb's law—have a very unusual property. A particle moving under either type of force always returns to its starting place with its initial velocity, provided that it lacks sufficient energy to move out to infinity. In other words, the path of a bound particle is always closed and its motion repeats indefinitely, no matter what its initial position or velocity.