I Is the Earth-Sun system an oscillator? What is an oscillator?

[DaveE]

I thinking of high Q oscillator examples* and pondered planetary orbits. The oscillators I am familiar with all shift energy either from place to place or different forms (mass-spring, L-C, E-B, etc.). But a planet in orbit (let's say circular) has potential and kinetic energy that never really changes. I guess the momentum vector circles around, but this seems different in kind to "normal" oscillators. Maybe it's just motion in curved space-time?

Thoughts? Is this an oscillator?
Bonus points for estimating the Q of this system, if it has one.

*Kerry Vahala has built optical resonators with Q=10¹¹. He was the guy down the hall in my undergraduate dorm, and one of the smartest people I ever met. He never left Caltech after he arrived as a Freshman about 45 years ago.

 

[Ibix]

It emits gravitational radiation, if that helps you. At least in theory - a massive 100W or so at $10^{-7}\mathrm{Hz}$ (from the Earth-Sun system) isn't detectable.

 

[Baluncore]

I guess the momentum vector circles around, but this seems different in kind to "normal" oscillators.

If a pendulum is an oscillator, then so is an orbital system.
Think of an elliptical orbit. The long axis is a pendulum, and the short axis is a pendulum, with an identical period.

 

[DaveE]

If a pendulum is an oscillator, then so is an orbital system.
Think of an elliptical orbit. The long axis is a pendulum, and the short axis is a pendulum, with an identical period.

I was thinking of a circular orbit, where the kinetic energy isn't ever converted to PE and vice-versa. I think that's pretty different than a pendulum.
What is an oscillator?

edit: So you are implying that it is like two coupled 1D pendulums? You'll need to elaborate for me since the PE and KE are scalars that don't change. I'm not sure about decomposing energy into orthogonal components.

 

[DaveE]

It emits gravitational radiation, if that helps you. At least in theory - a massive 100W or so at $10^{-7}\mathrm{Hz}$ (from the Earth-Sun system) isn't detectable.

Yes thanks, so it's an oscillator in the GR context. But that is insignificant in this context, this is a classical physics question.

 

[Baluncore]

Is a swinging pendulum an oscillator?

 

[JimWhoKnew]

I was thinking of a circular orbit, where the kinetic energy isn't ever converted to PE and vice-versa. I think that's pretty different than a pendulum.
What is an oscillator?

I'm sure you are aware that a planar circular orbit can be decomposed into orthogonal $X$ and $Y$ components that obey a simple harmonic motion each. Each component can be viewed as the usual swinging between KE and PE.

 

[DaveE]

I'm sure you are aware that a planar circular orbit can be decomposed into orthogonal $X$ and $Y$ components that obey a simple harmonic motion each. Each component can be viewed as the usual swinging between KE and PE.

Yes, of course. That works well for motion, but is it reasonable to talk about the KE or PE of the x or y component then?

 

[DaveE]

I guess energy isn't really the point. We can define a 2-D system state of a position and velocity vector then describe their change due to gravity and solve for motion. This is clearly oscillatory from a motion dynamic view. Energy is just a property that happens to be unchanged for this particular solution, it's not "driving" the system, that is done by forces and inertia. Got it now, thanks. It is actually pretty obvious from this viewpoint, LOL.

 

[Vanadium 50]

but is it reasonable to talk about the KE or PE of the x or y component then?

Probably not helpful. When the body is at the greatest extent in x, its velocity is pointed along y.

 

[Baluncore]

To form a circular orbit, the u and v pendulum axes will have equal amplitude and be in quadrature. Each axis can be analysed independently as a separate pendulum.

 

[sophiecentaur]

Yes thanks, so it's an oscillator in the GR context.

Is a swinging pendulum an oscillator?

I have a feeling that we are wasting our time (yet again) on a thread that is busting itself to justify an irrelevant use of the dreaded classification. Would any 'answer' actually improve our understanding of what actually goes on?

Fact is that, for a nearby observer, the gravitation potential of the system does actually change over the orbital cycle so the PE / KE idea is not relevant to understanding.