Does Jupiter orbit the Jupiter-Sun barycenter or not?

[whiskers]

It has happened to me. I'm sitting in a train waiting to move off, and then I notice the station moving, "silly me its the train moving instead", but for that moment in time your brain is fooled.

Fooled? Maybe not. You know what they say: "motion is relative."

[Robittybob1]

Fooled? Maybe not. You know what they say: "motion is relative."

Brushing up on electromagnetism, it was the relative motion of the magnet to the coil that created the current and it didn't matter if it was the coil that moved or the magnet that moved.

Sometimes you are correct.

I might admit if you were stuck in a spacecraft and had forgotten which way was up and things appeared to be moving around you for some strange reason you might not realise it was you who was falling into the black hole.

Edited February 24, 2015 by Robittybob1

[whiskers]

Brushing up on electromagnetism, it was the relative motion of the magnet to the coil that created the current and it didn't matter if it was the coil that moved or the magnet that moved.

Sometimes you are correct.

I might admit if you were stuck in a spacecraft and had forgotten where up was and things appeared to be moving around you for some strange reason you might not realise it was you who was falling into the black hole.

http://www.physicstutorials.org/home/mechanics/1d-kinematics/relative-motion

https://en.wikipedia.org/wiki/Galilean_invariance

[imatfaal]

I might admit if you were stuck in a spacecraft and had forgotten which way was up and things appeared to be moving around you for some strange reason you might not realise it was you who was falling into the black hole.

 

Yes you would. You would be accelerating and contrary to above assertions you can tell if you are accelerating. Gallilean relativity is about constant velocity motion - inertial frames are interchangeable and relative. But non-inertial frames - ie where you are accelerating / under influence of gravity (same thing more or less) - are not relative and the physics changes.

[Robittybob1]

 

Yes you would. You would be accelerating and contrary to above assertions you can tell if you are accelerating. Gallilean relativity is about constant velocity motion - inertial frames are interchangeable and relative. But non-inertial frames - ie where you are accelerating / under influence of gravity (same thing more or less) - are not relative and the physics changes.

Didn't Einstein say free fall is the same as inertial motion http://en.wikipedia.org/wiki/Equivalence_principle

so falling into a black hole, until you measure the tidal effects, I always understood that this was not acceleration. OK it took a lot of convincing but we only notice the full acceleration due to gravity when we are stationary.

"From this principle, Einstein deduced that free-fall is actually inertial motion. Objects in free-fall do not really accelerate."

That is, being at rest on the surface of the Earth is equivalent to being inside a spaceship (far from any sources of gravity) that is being accelerated by its engines.

Edited February 24, 2015 by Robittybob1

[swansont]

Didn't Einstein say free fall is the same as inertial motion http://en.wikipedia.org/wiki/Equivalence_principle

so falling into a black hole, until you measure the tidal effects, I always understood that this was not acceleration. OK it took a lot of convincing but we only notice the full acceleration due to gravity when we are stationary.

 

Invoking Einstein assures you are not talking about Galilean relativity

[Robittybob1]

 

Invoking Einstein assures you are not talking about Galilean relativity

Where does one start and the other finish? They blend together don't they? Relativistic effects are more noticeable at high speeds but really they are ever-present (but insignificant).

[swansont]

Where does one start and the other finish? They blend together don't they? Relativistic effects are more noticeable at high speeds but really they are ever-present (but insignificant).

 

When they are insignificant you can ignore them.

[Greg H.]

 

When they are insignificant you can ignore them.

Which is why Newtonion formulas still let us put satellites in orbit.

[Robittybob1]

 

When they are insignificant you can ignore them.

So were they principally the same, even though you ignore the insignificant correction? If you get to the point of being able to cruise close to a black hole I would tend to think you would have had to understood relativity, and the rocketeer would be wise no longer ignoring it.

[swansont]

A few posts back imatfaal made a point about Galilean systems, then you brought up Einstein as a point of contention. The phrase "Galilean system" specifically excludes any situation where Einstein's relativity is important. Before we continue on along some tangent, can you at least acknowledge whether you understand that or not?

[Robittybob1]

A few posts back imatfaal made a point about Galilean systems, then you brought up Einstein as a point of contention. The phrase "Galilean system" specifically excludes any situation where Einstein's relativity is important. Before we continue on along some tangent, can you at least acknowledge whether you understand that or not?

OK I didn't know that, but I'm not surprised. http://en.wikipedia.org/wiki/Galileo%27s_Leaning_Tower_of_Pisa_experiment

 

Galileo arrived at his hypothesis by a famous thought experiment outlined in his book On Motion.[5] Imagine two objects, one light and one heavier than the other one, are connected to each other by a string. Drop this system of objects from the top of a tower. If we assume heavier objects do indeed fall faster than lighter ones (and conversely, lighter objects fall slower), the string will soon pull taut as the lighter object retards the fall of the heavier object. But the system considered as a whole is heavier than the heavy object alone, and therefore should fall faster. This contradiction leads one to conclude the assumption is false.

So it appears Galileo at least thought about free fall.

[Robittybob1]

Now that I have studied a bit about the Lagrangian Points one would wonder how the star could orbit the Sun-planet barycenter if there were two planets of equal mass both at the L3 position of each other.

[pzkpfw]

Where's the barycenter in that scenario?

[pavelcherepan]

Now that I have studied a bit about the Lagrangian Points one would wonder how the star could orbit the Sun-planet barycenter if there were two planets of equal mass both at the L3 position of each other.

 

Are there only two planetary bodies in the system you're describing? If that's so and if their masses are equal then the barycenter would be at the center of the star, so that the star would be stationary in the reference frame centered at barycenter. If there are other bodies present then all of them (and the star too) will be orbiting the common barycenter which will be located well inside the star but not at the exact center.

[Robittybob1]

 

Are there only two planetary bodies in the system you're describing? If that's so and if their masses are equal then the barycenter would be at the center of the star, so that the star would be stationary in the reference frame centered at barycenter. If there are other bodies present then all of them (and the star too) will be orbiting the common barycenter which will be located well inside the star but not at the exact center.

I was harping back to my original idea that the 2 barycenters would move around the Sun, but once I had posted I realised your solution would be a solution but that was more the equivalent to the SS barycenter in this hypothetical 2 planet system.

[pavelcherepan]

I was harping back to my original idea that the 2 barycenters would move around the Sun, but once I had posted I realised your solution would be a solution but that was more the equivalent to the SS barycenter in this hypothetical 2 planet system.

 

I'd actually want to correct my earlier post. In fact the barycenter of this hypothetical system won't be at the center of the star. That would be the case if orbit was perfectly circular but since they are somewhat elliptical barycenter will shift towards one of the planets when it's in the pericenter and away from the planet at the apocenter and then slowly drift towards the center of the star (autumn and spring equinoxes) and then move to the other side.

[Robittybob1]

 

I'd actually want to correct my earlier post. In fact the barycenter of this hypothetical system won't be at the center of the star. That would be the case if orbit was perfectly circular but since they are somewhat elliptical barycenter will shift towards one of the planets when it's in the pericenter and away from the planet at the apocenter and then slowly drift towards the center of the star (autumn and spring equinoxes) and then move to the other side.

Fair enough, but in a simple 2 planet system the orbits could be virtually circular, there might be nothing left to perturb the orbits.

Just to be sure "Apocenter (1) away, Pericenter (2) close, and Focus (3).

You might have the barycenters moving in the wrong direction? They usually relate the movement of the barycenter wrt the Star at the focus.

[imatfaal]

Fair enough, but in a simple 2 planet system the orbits could be virtually circular, there might be nothing left to perturb the orbits.

 

For crying out loud how many times does this need to be said? Two planets and a Star IS NOT A SIMPLE SYSTEM. It is a three body problem which may or may not have an analytical answer (probably not). It could well have huge variance in outcomes due to miniscule changes in initial conditions. It is not amenable to calculation-enabling simplifications.

[Robittybob1]

 

For crying out loud how many times does this need to be said? Two planets and a Star IS NOT A SIMPLE SYSTEM. It is a three body problem which may or may not have an analytical answer (probably not). It could well have huge variance in outcomes due to miniscule changes in initial conditions. It is not amenable to calculation-enabling simplifications.

I read that the Lagrangian orbits were calculable and that is why we know about them. It was proposing a hypothetical system with a perfect balance. Do you think even that situation will be incalculable?