Which object is orbiting the other one?

[Kedas]

Can you say that nature doesn't make a difference between the two only that they orbit/accelerate each other.

 

Because the decision which object is orbiting which object is based on a third reference (frame).

(that is obvioulsy a free choice of the observer)

 

Is it possible to say that object A orbits object B more than the other way around? (assuming a reference frame was choosen)

[YT2095]

I think Mass is the frame of reference used, the body with the least mass is said to orbit the one of greater mass, that`s the way I understand to work anyway

[swansont]

This is speculation, but... I disagree slightly - more mass is a necessary but insufficient condition. I think if the barycenter is inside one of the planets (or maybe within ~ a diameter) then you talk about the other object orbiting it. Otherwise you say they orbit each other as a binary system.

[YT2095]

sure

I`de consider it a binary system also (in the classical sense), were both objects to be of equal mass.

I`m not sure diameter plays a role here though? as would our moon to become more massive than the Earth, but retain its volume, We (Earth) would then orbit the moon.

[Duke]

I tend to forget the word orbit, it was penned hundreds of years ago. both objects influence each other. One more than the other maybe. In which case i say mass.

[VendingMenace]

agreed, the term orbit is mearely one of convieneince.

 

What we tend to do is calculate the rotation of bodies about the center of mass of a system. As far as the solar system goes, we say everything orbts the sun, simply becuase most of the mass of the solar system is in the sun. So to a good first approximation the sun is the center of mass.

 

Of course, we choose to use the center of mass as our refernce frame, becuase it greatly simplifies our equations. HOwever, any frame is just as valid. We could (if we wished) choose the center of the universe as YT, and calculate how everything orbits him. Of course, this would be a major headache, but the important thing is, it could be done. Of course, i could also choose the center of everything to be me, and describe everything else's motion withe respect to me. SO you see, the world does revolve around me, no matter what my mother might have claimed to the contrary

[Kedas]

Assume two objects (with equal mass) in an empty universe.

 

If I'm present on object A then B will orbit around me and if I'm on B then A will orbit around.

 

Edit: I wrote this before I read the message of VendingMenace.

[YT2095]

ROFLOL @ Vending

I`ve been known to create some "Big Bangs" in my time, but not THAT BIG! besides, how heavy do you think I am? LOL

 

Kedas, I`de imagine that 2 objects of equal mass in the universe ALONE, would eventualy "find" each other through a gravitational "becconing" and then gradualy increase speed as they neared each other, collide, maybe fragment a little, and then rejoin, untill eventualy it would become as stable a mass as was structuraly possible, eventualy pulling into itself the little fragments that may have flown off in the 1`st encounter.

all this pre-suposes the lack of inertial force at the outset, if one were to be moving and the other stationary, eventualy they would get closer and reach a "Stable" orbit. and with you standing the one thereby increasing its mass on planet `A` then B would travel alot more in orbital around YOU, than you around IT

[Kedas]

I only want to address the relativity of orbiting, my own mass is the last thing I think about. Also I assumed that they were already orbiting.

VendingMenace said it in an entertaining way.

[AntiMagicMan]

Objects do not orbit other objects. They orbit the centre of mass of the system.

[Kedas]

Objects do not orbit other objects. They orbit the centre of mass of the system.

 

That is assuming gravity is the only force on it.

(it don't have to be objects in space)

[AntiMagicMan]

I believe we have used that assumption all the way through this discussion. Anyway it simply requires the main force between the two objects to follow an inverse square law.

[Kedas]

I believe we have used that assumption all the way through this discussion. Anyway it simply requires the main force between the two objects to follow an inverse square law.

 

You didn't read my first message, for all I care you are even allowed to hold those two object in your hand.

 

VendingMenace message sums it up. (more or less)

[AntiMagicMan]

But it is not true to say two objects orbit a common centre of mass if you are exerting forces upon them that do not correspond to an inverse square law.

[Kedas]

Yeah, 'orbit' should probably only be used when gravity is its reference frame but I don't know an other word for it. 'rotating' ?

[Dave]

If you have one body and hold it still, and then move another one around it, you're doing just that - moving it. It's not really doing anything special wrt the other body.

[AntiMagicMan]

That's the best way to describe it. Orbiting only means something in the context of the two bodies doing something to each other.

[YT2095]

the way I see it is like this, if there are 2 objects, one weighs 1000 grams and the other one 500 grams, the 500 gram object will orbit the 1kg object with 66.6% of the orbital effect and the 1kg object will have 33.3% of the effect.

but for ease, it`s said the 500gram object orbits the 1kg object