1. Hello,

Even after years of trying I'm still having a hard time understanding a simple physics question. So here it is:

If two objects are circling each other while always facing each other, are they still in motion relatively without a 3rd object or environment? How would we measure their orbital velocity if they are facing each other with absolute precision all the time?

My understanding says that they are stationary in relation to each other. So there is no motion?

Thanks.

-shami

2.

3. Because the objects are circling, they are accelerating, thus neither are an inertial frame of reference. Non-inertial motion can be detected and measured quite easily using equipment such as a gyroscope, newton meter or a sagnac interferometer.

4. so does that mean an object can be in spinning motion by itself without requiring a second object? it would be moving in relation to what? how can an object have acceleration without velocity?

and when you place a measurement device such as newton meter, aren't you adding a second object (or 3rd) to the system, which will work as a reference?

sorry, i'm still lost.

-shami

5. So does that mean an object can be in spinning motion by itself without requiring a second object?
Yes. A spinning object can detect that it is spinning without the need of any other external object.
It would be moving in relation to what?
It would be moving in relation to itself, because if a ball is spinning, the surface spins a lot faster than than a point which is closer to the rotation axis.
How can an object have acceleration without velocity?
Acceleration causes an object to change its velocity. If an object is stationary, it can still be accelerated, and at the very instant it is accelerated it would have acceleration but no velocity.
When you place a measurement device such as newton meter, aren't you adding a second object (or 3rd) to the system, which will work as a reference?
Not really. They are individual objects, but they are attached and can be considered to be one. The point is that there is no external reference point.

6. very interesting. i'll give those some serious thoughts. thanks waveman!

-shami

7. waveman, based on what you said, an object can have acceleration by itself. but there cannot be acceleration without force being applied if F = ma. and momentum is directly related to velocity, unless the wikipedia is wrong here (and the source as well):

http://en.wikipedia.org/wiki/Motion_(physics)

so if a spinning object is constantly being accelerated by itself, what force is being applied?

-shami

8. Originally Posted by BlackDog
waveman, based on what you said, an object can have acceleration by itself.
Yes, only for an instant. The point is that acceleration and velocity are two very diferent things.

There cannot be acceleration without force being applied if F = ma. and momentum is directly related to velocity.

so if a spinning object is constantly being accelerated by itself, what force is being applied?
Once a force has been applied to something, that object will continue moving until acted apon by another force, thats a pretty basic fact about motion. So once something has started spinning, it will continue to do so, unitl another force changes this.

9. waveman, sorry if i unintentionally gave the wrong impression, but i don't have a point. i'm simply trying to understand by asking questions.

So once something has started spinning, it will continue to do so, unitl another force changes this.
i understand. on a straight line, in such a case we'd have velocity. but to have acceleration on a straight line, we'd have to apply more force.

but in the case of spinning body, we'll have acceleration without velocity by applying force once. and we'll continue to have acceleration without applying any more force.

that's an interesting difference to me. i keep thinking acceleration is the change in velocity only. but in this case acceleration is something completely different.

okay!

-shami

10. But in the case of spinning body, we'll have acceleration without velocity by applying force once. and we'll continue to have acceleration without applying any more force.
I think your still confused. If something is rotating, it has both acceleration and velocity. I think you need to review some definitions:
Velocity - velocity is a vector, it has both quantity and direction. There are two ways to change the velocity of an object: changing its speed and changing its direction. When something is rotating, its speed is not changing, but its direction is, hence it is accelerating, because the velocity is changing.

that's an interesting difference to me. i keep thinking acceleration is the change in velocity only. but in this case acceleration is something completely different.
Thats what acceleration is, the rate at which velocity is changing, nothing more nothing less.

okay!

-shami

11. If you look at deformation of timespace as a contraction of timespace in one direction (vertical gravitational direktion) and an expansion in the opposite direction (and rest perpendicular to both directions) Something that falls always falls in the direction where timespace contracts. Then it doesn,t need an accelerating force cause it doesn,t accelerate, the contraction of time space only gives the idea of acceleration (if gravitational force is not a (real) force it,s only logical that the acceleration is not a real acceleration).
The curvation of spacetime then follows out of the different components/directions of spacetime (horizontal and vertical) even in "open space there" are direktions, this is observed all the time as a lot of stars can be considered as in open space.

Then spinning can also do without acceleration or a force. But how it can be determined (and therefor exist) in itself goes beyond my imagination. Because in itself something that spins can be motionless. A force that would be needed to change the spin (or make it spin) would deform it and in that it could be determined (would mean movement in itself) but without a force there is no accelerartion / deformation and Thus it ibecomes just movement but also not in itself detectable if a relation to the outside is absent.

12. Originally Posted by Waveman28
If something is rotating, it has both acceleration and velocity.
see, this is exactly where my confusion is. for something to have velocity, the velocity should be relative to something else. something being relative to "self" seems weird, cause everything should be stationary in relation to self.

and for the velocity to be changing direction continuously, it should require force to be applied continuously. so does a spinning object have velocity? how can velocity change if there is no velocity?

i can totally visualize something to be spinning in relation to another object. it's just hard to understand how one thing would be spinning by self. but at the same time, if something cannot be spinning by self, then my original question makes the world impossible. moon would be falling on earth!

-shami

13. so if a spinning object is constantly being accelerated by itself, what force is being applied?
I think I understand where your confusion is coming from. A spinning rigid body obeys the law of conservation of momentum - it's not going to spin faster unless a force is applied. You are thinking that at constant angular velocity there is still a constant acceleration because there is a constant change of direction, but this is true of every particle in that body. Particles on opposite sides of the body experience equal and opposite acclerations so the net acceleration of the whole body is zero. Hence F=MA still holds true.

Does this help?

14. bunbury thanks.

so does that mean, between two objects orbiting each other while facing each other with absolute precision, there would be 0 acceleration? by facing each other i mean aligned their sides with each other. see my original question?

-shami

15. Originally Posted by BlackDog
Originally Posted by Waveman28
If something is rotating, it has both acceleration and velocity.
see, this is exactly where my confusion is. for something to have velocity, the velocity should be relative to something else. something being relative to "self" seems weird, cause everything should be stationary in relation to self.

and for the velocity to be changing direction continuously, it should require force to be applied continuously. so does a spinning object have velocity? how can velocity change if there is no velocity?

i can totally visualize something to be spinning in relation to another object. it's just hard to understand how one thing would be spinning by self. but at the same time, if something cannot be spinning by self, then my original question makes the world impossible. moon would be falling on earth!

-shami
You need to look into Mach's Principle :

http://en.wikipedia.org/wiki/Mach%27s_principle

16. Depends where you're observing from, or effectively what your frame of reference is.

If you're observing from one of the faces, there'd be no relative motion.

Picture a universe with two particles (one of which has to be an observer or it'd be a third particle) and two dimensions - x (displacement across x) and t (time - constant). Both particles have only concepts of x and t as dimensions.

Particle A has velocity of .
Particle B has velocity of .

From either particle, they will be moving away from eachother at , and their absolute velocities become irrelevant, and effectively useless. You could say that the entire universe that we exist in is dead still (as a centre of mass), or you could say it's travelling at 1Pms-1 - they've both effectively be correct.

17. janus, wow! thanks! that's exactly what i was looking for! finally enlightenment! let me go try to understand it.

thanks to everyone else too.

-shami

18. unfortunately it appears to me that my question still remains unanswered.

either the definition of motion (change in position) needs to be changed, or there is no motion between two orbiting objects when they are facing each other with absolute precision. the objects must be orbiting each other even when there is no movement.

i can't see where the change in position is, in terms of relativity.

-shami

19. Originally Posted by BlackDog
waveman, based on what you said, an object can have acceleration by itself. but there cannot be acceleration without force being applied if F = ma. and momentum is directly related to velocity, unless the wikipedia is wrong here (and the source as well):

http://en.wikipedia.org/wiki/Motion_(physics)

so if a spinning object is constantly being accelerated by itself, what force is being applied?

-shami
At the quantum level, the spinning object is really multiple objects. Quantum objects have quantum spin, but it's very different from an ordinary gyroscopic effect.

I mean, if you take a ring made of iron, and start it spinning, it's only a "single object" in the sense that the solar system could be called a "single object". If you spin an individual particle like an electron, it behaves very differently from that.

20. you have defined a system as without motion in itself. And you also define it as rotating and thus allready relative namely to yourself when you imagine it. If the system exists for you and rotates it will also rotate relative to you as you will automatically also exist for the system you imagine. You can't imagine something and not be a part of it. That gives automatically what you imagine also the possibility to relate to you. As such what you imagine as rotating(it could be a cube as well)can be aware of it,s own rotation namely relative to who imagines it.

21. Ghrasp, unfortunately the definition is not limited only within my imagination. it is possible for two planets to align themselves in such a manner. you wouldn't need an observer in that case. yet they won't collide with each other because of inertia and gravity, just the way earth and moon are explained.

and from the readings it appears that einstein and newton were also troubled by this. i'm guessing there is a good explanation for this. i just don't see the answer!

-shami

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