The Swing of a Pendulum

origin said:
There is no contradiction. The 2 rockets have no relative velocity between them, then they accelerate at exactly the same rate and after a certain amount of time they stop accelerating. There is still no relative velocity between the rockets. The absolute velocity before the acceleration is unknown. After the acceleration the speed is:
[the unknown initial absolute velocity] + [velocity increase due to the acceleration] = Unknown absolute velocity.
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What do you mean they accelerate at the same rate? The distance between the rockets remains the same at ALL times!

If you want to talk about acceleration in this scenario you MUST talk about it in the absolute frame! The pendulum is swinging due to the change in velocity in the absolute frame!
 
Motor Daddy said:
How do you know if the velocity of the sphere changes or not since all you have is relative velocity?
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You would be able to tell if the velocity changed because that would be an acceleration and that acceleration would have an affect on which side of the sphere the light hit first - still won't help your search for an absolute velocity though
 
origin said:
You would be able to tell if the velocity changed because that would be an acceleration and that acceleration would have an affect on which side of the sphere the light hit first - still won't help your search for an absolute velocity though
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Maybe I didn't explain it well enough. Since you ONLY have relative velocity you can not talk about the acceleration of an object, you can only speak about the distance between at least two objects at different points in time.
 
Motor Daddy said:
What do you mean they accelerate at the same rate? The distance between the rockets remains the same at ALL times!
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If the distance is not changing between them they must be accelerating at the same rate and in the same direction.

If you want to talk about acceleration in this scenario you MUST talk about it in the absolute frame!
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Not at all.

The pendulum is swinging due to the change in velocity in the absolute frame!
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Nope, the pendulem moves (there is a force) because the local reference frame (the 2 ships) is undergoing acceleration.
 
Motor Daddy said:
Maybe I didn't explain it well enough. Since you ONLY have relative velocity you can not talk about the acceleration of an object, you can only speak about the distance between at least two objects at different points in time.
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You can detect the acceleration with a pendulem, force meter, or even your light sphere scenario - but you still never now your absolute velocity - only the change in velocity. You go from and unknown velocity + a known velocity increase to a new unknown velocity.
 
origin said:
You can detect the acceleration with a pendulem, force meter, or even your light sphere scenario - but you still never now your absolute velocity - only the change in velocity. You go from and unknown velocity + a known velocity increase to a new unknown velocity.
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No, that's what you do. I go from a known reference frame to known absolute velocities for all objects in space.
 
origin said:
You would be able to tell if the velocity changed because that would be an acceleration and that acceleration would have an affect on which side of the sphere the light hit first - still won't help your search for an absolute velocity though
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Negative - you were right the first time. Regardless of the motion of the sphere the light will *always* strike the inside wall of the sphere at ALL points at the exact same moment. The speed of light in a vacuum *never* changes. It's always c regardless of any motion of the sphere itself.

Of course Motor Daddy is wrong about his whole approach since he doesn't realize the above fact. (And that's only *one* of his many misunderstandings.) ;)
 
origin said:
Yep and the can easily compute their velocity change due to the acceleration.
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So what are the two initial velocities? According to the unit of measure of acceleration, let's use m/s^2, what do you base your acceleration calculations on? What distance changed?
 
Motor Daddy said:
No, that's what you do. I go from a known reference frame to known absolute velocities for all objects in space.
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No you really don't have any idea of an absolute velocity. What is your absolute velocity right now?
 
Motor Daddy said:
Two rockets in space and the distance between them is increasing at the rate of 20 m/s. How do you know which direction of travel they are each traveling?
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Who is "you?"

If "you" is the other rocket then it's easy. The other rocket is going in whatever direction it looks like it's going in. If you define an X-Y-Z system relative to your rocket, and the other rocket is getting farther in the Z direction, then you could say it's going in the Z direction. Or if you call that North you could say it's going North. Both are just as valid.

If it's someone on a different reference frame then you have to define what that one is. Once you do that, then you can figure out what direction they are traveling relative to that frame.

If at t=0 the distance between them is 100 meters, at t=1 the distance between them is 120 meters. Do you think it's possible that each rocket could be traveling in different directions, possibly even at the same speed (gasp!)?
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Not relative to each other, by definition. However you could choose a third observer who saw that they were going the same velocity relative to him, although in no case can they have the same velocity vector.

If unbeknownst to the rockets the rockets were each traveling away from the center of a huge sphere inside the sphere, would each rocket eventually hit the same point on the inside of the sphere or would the rockets eventually hit at opposite points of the sphere?Maybe they would hit 90 degrees apart, or 359 degrees apart.
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If they started from the center of a sphere, and they hit the same point on the sphere at the same time, then they have no relative motion. If they hit at a different points on the sphere, or hit the same point at a different time, then they do have relative motion.

Changing where the rockets depart the sphere changes the absolute velocity of the rockets, but the relative velocity always stays the same!
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If you define your problem as "two rockets have a relative velocity of X" that is of course true, but it's a meaningless statement; basically an identity. In the above problem there is also a velocity relative to the sphere - and if you want to do the math you can figure out what relative velocities each rocket will see in relation to the sphere.
 
Read-Only said:
Negative - you were right the first time. Regardless of the motion of the sphere the light will *always* strike the inside wall of the sphere at ALL points at the exact same moment. The speed of light in a vacuum *never* changes. It's always c regardless of any motion of the sphere itself.

Of course Motor Daddy is wrong about his whole approach since he doesn't realize the above fact. (And that's only *one* of his many misunderstandings.) ;)
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I was refering to a case where there is acceleration, not constant motion. The speed of light of course does not change.
 
Motor Daddy said:
Would you like a colored picture to help you understand?
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I have seen your pictures and they do not help you at all, they only showcase exactly where your ideas deviate from reality. I know exactly what you are saying and the problem is what you are saying deviates from reality.

I am really going to have to do some work now so have fun.
 
billvon said:
Which equipment would you need and how would you use it?
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I would need a sphere with a light source fixed to the center of the sphere. Then I would need almost exactly precise timers, all in sync at every point on the inside of the sphere. Then I would send a light signal from the center to all the timers which started simultaneously when the light signal was sent. Presumably the light would not hit all the timers simultaneously so I would transport the sphere to deep space and accelerate the sphere as required until all timers are struck by the light simultaneously, at which point I would have an absolute zero velocity sphere in space.
 
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