(split) Relativity on a plane

[Neddy Bate]

Nope. If you want to calculate the mass of a ball (known as the "invariant mass" in Relativity), the ball's center of momentum frame is the same as its rest frame.

I meant the center of momentum of the two ball system. If you want to calculate the relativistic increase in gravitational attraction between the balls, I think you need to do it from that frame, no?

 

[Neddy Bate]

I have become convinced that you are nothing but a Motor Daddy sockpuppet, the odds for different people to mangle relativity to the same exact extent are null.

Bull crap. I've disagreed with MD on a lot of things. He claims relativity is flat out wrong. He claims light only travels at c relative to an absolute rest frame. I know that's not how the real universe works.

As for your claim that I know nothing about relativity, you are simply wrong once again, as you so often are. Have you tried the bathroom scale experiment yet? I'm still waiting for you to post the two different scale readings that you were able to make the scale display at the same time.

 

[Tach]

Bull crap. I've disagreed with MD on a lot of things.

But you have the same exact misconceptions and the same exact total ignorance of the basics of SR.

He claims relativity is flat out wrong. He claims light only travels at c relative to an absolute rest frame. I know that's not how the real universe works.

Yet, in this thread you show total inability to understand the notion of relativistic measurements.

I'm still waiting for you to post the two different scale readings that you were able to make the scale display at the same time.

See, what did I tell you?

 

[Neddy Bate]

This must be the case, otherwise you can calculate different forces of attraction in different reference frames.

...which is precisely what SR does but you and your sockpuppet can't seem to accept.

So now you are claiming that the spring scale between the balls reads differently in different reference frames? Of course it doesn't. So we need to know which frame to calculate the force of attraction from. I think it's the frame in which the center of momentum of the 2-ball system is at rest.

 

[Tach]

So now you are claiming that the spring scale between the balls reads differently in different reference frames?

See? It is exactly this type of arguing that convinces me that you are a Motor Daddy sockpuppet. You take your misunderstandings and you formulate as if they were claimed by someone else.

So we need to know which frame to calculate the force of attraction fro.

We do. You don't (and you are clearly unable to learn).

 

[Neddy Bate]

But you have the same exact misconceptions and the same exact total ignorance of the basics of SR.

Wrong again. I can do Lorentz transforms, etc. I've never seen MD do them, because he thinks seems to think learning relativity would be a waste of time. Probably because of folks like yourself.

Yet, in this thread you show total inability to understand the notion of relativistic measurements.

I think I understand them pretty well. I understand time dilation, length contraction, and relativity of simultaneity. I don't believe that a spring scale can provide two different measurements of force at one time, but relativity certainly never claimed that it could. That is your own idea.

See, what did I tell you?

You claimed ball A applies a different force to the scale than ball B applies to it. All in one reference frame!!! So why don't you go demonstrate that is possible, using your bathroom scale? Oh, that;'s right, because it's impossible.

 

[CptBork]

I meant the center of momentum of the two ball system. If you want to calculate the relativistic increase in gravitational attraction between the balls, I think you need to do it from that frame, no?

Newton's law of gravity doesn't apply in Relativity, FYI. To say that the problem is with faster than light gravitational signals oversimplifies things. If you calculate the forces and accelerations from Newton's law in one frame, using the inertial masses and the distance between the two balls, and then do the same calculation from a boosted frame, the results don't transform consistently from one frame to the other.

 

[Tach]

You claimed ball A applies a different force to the scale than ball B applies to it.

Can you disengage your motor moth for a second and think about the fact that A doesn't move wrt his scale whilst B moves (at speed v)? Why would the forces be the same? Because you reason exactly like Motor Daddy?
You claimed that you understand Lorentz transforms, there is a whole introductory chapter that explains transformation of force. Instead of spending several days posting nonsense in this thread you could have gone and studied the respective chapter. Much better use for your time.

 

[Tach]

Newton's law of gravity doesn't apply in Relativity, FYI. To say that the problem is with faster than light gravitational signals oversimplifies things. If you calculate the forces and accelerations from Newton's law in one frame, using the inertial masses and the distance between the two balls, and then do the same calculation from a boosted frame, the results don't transform consistently from one frame to the other.

He's not assuming universal attraction per se. If he did, the two balls couldn't move the way they move in his silly exercise. He's assuming two identical forces that are orthogonal to the direction of motion.

 

[Neddy Bate]

Newton's law of gravity doesn't apply in Relativity, FYI. To say that the problem is with faster than light gravitational signals oversimplifies things. If you calculate the forces and accelerations from Newton's law in one frame, using the inertial masses and the distance between the two balls, and then do the same calculation from a boosted frame, the results don't transform consistently from one frame to the other.

But Newton's laws hold fairly well in each individual rest frame, right? All I was trying to do was calculate the force measurement that would be displayed on the spring scale between the two balls. This measurement could be taken in the frame of either ball, the situation is perfectly symmetrical.

The spring scale's size and mass are assumed to be negligible. At the instant when the two balls of identical mass and radius pass each other at a speed of $$v$$ tangent to their surfaces, can you tell me what the gravitational force of attraction would be?

 

[Neddy Bate]

He's not assuming universal attraction per se. If he did, the two balls couldn't move the way they move in his silly exercise. He's assuming two identical forces that are orthogonal to the direction of motion.

Yes, exactly. The moving ball could be guided by a straight track right up until the moment that it passes by the other ball. It's not a difficult arrangement.

 

[Neddy Bate]

Can you disengage your motor moth for a second and think about the fact that A doesn't move wrt his scale whilst B moves (at speed v)? Why would the forces be the same? Because you reason exactly like Motor Daddy?

The force applied to the scale by A must be equal and opposite to the force applied to the scale by B. Otherwise, there is a net force, which would cause the scale to accelerate. Go try it with your bathroom scale. Is that what you are claiming happens? The scale accelerates?

You claimed that you understand Lorentz transforms, there is a whole introductory chapter that explains transformation of force. Instead of spending several days posting nonsense in this thread you could have gone and studied the respective chapter. Much better use for your time.

The Lorentz transforms I know transform coordinates, not forces.

 

[Tach]

The force applied to the scale by A must be equal and opposite to the force applied to the scale by B.

]

Scale A is comoving with ball A.
B is moving wrt A and the scale A.
How does B act on the scale at A? Through telepathic waves?

Scale B is comoving with ball B.
A is moving wrt B and the scale B.
How does A act on the scale at B? Through telepathic waves?

The Lorentz transforms I know transform coordinates, not forces.

This kind of ignorance is your problem. Until you learn the appropriate chapter of SR, you will continue to spew nonsense.

 

[Neddy Bate]

Scale A is comoving with ball A.
B is moving wrt A and the scale A.
How does B act on the scale at A? Through telepathic waves?

Scale B is comoving with ball B.
A is moving wrt B and the scale B.
How does A act on the scale at B? Through telepathic waves?

Telepathic waves? LOL. So you still don't even understand the problem? The balls pass by each other close enough that the only thing that can fit between them is the extremely tiny, massless spring scale. You only need one scale; because of symmetry, there can be no difference in results obtained by two scales.

When ball B is located at:
$$(x,y,z)=(0,2r,0)$$
and ball A is located at:
$$(x,y,z)=(0,0,0)$$

The distance between the centers of the balls is exactly 2r which means the surfaces of the balls are virtually touching each other. That is, they would be touching if there hadn't been a tiny scale slipped between them. In other words, both balls touch the scale at the same time.

This kind of ignorance is your problem. Until you learn the appropriate chapter of SR, you will continue to spew nonsense.

But you used the force transform when you presented your solution to this problem, and yet your solution did not make any sense. You arrived at the conclusion that the two balls apply different forces to the scales, according to each reference frame.

 

[Motor Daddy]

Newton's law of gravity doesn't apply in Relativity, FYI. To say that the problem is with faster than light gravitational signals oversimplifies things. If you calculate the forces and accelerations from Newton's law in one frame, using the inertial masses and the distance between the two balls, and then do the same calculation from a boosted frame, the results don't transform consistently from one frame to the other.

What, you can pick and choose which laws of physics are the same in all frames and which aren't? I thought all the laws of physics are supposed to be the same in all frames in SR?

 

[Neddy Bate]

Since Tach is having trouble understanding the ball scenario, here is a drawing that might help. The red thing is the scale.

 

[CptBork]

What, you can pick and choose which laws of physics are the same in all frames and which aren't? I thought all the laws of physics are supposed to be the same in all frames in SR?

Yeah, and that's why you can't use Newtonian gravity in SR (you can if you're describing non-relativistic objects), as they explain in undergrad.

 

[Tach]

But you used the force transform when you presented your solution to this problem, and yet your solution did not make any sense.

Tough, you need to engage brain.

 

[James R]

Let me know when you've tried the bathroom scale experiment. A spring scale cannot display two different forces at the same time.

Right. It can't display two different readings at the same time. But that doesn't mean the force on it isn't different in two different frames.

For another example: a ruler can't display two different readings at the same time. Yet if you take that ruler and measure the length of an object in two different frames, you'll get different answers.

 

[Neddy Bate]

But you used the force transform when you presented your solution to this problem, and yet your solution did not make any sense.

Tough, you need to engage brain.

Tach, now that you can see this drawing:

Do you still want to claim that the blue ball exerts a different amount of force on the scale than the green ball?