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Old 21st March 2020, 06:14 AM   #41
Robin
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Another example. You have remote control of a robot in a small space station. Your only information is from a camera mounted on the robot.

You see the space station spinning around the robot but from just the camera information you can't tell if the robot is spinning or the space station is.

So how do you tell which is spinning?

You instruct the robot to hold some small object in front of the camera and let it go.

If the object remains in front of the camera then the space station is spinning.

If the object moves out of view and then back in again from.the other side then you know the robot is spinning.

So you can tell which will be experiencing centripetal and centrifugal forces using only information from within the space station and you don't need to know anything at all about any other mass outside of the station, never mind distant stars.
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Old 21st March 2020, 08:43 AM   #42
Cheetah
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Originally Posted by Robin View Post
There is no fact of the matter. What does that have to do with what I said?


Your pictures are to simplistic. They don't take into account what it means for something to rotate.
It's the same thing. The principle is identical.

Lets assume either A or B is rotating and the other stationary. I'm just using the same pics:

From the reference frame of B:
Attachment 41695
A appears to be rotating.


From the reference frame of A:
Attachment 41696
B appears to be rotating.

See?
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Old 21st March 2020, 12:25 PM   #43
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Originally Posted by Robin View Post
Another example. You have remote control of a robot in a small space station. Your only information is from a camera mounted on the robot.

You see the space station spinning around the robot but from just the camera information you can't tell if the robot is spinning or the space station is.

So how do you tell which is spinning?

You instruct the robot to hold some small object in front of the camera and let it go.

If the object remains in front of the camera then the space station is spinning.

If the object moves out of view and then back in again from.the other side then you know the robot is spinning.

So you can tell which will be experiencing centripetal and centrifugal forces using only information from within the space station and you don't need to know anything at all about any other mass outside of the station, never mind distant stars.
You've assumed that the distant stars have no influence on the observations, conducted a thought experiment where there's no influence demonstrated and then concluded that there's no influence.

Do you see any problem with that?
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Old 21st March 2020, 03:47 PM   #44
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According to this is actually still not completely understood. Hence it's still a principle and not a law.

YouTube Video This video is not hosted by the ISF. The ISF can not be held responsible for the suitability or legality of this material. By clicking the link below you agree to view content from an external website.
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Old 22nd March 2020, 05:22 AM   #45
HansMustermann
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Originally Posted by Robin View Post
Frankly the problem is that you can't seem to conceptualise a spinning coordinate system to realise that there really are differences between a system that is rotating and a system that is not spinning viewed by a rotating observer.
And that is the whole bloody POINT fer fork's sake. Both look the same, so why is there still a difference? THAT is what Mach was having a brainfart about.

Edit:

If my coordinate system sits still and the object is orbiting me with angular speed ω, in my coordinate system its coordinates go:
x = r*cos(ωt)
y = r*sin(ωt)
If the object sits still and my coordinate system rotates with the same angular speed in the opposite direction
x = r*cos(ωt)
y = r*sin(ωt)
It's IDENTICAL in my coordinate system. But the two behave differently. Yes, there is a difference between the two. That's the POINT. But what was confusing mach is basically: WHY?
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Old 24th March 2020, 08:55 PM   #46
Robin
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Originally Posted by sphenisc View Post
You've assumed that the distant stars have no influence on the observations, conducted a thought experiment where there's no influence demonstrated and then concluded that there's no influence.

Do you see any problem with that?
Do you mean apart from the problem that what you just said is a bizarre misrepresentation of what I said?

I would ask you to at least spend a few seconds thinking about what you are going to write before you post
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Old 24th March 2020, 09:00 PM   #47
Robin
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Originally Posted by HansMustermann View Post
And that is the whole bloody POINT fer fork's sake. Both look the same, so why is there still a difference? THAT is what Mach was having a brainfart about.

Edit:

If my coordinate system sits still and the object is orbiting me with angular speed ω, in my coordinate system its coordinates go:
x = r*cos(ωt)
y = r*sin(ωt)
If the object sits still and my coordinate system rotates with the same angular speed in the opposite direction
x = r*cos(ωt)
y = r*sin(ωt)
It's IDENTICAL in my coordinate system. But the two behave differently. Yes, there is a difference between the two. That's the POINT. But what was confusing mach is basically: WHY?
But the two do not behave diffrently, as I pointed out (over and over again).

In one frame objects with no forces acting on them move in a straight line. In the other objects with no forces upon them move in a circle or spiral.

i illustrated this earlier with a system of particles rotating around each other and the same system with observer rotated so that the particles do not rotate around each other.

In the one with the observer rotated the free particles move in a spiral - the frame where the objects rotate around each other the free particles move in a straight line.

No matter how many times you repeat it, it is simply demonstrably wrong that an object rotating is equivalent to a static object with the observer rotated.
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Old 24th March 2020, 09:10 PM   #48
Robin
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Originally Posted by HansMustermann View Post
And that is the whole bloody POINT fer fork's sake. Both look the same, so why is there still a difference? /
They don't look the same. That is the point.

Quote:
If my coordinate system sits still and the object is orbiting me with angular speed ω, in my coordinate system its coordinates go:
x = r*cos(ωt)
y = r*sin(ωt)
If the object sits still and my coordinate system rotates with the same angular speed in the opposite direction
x = r*cos(ωt)
y = r*sin(ωt)
It's IDENTICAL in my coordinate system. But the two behave differently. Yes, there is a difference between the two. That's the POINT. But what was confusing mach is basically: WHY?
So let's try that:

So here we have an object rotating:



And here we have the same object with the frame rotated in the opposite direction just as you suggest.



That is obviously not equivalent to a static object. With a static object the free objects nearby would describe straight lines.

No, a rotating object is demonstrably not equivalent to a static object with observer rotated.
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Old 24th March 2020, 09:25 PM   #49
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It is odd that everybody here states that a rotating object is exactly equivalent to a static object with observer rotated.

I have many times stated the reasons that they are different and no-one here has even acknowledge those reasons I gave, never mind addressed them.

They just go back to asserting that a rotating object is exactly equivalent to a static object with observer rotated.
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Old 24th March 2020, 09:53 PM   #50
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Originally Posted by Cheetah View Post
It's the same thing. The principle is identical.

Lets assume either A or B is rotating and the other stationary. I'm just using the same pics:

From the reference frame of B:
Attachment 41695
A appears to be rotating.


From the reference frame of A:
Attachment 41696
B appears to be rotating.

See?
You haven't addressed anything I have said.

Does the rotation in question happen:

A) By a magical spell or
B) Some other reason?

Answer me that one.

If you are going to say "It doesn't matter" then you are not addressing what I said.

There might be a superficial similarity to the two cases, but if you look closely that similarity will disappear.

You are using a simplistic diagram to appeal to the superficial similarity between the cases, but in fact those objects (unless they are supposed to be quarks) will be aggregates of many smaller objects with forces between them. Without those forces rotation would not even happen.

So tell me what is the force that is preventing those objects from simply continuing in a straight line, and I will tell you the difference between the two cases.

And, in fact, I have already given you one method for telling the difference.
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Old 24th March 2020, 10:38 PM   #51
HansMustermann
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Originally Posted by Robin View Post
But the two do not behave diffrently, as I pointed out (over and over again).

In one frame objects with no forces acting on them move in a straight line. In the other objects with no forces upon them move in a circle or spiral.
Well, that IS behaving differently, innit? Same no applying forces, different results. And Mach is just basically asking: err, why? That's it. That's the whole story.
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Old 24th March 2020, 10:56 PM   #52
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Originally Posted by HansMustermann View Post
Well, that IS behaving differently, innit? Same no applying forces, different results. And Mach is just basically asking: err, why? That's it. That's the whole story.
No, it's the same behavior, just described in different language.
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Old 24th March 2020, 11:08 PM   #53
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I think the youtube video I linked to explains it better.
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Old 25th March 2020, 06:39 AM   #54
Robin
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Originally Posted by HansMustermann View Post
Well, that IS behaving differently, innit? .
No, the curved path taken by objects with no forces acting on them in the rotating frame is just a consequence of the co-ordinate transform.

If I see a video of an object moving in a straight line and then watch again while rotating my screen Is will see the same object describe a spiral. That does not mean the object is behaving differently. It is behaving the same.

And surely there is no mystery as to why the object that moves in a straight line when the screen is held still, moves in a spiral when the screen is rotated.
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Old 25th March 2020, 10:14 AM   #55
HansMustermann
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There isn't for you. There sure was for Mach.
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