[Jaaanosik (OP)]

Do you understand what it means?

Yes, it means that executing a rotation does not require net momentum (unlike your engine, which does require net momentum).

The cat is isolated system, correct?

For the sake of discussion, yes.

How the cat could start the rotation without an external impulse?

Because net momentum isn't needed for a rotation (but net momentum is needed for your spaceship to go forward). I've already said that before.

The thing about the gyroscope is ultimately irrelevant. Even if I was utterly unable to explain anything about gyroscopes, I can still point to Noether's theorem as proof that you can't interconvert angular and linear momentum.

It's getting late here, 2:20am, I am going to bed.
Still, I think we made a good progress.
I'll just say an isolated system is not a closed system.
The isolated system can generate momentum, the closed system requires an external momentum.
You are mistaken about the wobble.
Check how big wobble is for the CD players.
Here is a quote from wiki about CMGs: 'large CMGs have produced thousands of newton meters of torque.'
What is this torque? How is this torque used on ISS?
Jano

[Kryptid]

I'll just say an isolated system is not a closed system

According to the definitions that you have provided in your Wikipedia link, an isolated system is even more cut off from the outside world than a closed system is. A closed system allows for the transfer of energy but not matter, whereas an isolated system does not allow for the transfer ot either energy or matter.

The isolated system can generate momentum

How is it going to generate momentum when (1) it can't receive momentum from an outside source, and (2) momentum is conserved?

You are mistaken about the wobble.
Check how big wobble is for the CD players.

Even if I am, so what? It won't help your engine work.

What is this torque?

https://en.wikipedia.org/wiki/Torque

How is this torque used on ISS?

I presume it is used to reorient the ISS and make it stable against changes in orientation. But that doesn't change the net angular momentum of the ISS nor does it help your engine work.

You have agreed with me that your ship starts with zero angular momentum and zero linear momentum. You have agreed that conservation of momentum is true. The ship is an isolated system. Putting these three things together, the ship cannot ever have a net momentum that isn't zero. There is no angular momentum present, so you can't argue that it can be changed into linear momentum because zero can't become a positive number. No momentum is entering the ship from an outside source nor is momentum leaving the ship to an outside source. So tell me already: how can zero net momentum in an isolated system turn into positive net momentum without violating conservation of momentum?

[Jaaanosik (OP)]

I'll just say an isolated system is not a closed system

According to the definitions that you have provided in your Wikipedia link, an isolated system is even more cut off from the outside world than a closed system is. A closed system allows for the transfer of energy but not matter, whereas an isolated system does not allow for the transfer ot either energy or matter.

The isolated system can generate momentum

How is it going to generate momentum when (1) it can't receive momentum from an outside source, and (2) momentum is conserved?

The cat is a system.
We can call it whatever - closed/isolated, I do not care.
What is important is that the system can turn 180 degrees by itself and it costs energy.
Are we sure the cat generated no net angular momentum?



The orientation of the body seems to be different.

You are mistaken about the wobble.
Check how big wobble is for the CD players.

Even if I am, so what? It won't help your engine work.

What is this torque?

https://en.wikipedia.org/wiki/Torque

How is this torque used on ISS?

I presume it is used to reorient the ISS and make it stable against changes in orientation. But that doesn't change the net angular momentum of the ISS nor does it help your engine work.

You have agreed with me that your ship starts with zero angular momentum and zero linear momentum. You have agreed that conservation of momentum is true. The ship is an isolated system. Putting these three things together, the ship cannot ever have a net momentum that isn't zero. There is no angular momentum present, so you can't argue that it can be changed into linear momentum because zero can't become a positive number. No momentum is entering the ship from an outside source nor is momentum leaving the ship to an outside source. So tell me already: how can zero net momentum in an isolated system turn into positive net momentum without violating conservation of momentum?

Please, have a look here:


Just one side. The CMG will not rotate, the spaceship will rotate.
There is an accelerometer attached to the CMG body - 'no acceleration'.
There is an accelerometer attached to the spaceship - there is an acceleration during the reeling in.
'No acceleration' - meaning if the motion is done right with the VSCMGs then there is no acceleration measured.
https://en.wikipedia.org/wiki/Control_moment_gyroscope#Variable-speed
The VSCMGs can eliminate the wobble.
We can do the reeling in - time interleaved left, right short impulses - the same result, no acceleration on both VSCMG systems but the spaceship accelerates.
We stop the CMG rotations before they hit the spaceship back.
What would be the result? A forward net momentum,
Jano

Edit: Stopping the CMGs is important to get rid of the big torque force vectors.
The torque vectors will not cause any havoc when they are not present.

[Jaaanosik (OP)]

CMGs in action:

Jano

[Kryptid]

Are we sure the cat generated no net angular momentum?

Yes, because Noether's theorem says so. Do you even know what a conservation law is?

The orientation of the body seems to be different.

Of course it's different. Changing orientation doesn't require net momentum.

We stop the CMG rotations before they hit the spaceship back.

What force is stopping the CMG?

What would be the result? A forward net momentum

I don't really need to explain to you why zero angular momentum plus zero linear momentum equals positive linear momentum is bad math, do I?

As long as you keep arguing that 0 + 0 = positive number I am going to stand against it because it is wrong.

[Jaaanosik (OP)]

Are we sure the cat generated no net angular momentum?

Yes, because Noether's theorem says so. Do you even know what a conservation law is?

The orientation of the body seems to be different.

Of course it's different. Changing orientation doesn't require net momentum.

We stop the CMG rotations before they hit the spaceship back.

What force is stopping the CMG?

What would be the result? A forward net momentum

I don't really need to explain to you why zero angular momentum plus zero linear momentum equals positive linear momentum is bad math, do I?

As long as you keep arguing that 0 + 0 = positive number I am going to stand against it because it is wrong.

Kryptid,
did you check the last video?
Anything to say?
Jano

[Kryptid]

Anything to say?

No. The Cubli doesn't violate conservation of angular momentum (nothing ever does), so there is nothing to say.

[Jaaanosik (OP)]

Anything to say?

No. The Cubli doesn't violate conservation of angular momentum (nothing ever does), so there is nothing to say.

Is Cubli a closed system?
If yes then it needs an external force to change the momentum.
Do you see any external force that causes the jump up?
Jano

[Kryptid]

Is Cubli a closed system?

Depends on which definition of "closed system" you are going for.

If yes then it needs an external force to change the momentum.

The momentum doesn't change...

Do you see any external force that causes the jump up?

No, because no such force is needed. I already told you once that changing orientation does not require a change in net angular momentum.

Momentum is conserved. Period. If you think you've found a case where it isn't, it's only because your analysis of it was either flawed or incomplete. We know that beforehand even without having to get into the details of how the system works. Noether's theorem guarantees it.

[Jaaanosik (OP)]

Is Cubli a closed system?

Depends on which definition of "closed system" you are going for.

The textbook definition for now.
Cubli is not a closed system. Agreed?

If yes then it needs an external force to change the momentum.

The momentum doesn't change...

Do you see any external force that causes the jump up?

No, because no such force is needed. I already told you once that changing orientation does not require a change in net angular momentum.

Momentum is conserved. Period. If you think you've found a case where it isn't, it's only because your analysis of it was either flawed or incomplete. We know that beforehand even without having to get into the details of how the system works. Noether's theorem guarantees it.

Cubli is not  a closed system.
CMGs are not a closed system.
There is no point in talking that a momentum is conserved if the system is not closed.
Jano

[Kryptid]

The textbook definition for now.

The one that says a closed system doesn't allow matter to be transferred but energy can?

Cubli is not a closed system. Agreed?

If it was an open system, then matter would be transferred in and out of it. What matter is being transferred in and out of the Cubli?

CMGs are not a closed system.

Of course not. They are interacting with the other components of the ship. But the other components of the ship don't have any momentum that they can give to the CMGs. The CMGs don't have any momentum to give to the other components of the ship either (remember, you agreed that the system has zero momentum before the engine is cut on).

There is no point in talking that a momentum is conserved if the system is not closed.

You know why that is, don't you? It's because an open system can change its momentum by interacting with outside systems. Your ship is not interacting with other systems because it is an isolated system. No mechanism exists by which it can receive momentum from somewhere else. And if you did rely on receiving momentum from an outside source, then that would pretty much mean that you have abandoned your claim that you can give the ship net momentum just by manipulating its internal components.

To drive this home, consider a universe that is completely empty of all energy and matter with the exception of your spaceship and an astronaut that is floating right behind it. The spaceship is turned off and the astronaut sees the ship as stationary in their reference frame. Since the astronaut obviously sees themself as being stationary as well, then they know that the total momentum in their universe is zero (the ship's momentum plus the astronaut's momentum equals zero).

Now you turn the engine on. According to you, the ship will begin to move forward and leave the astronaut behind. The astronaut goes about measuring the total momentum of the universe a second time. This time, the moving ship does have momentum from the astronaut's reference frame. They take the measurements, do the calculations and find that the ship's momentum plus the astronaut's momentum is now a positive number. This means that a closed system (the entire universe) has experienced an increase in net momentum.

We know, of course, that such a thing is impossible because momentum is a conserved quantity. A closed system cannot go from zero momentum to positive momentum. This is how we know that the scenario I just described cannot happen in the real world. It isn't possible for the ship to move because it will increase the total momentum of a closed system.

[Jaaanosik (OP)]

The textbook definition for now.

The one that says a closed system doesn't allow matter to be transferred but energy can?

Cubli is not a closed system. Agreed?

If it was an open system, then matter would be transferred in and out of it. What matter is being transferred in and out of the Cubli?

This is from the textbook:

Thus, if there is no angular impulse about a fixed point (or about the mass center), the angular momentum of the system about the fixed point (or about the mass center) remains unchanged.

This is the definition of the closed system for the angular momentum.
No angular impulse - conservation of the angular momentum.
It is the angular impulse that makes the Cubli jump up, the definition for the conservation of the angular momentum does not hold anymore.
If the definition of the closed system has to include the conservation of the angular momentum then Cubli is an open system.

CMGs are not a closed system.

Of course not. They are interacting with the other components of the ship. But the other components of the ship don't have any momentum that they can give to the CMGs. The CMGs don't have any momentum to give to the other components of the ship either (remember, you agreed that the system has zero momentum before the engine is cut on).

So when CMGs try to rotate the ISS then the ISS is not resisting the rotation? The angular momentum in opposite direction?
The CMGs balance the disturbances and CMGs make sure the attitude is steady in the ECI frame.
CMGs keep the rotation of the ISS steady in the ECI frame.
The disturbances are external impulses to the ISS and CMGs work against them.
What makes you say the ISS has no momentum to give to CMGs?
What are those disturbances?
Jano

[Jaaanosik (OP)]

...

There is no point in talking that a momentum is conserved if the system is not closed.

You know why that is, don't you? It's because an open system can change its momentum by interacting with outside systems. Your ship is not interacting with other systems because it is an isolated system. No mechanism exists by which it can receive momentum from somewhere else. And if you did rely on receiving momentum from an outside source, then that would pretty much mean that you have abandoned your claim that you can give the ship net momentum just by manipulating its internal components.

To drive this home, consider a universe that is completely empty of all energy and matter with the exception of your spaceship and an astronaut that is floating right behind it. The spaceship is turned off and the astronaut sees the ship as stationary in their reference frame. Since the astronaut obviously sees themself as being stationary as well, then they know that the total momentum in their universe is zero (the ship's momentum plus the astronaut's momentum equals zero).

Now you turn the engine on. According to you, the ship will begin to move forward and leave the astronaut behind. The astronaut goes about measuring the total momentum of the universe a second time. This time, the moving ship does have momentum from the astronaut's reference frame. They take the measurements, do the calculations and find that the ship's momentum plus the astronaut's momentum is now a positive number. This means that a closed system (the entire universe) has experienced an increase in net momentum.

We know, of course, that such a thing is impossible because momentum is a conserved quantity. A closed system cannot go from zero momentum to positive momentum. This is how we know that the scenario I just described cannot happen in the real world. It isn't possible for the ship to move because it will increase the total momentum of a closed system.

Kryptid,
There is an impulse that propels the spaceship away.
Is it an internal or an external impulse? Is it important?
The system is not closed anymore.
The momentum is frame dependent.
Dynamics/boundaries changed, the astronaut sees dv/dt change of the spaceship in his frame.
If the Universe, the spaceship and the astronaut are one frame in the beginning.
Then the spaceship accelerates.
What is the accelerometer going to say that is attached to the spaceship, the Universe accelerometer and the astronaut accelerometer?
I think you should rethink and rewrite what you wrote, specifically think about the frame dependency of the momentum,
Jano

[Bobolink]

Do you see any external force that causes the jump up?

I saw where it used gravity and ground to push against.  Of course the conservation of momentum wasn't broken.  You realize that would not work in a zero-g environment with no wall to work against.  You know why?  That's right the conservation of momentum.

You are tilting at windmills.

If you spent half this effort learning physics you would be very knowledgeable guy...

[Kryptid]

This is a completely worthless discussion. You are just as bad at physics as Dave Lev is. You don't understand what conservation laws are. You don't understand what a closed system is. And nothing I can do will be able to alleviate that ignorance. So I'm ending my attempt to educate you.

You might as well be arguing that you can charge up a dead battery using another dead battery, because if you believe that momentum can be created, then you might as well believe that energy can be created too. Conservation laws have no meaning to you.

[Jaaanosik (OP)]

This is a completely worthless discussion. You are just as bad at physics as Dave Lev is. You don't understand what conservation laws are. You don't understand what a closed system is. And nothing I can do will be able to alleviate that ignorance. So I'm ending my attempt to educate you.

You might as well be arguing that you can charge up a dead battery using another dead battery, because if you believe that momentum can be created, then you might as well believe that energy can be created too. Conservation laws have no meaning to you.

Kryptid,
You missed that momentum is frame dependent?
When your spaceship accelerates then gas is flying out on the other side.
Momentum is conserved for those 'two entities'.
Both entities had zero momentum in the astronaut frame before but now both entities have momentum left/right in the astronaut frame.
This is  you: "They take the measurements, do the calculations and find that the ship's momentum plus the astronaut's momentum is now a positive number."
It has to be positive number because the gas and astronaut is negative number, isn't it?
The spaceship is positive number, the astronaut is unchanged (0) and the gas is negative number in the Universe frame.

You screwed up royally my friend. The spaceship and the astronaut is open system, not closed, you left out the engine gas.
The spaceship has a positive net momentum in the astronaut frame.
Are you going to admit that you made a mistake?
Jano

[Kryptid]

you left out the engine gas.

Your design doesn't have engine gas.

[Jaaanosik (OP)]

This is a completely worthless discussion. You are just as bad at physics as Dave Lev is. You don't understand what conservation laws are. You don't understand what a closed system is. And nothing I can do will be able to alleviate that ignorance. So I'm ending my attempt to educate you.

You might as well be arguing that you can charge up a dead battery using another dead battery, because if you believe that momentum can be created, then you might as well believe that energy can be created too. Conservation laws have no meaning to you.

Bobolink,
What happens when the Cubli rotates in zero-g?
Will the outside frame rotate 45 degrees as in the video if the frame is not attached to anything?
Yes, no?
Jano

[Jaaanosik (OP)]

you left out the engine gas.

Your design doesn't have engine gas.

I missed that you had my design of the spaceship in mind.
Then the question is if the CMGs center of mass would have unchanged position in the astronauts frame during/after reeling in the spaceship.
Imagine Cubli doing 45 degree impulse (jump up) as in the video.
The jump up 45 degrees and then it stops.
I am sure you are going to question the stopping part.
That's where the wobble example shows it is possible,
Jano

Edit:

The 6:03min into the video is very interesting part.

[Kryptid]

I am sure you are going to question the stopping part.

Nope. I'm not questioning anything else. I'm going to leave you to your ignorance now.