Does a spinning disk gain relativistic mass

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Phase 1 The spinning disk is moving downwards in a container, its spin is slowed to a stop.

Phase 2 The disk is accelerated upwards with a force, it slows and starts moving upwards.

Phase 3 The disk is spun up giving it more relativistic mass.

Phase 4 The disk is accelerated downwards with a force, it slows and starts moving downwards.

So Phase 2 would accelerate the system downwards, and Phase 4 would accelerate the system upwards.

Phase 4 would need more force than Phase 2, That would make the system accelerate.

I don't see how Phase 1 and Phase 3 would make any difference.

You're still ignoring the rest of the mass of the system and specifically what has the energy when the wheel is at rest. Energy is conserved. It doesn't just show up to spin the wheel up and disappear when the wheel slows.

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Ok, then transfer the energy into a disk that is attached to the container using AC current so there is no electronic mass transfer.

This disk can spin when the moving disk isn't and not spin when the moving disk is.

This will help dampen Phase 2.

Or even better, have another disk doing the same thing but going down when the other is going up.

so Phase 1 and Phase 3 would be just transferring spin from one disk to the other as it goes past.

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Ok, then transfer the energy into a disk that is attached to the container using AC current so there is no electronic mass transfer.

You still have energy transfer, which represents mass.

Is there some reason why you make specific mention of AC? DC has no net mass transfer, either.

Anyway, I'm still not seeing any analysis showing why the CoM should move. We'll eventually arrive at the discussion that the disk must deform as it speeds up, and that's not going to be a trivial CoM calculation.

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You still have energy transfer, which represents mass.

Is there some reason why you make specific mention of AC? DC has no net mass transfer, either.

Anyway, I'm still not seeing any analysis showing why the CoM should move. We'll eventually arrive at the discussion that the disk must deform as it speeds up, and that's not going to be a trivial CoM calculation.

Thinking through this without implying any certainty (read- JC not sure what he is talking about)

I don't think it will work. I think Conservation of momentum must hold...

having said that:

...but if you are transferring energy by pure torque (could be balanced torque/counter torque double flywheel discs on same axis, so the main body of the system remains stationary), you are displacing energy along the axis of rotation...

If you can do that with no balancing opposite effect, you have introduced a mechanism to displace the center of mass of a closed (isolated) system, contrary to the law of conservation of momentum.

and having said that;

Since you cannot do that...what is the balancing opposite effect? The main body (slightly lighter, having lost some energy) should be displaced just enough to maintain conservation of momentum when it pushes energy along the axis (so in fact does not remain stationary)

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o, i was thinking that dc current pushed electrons which have mass one way.

The energy would transfer from one disk to the other as they went past the middle, so no net energy transfer in any direction.

I would think that the CoM would be in the middle of the system when both disks pass each other, and then go towards the top where the spinning disk goes.

There could be a counter weight that moves to keep the CoM in the middle.

The issue is that the force needed to repel the spinning disk would be more than the for the non spinning disk, and since there would be an opposite reaction, then the container would gain speed upwards.

Edited by 514void
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o, i was thinking that dc current pushed electrons which have mass one way.

DC circuits are a loop. One wire in, one wire out.

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cool, so what is the issue with energy transfer?

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cool, so what is the issue with energy transfer?

You are using energy to make your disk spin. This extra energy is what is called "relativistic mass". This energy has to come from somewhere (a battery? a tank of gas?). When you slow the disk down again, the energy has to go somwhere.

All you are doing is moving energy around. Which is no different from moving mass around and, therefore, will no provide you with any form of propulsion. TANSTAAFL

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ok, I can see where you are confused, it takes energy to spin up a disk, but there is no propulsion from this bit, the propulsion is from reacting to this energy with a force.

What is TANSTAAFL?

Edited by 514void
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ok, I can see where you are confused, it takes energy to spin up a disk, but there is no propulsion from this bit, the propulsion is from reacting to this energy with a force.

How do you exert the force?

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whichever way, with a spring or a mechanical arm or EM field or something else.

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So what happens when the spring pushes against the mass? What's on the other end of the spring?

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ok, I can see where you are confused, it takes energy to spin up a disk, but there is no propulsion from this bit, the propulsion is from reacting to this energy with a force.

What is the point of spinning the disk? To increase its mass, I assume. Where does that extra mass come from? The energy source.

(And swansont has asked the other obvious question.)

What is TANSTAAFL?

There ain't no such thing as a free lunch.

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Ok, then transfer the energy into a disk that is attached to the container using AC current so there is no electronic mass transfer.

This disk can spin when the moving disk isn't and not spin when the moving disk is.

This will help dampen Phase 2.

Or even better, have another disk doing the same thing but going down when the other is going up.

so Phase 1 and Phase 3 would be just transferring spin from one disk to the other as it goes past.

No matter how you store the energy, it represents relativistic mass and moving it from one end of the container to the other is just like moving mass from one end to the other.

So, let's assume that your energy source is located half way between the ends. As this energy source transfers energy to the spin the disk at the top, you are also transferring relativistic mass from the center to the top. The container will shift downward to compensate in order to keep the CoM stationary in an inertial frame.

You move the spinning disk downward, and the container moves up in response, again without the CoM moving with respect to the inertial frame.

The disk reaches the bottom and slows down transferring its energy back to the energy source. Moving the energy back up to the middle causes the container to shift down.

Move the disk back up to the to and the container shifts down some more. In neither Case does the CoM shift in the Inertial frame.

When it gets back to the top, you are back where you started with no change in the CoM.

In essence, moving the rest mass of the disks from top to bottom and back will not shift the CoM, and moving the energy and its relativistic mass from center to top, from top to bottom and from bottom top center has no effect on the net shift on the CoM.

It does not matter what form the energy has, kinetic or electric, it still has an equivalent relativistic mass and it moving around in the container has the same effect as moving a rest mass would.

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No matter how you store the energy, it represents relativistic mass and moving it from one end of the container to the other is just like moving mass from one end to the other.

So, let's assume that your energy source is located half way between the ends. As this energy source transfers energy to the spin the disk at the top, you are also transferring relativistic mass from the center to the top. The container will shift downward to compensate in order to keep the CoM stationary in an inertial frame.

You move the spinning disk downward, and the container moves up in response, again without the CoM moving with respect to the inertial frame.

The disk reaches the bottom and slows down transferring its energy back to the energy source. Moving the energy back up to the middle causes the container to shift down.

Move the disk back up to the to and the container shifts down some more. In neither Case does the CoM shift in the Inertial frame.

When it gets back to the top, you are back where you started with no change in the CoM.

In essence, moving the rest mass of the disks from top to bottom and back will not shift the CoM, and moving the energy and its relativistic mass from center to top, from top to bottom and from bottom top center has no effect on the net shift on the CoM.

It does not matter what form the energy has, kinetic or electric, it still has an equivalent relativistic mass and it moving around in the container has the same effect as moving a rest mass would.

Thanks Janus, that is what I suspected though was not absolutely sure.

So there must be an axial force (as well as the torque, and separate from the one proposed by 514 for acceleration), however slight, that would push the driving energy along the axis of rotation, which would then reverse and then disappear as the energy is transferred?

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a mechanism (maybe a spring) would push against the container and the spinning disk at the top,

and another mechanism would push against the container and the non spinning disk at the bottom.

It would push against the axle of the disk without affecting its spin.

the forces from these mechanical interactions would cancel out rest mass, but there would be relativistic mass to counter by the top mechanism.

The CoM (Center of Mass) could be kept in one place in the system with a counter weight, but the difference in forces that change the disks direction would create a net force on the system to move it upwards.

In regards to Janus saying that moving energy to a disk would move the container,

what would happen if the energy transfer was from slowing a spinning disk to a stop, and the energy was transferred to a non spinning disk to spin it.

would these disks accelerate in a certain direction?

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The CoM (Center of Mass) could be kept in one place in the system with a counter weight, but the difference in forces that change the disks direction would create a net force on the system to move it upwards.

No, it wouldn't. Each of the forces on the disks also exert forces on the container, not on each other. The container will shift in response to any difference in position of the masses. You don't get to dictate that the action and reaction forces will be different. The will always be equal.

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the disk that is spinning will have extra relativistic mass, so the force will be greater.

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the disk that is spinning will have extra relativistic mass, so the force will be greater.

<sigh> But that extra mass will have come from somewhere. So you have already moved the mass in one direction, now you are just going to move it back again.

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the disk that is spinning will have extra relativistic mass, so the force will be greater.

And so will the force that's exerted on the container.

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yes, the relativistic mass is gained without any force exerted on the container, and the force that is exerted on the container will be greater than the force on the opposite side of the container. I will make a diagram so that you can understand it. (might take a while, I'm not so good at diagrams.)

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yes, the relativistic mass is gained without any force exerted on the container, and the force that is exerted on the container will be greater than the force on the opposite side of the container. I will make a diagram so that you can understand it. (might take a while, I'm not so good at diagrams.)

You're missing the point. It doesn't matter if those forces match or not. All that matters is that the force on the container matches the force on the mass, which it will.

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Its like 2 people on a long skateboard

person 1 throws an object.

while its travelling to person 2 it gains mass without it changing speed.

person 2 catches it and throws it back.

it loses mass without changing its speed.

person one catches it.

They would accelerate.

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Its like 2 people on a long skateboard

person 1 throws an object.

while its travelling to person 2 it gains mass without it changing speed.

person 2 catches it and throws it back.

it loses mass without changing its speed.

person one catches it.

They would accelerate.

Where is this extra mass coming from? You are failing to take the entire system into account.

The extra mass comes from the energy you give the disk. That energy must have come from somewhere, e.g. a battery. So the battery will lose an equal amount of mass. When you take that into account, there is no net change in position or velocity.

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Its like 2 people on a long skateboard

person 1 throws an object.

while its travelling to person 2 it gains mass without it changing speed.

person 2 catches it and throws it back.

it loses mass without changing its speed.

person one catches it.

They would accelerate.

In their rest frame, the would simply wiggle. The person throws the rock, and the skateboard recoils. The rock would not gain relativistic mass after leaving the first thrower, as it doesn't change speed in flight. When the second person catches the rock, the skateboard will stop. Throwing it back reverses the motion. At no point does the CoM change.

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