Does a spinning disk gain relativistic mass

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it would gain relativistic mass after leaving the first thrower with spin.

Edited by 514void

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So you have two equal rest masses moving up and down the whole length of your container. No net effect.

You also have the energy (extra "mass") in the spinning wheel moving up and down in the top half of the container. No net effect.

(This is actually clearer if you cover up either the top or bottom half of your diagram.)

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You can analyze each wheel independently. Neither one will cause the CoM to change.

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Yep. However you analyse it ... nothing happens.

Although, eventually the whole thing will stop when you run out of energy. And then you will have wasted a huge amount of energy not going anywhere. A bit like this thread.

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umm, you do realise that a heavier object is harder to change direction right?

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umm, you do realise that a heavier object is harder to change direction right?

So? You have to reverse the direction of that extra mass (energy) at the top when the wheel reverse direction and at the middle, when you transfer the energy from the downward-moving wheel to the upward moving one.

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yes, and spinning an object will not cause it to speed up or slow down.

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umm, you do realise that a heavier object is harder to change direction right?

Yes, and it has no impact on the conclusion.

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yes, and spinning an object will not cause it to speed up or slow down.

That seems irrelevant. You have transferred the energy from one disk to the other. Therefore the energy (which is where your "relativistic mass" comes from) is reversing direction at the top and at the center.

I think I will give up. You clearly can't see that all you are doing is moving a couple of masses and some energy backwards and forwards which will have no net effect.

Edited by Strange
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it is reversing direction at the top with a force which will accelerate the system.

it is reversing direction in the middle with no net force. (this bit is the key)

Edited by 514void
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it is reversing direction in the middle with no net force. (this bit is the key)

Why does it have no net force? (This may be the key bit you don't understand.)

You are reversing the direction of your relativistic mass (aka energy) at that point.

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it is reversing direction at the top with a force which will accelerate the system.

Stating this doesn't make it true. All forces come in action/reaction pairs. The force exerted on the mass is the sam as the force exerted by the mass. Thus, whatever is pushing on the mass feels the same force, in the opposite direction. The rest of the mass of the system moves opposite to the spinning mass, such that the CoM doesn't move.

You can't change the answer by ignoring half the problem.

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it would gain relativistic mass after leaving the first thrower with spin.

OK. Where does the energy come from to spin the wheels? Where is the source of that energy? How do you account for the mass inertia of that energy? How does it get displaced to the double wheel system?

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Why does it have no net force? (This may be the key bit you don't understand.)

You are reversing the direction of your relativistic mass (aka energy) at that point.

well if spinning something created a net force in a certain direction, then which direction would that be?

I assume you mean that the disk would slow down when you spin it up and speed up when you slow it down.

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well if spinning something created a net force in a certain direction, then which direction would that be?

I assume you mean that the disk would slow down when you spin it up and speed up when you slow it down.

No I don't mean that at all. Of course spinning something will not create a net force. Do you think it will? Why? And why would the disk speed up or slow down?

I mean that you are just moving stuff backwards and forwards. The "stuff" is the rest mass of the wheels and the energy that gives them increased "relativistic mass".

Moving stuff backwards and forwards will not propel you anywhere.

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OK. Where does the energy come from to spin the wheels? Where is the source of that energy? How do you account for the mass inertia of that energy? How does it get displaced to the double wheel system?

there would be motors i suppose that would spin the double wheels to speed them up and maybe use them as generators to slow them down.

any extra energy would be provided by a power source, maybe put that in the middle somewhere.

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there would be motors i suppose that would spin the double wheels to speed them up and maybe use them as generators to slow them down.

any extra energy would be provided by a power source, maybe put that in the middle somewhere.

You say, "any extra energy" as if you think the motors spinning the wheels don't need any energy.

And don't you see that if you are transferring the energy from the power source to the wheels and back again, there is no net change?

Most importantly: do you understand that (relativistic) mass and energy are the same thing?

Transferring energy to and from the power source or between the wheels is exactly the same as moving mass between them. Do you understand that?

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No I don't mean that at all. Of course spinning something will not create a net force. Do you think it will? Why? And why would the disk speed up or slow down?

I mean that you are just moving stuff backwards and forwards. The "stuff" is the rest mass of the wheels and the energy that gives them increased "relativistic mass".

Moving stuff backwards and forwards will not propel you anywhere.

the forces on the rest masses would cancel themselves out.

there is no net force in moving the energy from downwards to upwards.

there is a net force moving the energy from upwards to downwards.

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the forces on the rest masses would cancel themselves out.

OK

there is no net force in moving the energy from downwards to upwards.

there is a net force moving the energy from upwards to downwards.

Why is there a net force moving the energy one way and not the other??

Edited by Strange
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You say, "any extra energy" as if you think the motors spinning the wheels don't need any energy.

And don't you see that if you are transferring the energy from the power source to the wheels and back again, there is no net change?

Most importantly: do you understand that (relativistic) mass and energy are the same thing?

Transferring energy to and from the power source or between the wheels is exactly the same as moving mass between them. Do you understand that?

I realise that in 100% efficient energy transfer you would not need extra energy.

the energy transfer in a 100% efficient system would just be from the spinning wheels to the non spinning wheels to swap their spins.

so the energy transfer to spin and unspin the wheels would create a net force?

how would this manifest?

OK

Why is there a net force moving the energy one way and not the other??

because spinning up things creates no translational force.

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so the energy transfer to spin and unspin the wheels would create a net force?

Transferring energy is identical to transferring the equivalent mass.

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Transferring energy is identical to transferring the equivalent mass.

so you are saying that transferring spinning energy from one disk to the other would speed up one and slow down the other translationarily?

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so you are saying that transferring spinning energy from one disk to the other would speed up one and slow down the other translationarily?

What effect would moving a mass between them have?

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What effect would moving a mass between them have

If anything it would accelerate them away from where the wire was attached to the motor?

I don't think that is right.

Anyhow giving a disk relativistic mass by giving it spin wouldn't change its translatory speed, or otherwise there would be some preferred frame to speed up or slow down against.

Edited by 514void
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it would gain relativistic mass after leaving the first thrower with spin.

Here's what you are missing: The spinning wheels which are as they are moving down to the center have downward momentum, due to both the rest mass of the wheels moving downward and the relativistic mass from their spin. When the two sets of wheels meet and you transfer the energy from one set of wheels to the other, you also transfer the downward momentum of the relativistic mass at the same time. This will work against the upward momentum of the wheels that have taken on this energy, and they will slow down. For example, if the rest mass momentum and relativistic mass momentum from spinning were, after the transfer of spin from the one set of wheels to the other, the set receiving the energy would stop its upward movement. The only way it can continue its upward movement at the same speed it had after the transfer is for it to receive an additional push from the container.

In that case all you have is a system were you have two "masses" moving back and forth. With the rest masses moving back and forth the whole length of the container and the relativistic masses moving back and forth for half the container. Each set of masses carries its own momentum and reversing the direction of these masses both have an effect on the container.

You cannot just wish away this transfer of momentum of the passing wheels.

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