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Thought Experiment


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#1 Lowemack

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Posted 17 February 2017 - 01:15 PM

Imagine a children's roundabout, with vertical wheels around the circumference, pointing towards the centre. Each wheel has a weight on it's outside edge making each wheel unbalanced. If we spin all these wheels together in synchronisation, and the weights go up on the outside and the down nearer the centre, would there be a slight upward lift when the weights are moving up on the outside, and a slight downward pressure when they are going down towards the centre.

 

If the answer is yes, then my thought is that if we could keep the unbalanced weight on the outside, always going up then we would have a slight lift.

 

So, if remove the weights so the wheels are balanced, spin the wheels, then we spin the roundabout really fast, the outside edge of all the wheels would be heavier than than the inside edge of the wheels due to the fact that the linear velocity on the outside is greater than towards the centre and this would cause it to be heavier because relativity says faster things get heavier. This would give a constant lift.

 

The lift would only be small, but any non propellant thrust in space is really useful.

 

Also, apart from overcoming friction, it does not need any energy, once it is up to speed.

 

 


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#2 J.C.MacSwell

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Posted 17 February 2017 - 01:44 PM

You cannot change the velocity of the centre of gravity of an isolated system. You cannot even get it to wobble, even temporarily or slightly.

 

Not even in a thought experiment except with wrong assumptions or bad math. So if you come up with some contraption that appears like it might...you look for your error.


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#3 Bender

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Posted 17 February 2017 - 01:55 PM

Why would this be any different from your thread about "New non propellant thruster idea", where I already explained why it does not work.

 

To put it another way: if the wheels exert an upwards force on the roundabout, the roundabout exerts an equal and opposite force on the wheels. When you look at the total system, these forces cancel each other out.


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#4 Lowemack

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Posted 17 February 2017 - 02:11 PM

You cannot change the velocity of the centre of gravity of an isolated system. You cannot even get it to wobble, even temporarily or slightly.

 

Not even in a thought experiment except with wrong assumptions or bad math. So if you come up with some contraption that appears like it might...you look for your error.

Would a rotating unbalanced wheel not wobble?


Why would this be any different from your thread about "New non propellant thruster idea", where I already explained why it does not work.

 

To put it another way: if the wheels exert an upwards force on the roundabout, the roundabout exerts an equal and opposite force on the wheels. When you look at the total system, these forces cancel each other out.

I had to modify it slightly as I got the direction of thrust wrong, it would be tangential not centrifugal.


Try this similar thought experiment.

 

I am in  a space ship and I throw a cannon ball to the rear. I generate thrust until the canon ball hits the back of the space ship, then we will stop. Then if I walk to the cannon ball and bring it back, the ship will be back at it's start position.

 

What if before I throw the cannon ball I spin it really fast so it gains relativistic mass, then I throw it and before it hits the back of the space ship i can stop it spinning so it will have less mass, we must then generate overall thrust.

 

Rotating balls or stopping them spinning will not alter the spaceships linear momentum. 


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#5 Strange

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Posted 17 February 2017 - 02:26 PM

Would a rotating unbalanced wheel not wobble?

 
 
The wheel will wobble, but its centre of mass won't.
 

What if before I throw the cannon ball I spin it really fast so it gains relativistic mass, then I throw it and before it hits the back of the space ship i can stop it spinning so it will have less mass, we must then generate overall thrust.

 
This was also explained in your other thread. To spin the ball you need to add energy. To stop it spinning you have to remove that energy. So you have also transferred the energy from the front to the rear along with the mass of the ball.

 

You can't violate conservation of momentum.


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#6 J.C.MacSwell

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Posted 17 February 2017 - 02:37 PM

Would a rotating unbalanced wheel not wobble?


 

An isolated rotating unbalanced wheel must wobble, but as per Strange the centre of mass (and energy) will not.

 

So there is no need to look at a cycle of events where you shift an isolated systems mass/energy at one point, change something, reverse it, and ratchet yourself in any new direction...you simply can't to it at any point in the cycle.


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#7 Lowemack

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Posted 17 February 2017 - 03:41 PM

All the wheels are rotating and also rotating around the roundabout. They will all, constantly wobble upwards, because it is always the outside edge of each wheel that is heavier, because it has some extra relativistic mass than the inside, downward moving part.


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#8 Strange

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Posted 17 February 2017 - 03:54 PM

All the wheels are rotating and also rotating around the roundabout. They will all, constantly wobble upwards, because it is always the outside edge of each wheel that is heavier, because it has some extra relativistic mass than the inside, downward moving part.

 

 

Nope.


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#9 Lowemack

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Posted 17 February 2017 - 04:21 PM

Do you agree with the first paragraph in this post. If the whole roundabout was on a scale, would the reading decrease as the weights were going up and decrease when they are going down?


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#10 J.C.MacSwell

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Posted 17 February 2017 - 04:25 PM

Do you agree with the first paragraph in this post. If the whole roundabout was on a scale, would the reading decrease as the weights were going up and decrease when they are going down?

Yes. As the scale is not part of the system, the system is not isolated.

 

Isolated, there can be no net force on the system at any point in time. So the answer would be no without interaction with the scale or some other outside force.

 

The wheels can exert a force on the remainder of the system, only to the degree that the remainder of the system exerts an equal but opposite force on the wheels, so the momentum balance does not change without some external interference.


Edited by J.C.MacSwell, 17 February 2017 - 05:44 PM.

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#11 Bender

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Posted 17 February 2017 - 08:58 PM

Do you agree with the first paragraph in this post. If the whole roundabout was on a scale, would the reading decrease as the weights were going up and decrease when they are going down?

The surface of the roundabout will go up and down, but the centre of gravity of the entire system wouldn't.

 

If you change the weight distribution in the wheels is such a way that the unbalance is always at the same point, the roundabout wouldn't even do that.

 

Or, in other words: conservation of momentum.


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