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Mess with your minds: A wind powered craft going faster than a tail wind speed.

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electrodacus:

--- Quote from: ejeffrey on September 18, 2021, 05:15:11 pm ---
Sure, it was your demo proposal, not mine.  You claimed this was a demo that would satisfy you and that it was impossible for the cart to go the opposite direction from the applied force. It clearly is possible as I have demonstrated and as I have explained both in terms of forces and torques as well as power transfer.

You can alter it to go the other way if you want, or faster than the paper in either direction with a suitable choice of gear ratio.  The traction requirement becomes larger so you will probable need to add some weight to the cart and pull carefully.  I can sort of get that to work with the same cart just putting the paper under the other wheel but it's hard to do without slip.

--- End quote ---

Yes it is impossible without energy storage to move the vehicle from left to right.  The force at wheel difference is a bad explanation as is about power at the wheel not force.
Vehicle can not travel faster than paper unless it uses energy storage. It may be that even in my video for some fraction of seconds vehicle was moving faster than paper but average speed was well below paper speed.  No amount of weight added to the cart will allow you to drive in average faster than the paper.
You can not put the paper under the other wheels as then vehicle will not move opposite to paper direction but it will move in the same direction as the paper (and I did just confirmed by testing on my model same one in the slow motion video).



--- Quote from: ejeffrey on September 18, 2021, 05:15:11 pm ---

--- Quote ---If you want to see things in modified reference frame

--- End quote ---

I would caution you against trying to think in multiple reference frames at once,.  It's true that you can solve the problem in any reference frame you want and get the right answer but in practice it is an easy way to get led astray.  This is because it is easy to come up with "invariants" that are not actually true in other frames.

One example is that the change of frame itself does not conserve energy.  In fact in any reference frame other than the lab frame the kinetic energy of the ground is "infinite" so you basically can't use global conservation of energy. Also if you solve part of the problem in one frame and switch you will get inconsistent results.  Another problem people have is that they accidentally use an accelerating reference frame such as a vehicle that isn't moving at constant speed.

All this is why I stick to the lab frame in all of these discussions.

--- End quote ---

You only get inconsistent results if you do not know how to calculate in a modified frame of reference.  Results in any reference frame will be exactly the same if you do things correctly.
You do not do the calculations correctly that is why I insist you do not modify the reference frame as it is the easier way to not mess things up.

Kleinstein:

--- Quote from: electrodacus on September 18, 2021, 05:34:37 pm ---Yes it is impossible without energy storage to move the vehicle from left to right.  The force at wheel difference is a bad explanation as is about power at the wheel not force.
Vehicle can not travel faster than paper unless it uses energy storage.

--- End quote ---
:bullshit: :bullshit: :horse:

Using force is the easier way and the normal way one does the calculation if one understand what force is.
Using power works, but has to take the different velocities into account and than gets the same result as using power.

PlainName:

--- Quote ---Yes it is impossible without energy storage to move the vehicle from left to right.
--- End quote ---

I see two situations here:

1. There is no energy storage and you are wrong.

2. There is energy storage but it doesn't matter because the vehicle moves from left to right as long as the paper is moving.

That's the crucial thing, is it? That the movement is not a temporary 'until the energy is gone' thing. Even if the alleged energy store was storing and releasing, storing and releasing, the thing is moving left to right without at any point moving right to left. And it will keep doing that for as long as the paper holds out. So how it does that in movement terms, in what way the energy is tranferred, is just a sideshow and irrelevant.

You entire supposition of the energy store is that it cannot last indefinitely, so the Blackbird can only slow down to below wind speed once it has used the store to get above wind speed. But here your store (that is, your energy store, wherever you happen to think it is) is able to keep supplying the left-to-right motion forever.

electrodacus:

--- Quote from: Kleinstein on September 18, 2021, 07:24:04 pm ---
Using force is the easier way and the normal way one does the calculation if one understand what force is.
Using power works, but has to take the different velocities into account and than gets the same result as using power.

--- End quote ---

I understand forces what most people do not understand is how forces act against the vehicle.
Power include the force and speed so is the other way around. You need to think about speed when looking at forces.
There are to many things you do not understand thus the reason it is hard for me to explain how things work.
You are just looking at real world examples and you do not understand how they work or why.

Imagine you have two electric motors on that vehicle both same exact model.
One motor is connected to front wheel trying to move the vehicle from right to left and one connected to the back wheel trying to move the vehicle from left to right (so opposite direction).
Front motor is directly connected 1:1 gear ratio and the other is connected trough a 2:1 gear ratio with 90% efficiency.
Both motors are getting 100W and say they are 100% efficient so 100W is also available at the shaft.
Now the front wheel will see 100W since is direct drive (hub motor if you prefer) and the back wheel spins at half the motor RPM so just 1.8x the force as it needs to go trough that 90% efficient gear.

Say wheels are not able to slip what direction will that vehicle travel ?
I say since there is 100W available at the front wheels they will move the vehicle backwards for right to left.
You say since force is higher at the back wheel 1.8x higher to be exact vehicle will move from left to right and you will be wrong.

In real world there are many more factors than the theoretical problem I explained above including possibly wheel slip (there is a threshold force for a wheel to start slipping and when that happens force possible at that wheel is lower as there is a hysteresis). That effect is seen in my slow motion video since as soon as the front wheel starts to slip the back wheels can just start pushing the vehicle forward using the stored energy.

electrodacus:

--- Quote from: dunkemhigh on September 18, 2021, 07:51:16 pm ---
--- Quote ---Yes it is impossible without energy storage to move the vehicle from left to right.
--- End quote ---

I see two situations here:

1. There is no energy storage and you are wrong.

2. There is energy storage but it doesn't matter because the vehicle moves from left to right as long as the paper is moving.

That's the crucial thing, is it? That the movement is not a temporary 'until the energy is gone' thing. Even if the alleged energy store was storing and releasing, storing and releasing, the thing is moving left to right without at any point moving right to left. And it will keep doing that for as long as the paper holds out. So how it does that in movement terms, in what way the energy is tranferred, is just a sideshow and irrelevant.

You entire supposition of the energy store is that it cannot last indefinitely, so the Blackbird can only slow down to below wind speed once it has used the store to get above wind speed. But here your store (that is, your energy store, wherever you happen to think it is) is able to keep supplying the left-to-right motion forever.

--- End quote ---

1. Sorry but how can you say there is no energy storage when is so clear seen in video and is clear to see the stretching of the rubber belt.
2. Look more closely at the video and read about stick-slip hysteresis if you did not heard about that before. The combination of those two the energy storage and stick slip hysteresis is what makes the vehicle move forward you just need to understand what you see in the video.

Vehicle is powered by the paper (hope you agree with that) so it can move forever but speed of the paper will always be higher than average speed of the vehicle. Vehicle speed can exceed temporarily the speed of the paper due to energy storage but the average speed is much lower.
What you see represented there is a the equivalent of the blackbird version that uses the propeller as a wind turbine and drives against the wind direction.

Please explain how vehicle is able to move from left to right.
Here is my explanation
Front wheel starts to spin while back wheel is locked and this is only possible if there is energy storage.
The belt acts with exactly the same force against the large pulley on the back wheel as it is against the small pulley on the front wheel.
Since the front wheel pulley is smaller the front wheel is the one that rotates but not slipping up to this point.
When enough energy is stored in to the belt the force that belt acts against the wheels will make the front wheel slip.
There is a minimum force threshold needed for the wheel to start slipping but once that happens the force needed to keep the wheel slipping (you may have experienced this if you ever drove a car on ice or snow).
Because of this since the back wheel is not slipping it can push the vehicle forward using the energy stored in the rubber belt and when the rubber belt energy drops low enough that it can no longer push the vehicle the front wheel stops slipping and things as seen in the video repeat.

If you do not agree with my explanation of what happen to the vehicle in my slow motion video please provide your explanation in a similar way.

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