General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
electrodacus:
--- Quote from: Kleinstein on August 31, 2021, 08:38:28 pm ---The red parts are wrong, get the 2 points right, and you should be allright.
1) When the vehicle moves left to right, the wheel will spin faster (unless it slips). One does not have to power the wheel, it is enough to move the vehicle. After all this is the G wheel and not the motor. So the generator does not have to more the vehicle, it just follows where the motor says is should go.
2) the M wheel can more the vehicle to the right, if it can provide enough force to do so. The force needed is the breaking force of the motor. This force not power power that needs to be overcome. The lower the speed of the vehicle relative to the fixed frame, the more force the motor can provide from a given power.
The final point is, that for the case that the motor drives to the right and the generator brakes, the power available to the generator is higher than the power needed by the motor. If you don't believe: take the simple example of the motor in stop (no power needed) and the generator is still driven by the treadmill speed. So there is plenty of power to spare and slowly move to the right.
Do the math and you see that it even works at higher speed if the efficiency is good (e.g. > 50%).
--- End quote ---
No the points are not wrong.
1)What do you imagine will make the G wheel spin faster than the treadmill ? The particular vehicle in my diagram will never move from left to right (I'm referring to that one without the spiral spring).
You are probably forgetting that motor wheel is powered only by the G wheel so power at the motor will be lower than power generated by G wheel.
Maybe you do not understand what generating a certain power at G wheel means (it means wheel will need to be breaking) and breaking G wheel means the treadmill will push the vehicle backwards and then M wheel can not push the vehicle back since it has only a bit less power than it was generated by G wheel. This is conservation of energy and was never broken.
2) It can not move the vehicle from left to right as it has less power than generated by G. If you want to compare forces only then you need to consider a gear ratio of 1:1. If you do not have the 1:1 gear ratio then you will need to consider the gear ratio and not only the force as force at the wheel acts against the chain / gear box and not directly against the vehicle.
That final point you try to make is valid for the dragged paper discuses by others here and not a treadmill. So that paper dragged from right to left is not the same with a treadmill.
So I think we first need to settle the fact that treadmill is not the same with a dragged surface. Understanding this may be the key to understanding the rest of the problem.
Kleinstein:
The vehicle will only see the upper side of the belt of the treadmill and it can not tell appart if the is a belt, or a piece of paper or rubber pulled by a human.
It is just stupid to insist on a difference between the paper and belt of the treadmill. For the sake of the discussion they are the same: a surface that is moved to the left.
------
for your response of the 2 simple points:
1) The vehicle moving to the right is the assumption you introduce in the sentence. So for the point 1) you assume it moves to the right - no matter what.
If the vehicle moves to the right, the wheel has to rotate faster - that is the geometriy and condition for the wheel no to slip. That is kind of the definition of a wheel. The treadmil and the motor combined drive the wheel - some of the power comes from the treadmill, that in the simple picture has plenty of power. Using the conservation of energy is a tricky argument in this comtext, as the treadmill constantly provides power. The conservation of energy does not prohibit the movement to the right - that is kind of handled in the 2.nd point.
2) The motor does not need more power than the generator to move the vehicle. It only has to have the same (or more if there is friction) force.
The force is power divided by speed. So if you keep the speed low enough there is always enough force.
(knowing the difference between power and force is really important here)
Gear ratio is another way to express the speed of the motor. The gear ratio of 1:1 is the critical case that does not work - so don't use it. But a gear ration so that the motor side is rotating slower (e.g. half the speed) does, and it provides enough force to do this against the generator pulling in the other direction.
If the final point works for the dragged paper, it also work for the treadmil, as the 2 cases are exactly the same. How should the vehicle tell the difference ?
electrodacus:
--- Quote from: Kleinstein on August 31, 2021, 09:21:32 pm ---The vehicle will only see the upper side of the belt of the treadmill and it can not tell appart if the is a belt, or a piece of paper or rubber pulled by a human.
It is just stupid to insist on a difference between the paper and belt of the treadmill. For the sake of the discussion they are the same: a surface that is moved to the left.
--- End quote ---
That is your wrong intuition that they will be the same but they are not even close.
--- Quote from: Kleinstein on August 31, 2021, 09:21:32 pm ---for your response of the 2 simple points:
1) The vehicle moving to the right is the assumption you introduce in the sentence. So for the point 1) you assume it moves to the right - no matter what.
If the vehicle moves to the right, the wheel has to rotate faster - that is the geometriy and condition for the wheel no to slip. That is kind of the definition of a wheel. The treadmil and the motor combined drive the wheel - some of the power comes from the treadmill, that in the simple picture has plenty of power. Using the conservation of energy is a tricky argument in this comtext, as the treadmill constantly provides power. The conservation of energy does not prohibit the movement to the right - that is kind of handled in the 2.nd point.
2) The motor does not have the same power as the generator to more the vehicle. It only has to have the same (or more if there is friction) force.
The force is power divided by speed. So if you keep the speed low enough there is always enough force.
(knowing the difference between power and force is really important here)
Gear ratio is another way to express the speed of the motor. The gear ratio of 1:1 is the critical case that does not work - so don't use it. But a gear ration so that the motor side is rotating slower (e.g. half the speed) does, and it provides enough force to do this against the generator pulling in the other direction.
If the final point works for the dragged paper, it also work for the treadmil, as the 2 cases are exactly the same. How should the vehicle tell the difference ?
--- End quote ---
This depends so much on you understanding that treadmill is something different that it will not make sense for me to try and answer them until you can understand that treadmill can not be replaced by a dragged piece of paper.
Alex Eisenhut:
Are you telling me a person standing on a stopped treadmill is going to have to run in another direction when the treadmill starts, than someone who has the rug under them pulled in the same direction?
electrodacus:
--- Quote from: Alex Eisenhut on August 31, 2021, 09:32:16 pm ---Are you telling me a person standing on a stopped treadmill is going to have to run in another direction when the treadmill starts, than someone who has the rug under them pulled in the same direction?
--- End quote ---
A person on a treadmill is not between two separate mediums at the same time unless it has a foot on treadmill and one on the ground.
Not sure if this sort of analogy is good for anything.
A person on a treadmill that is turned on is not powered by the treadmill it will need to use his own calories just to stay on the treadmill.
But if you turn OFF the treadmill and instead someone pushes the treadmill then person on treadmill has no need to use any of his calories.
This at least shows that a running treadmill is not the same with a powered OFF treadmill that is pushed backwards.
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