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Newton's third law problem.
electrodacus:
--- Quote from: Nominal Animal on November 28, 2022, 09:59:18 pm ---Here's some of the actual physics related to this phenomena, for anyone interested:
Let \$v_v\$ be the vehicle speed with respect to ground, and \$v_w\$ be the wind speed with respect to ground.
For a vehicle that is in continuous contact with ground (through e.g. wheels), the amount of power available from the wind is
$$P_\text{in} = \frac{\kappa}{2} \rho A_\text{in} v_w^3 \tag{1}\label{1}$$
where \$\kappa\$ is the extraction efficiency, up to about 0.593 in theory (Betz's coefficient), and up to about 0.475 in existing devices; \$\rho\$ is the density of air; and \$A_\text{in}\$ is the cross-sectional area of the extraction device.
In addition to the ground friction losses, the amount of power lost via drag is
$$P_\text{out} = \frac{C_D}{2} \rho A_\text{out} \lvert v_w - v_v \rvert^3 \tag{2}\label{2}$$
--- End quote ---
OK so you agree with the correct equation for power available to any wind powered vehicle. Keep in mind that first equation is only for a stationary vehicle.
All you did is add the efficiency while I took the ideal case with 100% efficiency to be generous.
Glad you find the equations somewhere and agree with what I'm saying.
electrodacus:
--- Quote from: Kleinstein on November 28, 2022, 10:12:31 pm ---
The mechanical power is force times speed. So how on earth can you get 300 W with 0 speed as one factor.
A limited force and zero speed naturally gives zero power - where is the problem ?
--- End quote ---
Wind speed is not zero is 30km/h
Vehicle speed is zero.
electrodacus:
--- Quote from: Kleinstein on November 28, 2022, 10:08:57 pm ---So the wrong point is assuing the essentially (vehicle much slower than the wind) constant power from the kinetic energy of the wind as the power needed to move the vehicle.
--- End quote ---
That is exactly the case when vehicle drives upwind. Once you understand that you will understand all that I was saying.
To give you maybe a more intuitive example.
Imagine the vehicle is colliding with balls say 8 balls of 1.2kg each per second. That amount of lost kinetic energy will need to be replaced by the motor in order to maintain speed.
IanB:
--- Quote from: Kleinstein on November 28, 2022, 10:08:57 pm ---Common languish english seems to use the word "power" also for a few different things than the physical power.
--- End quote ---
I'm not sure how it translates in German, but in English technical language, thermodynamics has the two concepts of work and heat (first law: ΔU = Q - W). Mechanical work is force times distance, while heat relates to the molecular energy of materials. It gets confusing with power, because power applies to the rate of doing work, and also to the rate of transfer of heat (there are no different words for mechanical power and thermal power). Thus, you can have a 10 kW gas furnace that does no work at all.
Because of this, careful thermodynamic arguments tend to use Work and Heat as the terms of reference in order to be precise.
PlainName:
--- Quote from: electrodacus on November 28, 2022, 07:20:53 pm ---There is a simple question.
What is the wind power available to a wind powered vehicle ?
My answer (and not just mine) is this https://scienceworld.wolfram.com/physics/DragPower.html
That equation alone is what proves "your" explanation of how this vehicle's work wrong.
--- End quote ---
No it doesn't. Let's just assume that the equation is correct (there is some doubt) - you're not applying it appropriately. Specifically, you're ignoring the thrust from the propeller. There is thrust (it's a turning propeller, after all) so where does that figure in your equation?
--- Quote ---All you need to do to prove me wrong is do an experiment that shows that equation is incorrect.
--- End quote ---
Pointless. There are now a large number of such experiments showing that the thing works - pick any of those and there is your proof. All you're saying there is "I want you to waste your time and then I'll find some dubious reason why I'll still ignore the evidence." If the equation says to you that those experiments that work can't actually work, then either the equation is wrong or you're using the wrong one.
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