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Newton's third law problem.
Kleinstein:
The equation / expression alone does not prove anything: one has to use the formulas in the right context. Just because a formula applies to one context does not mean it is also correct in another context.
An important point constantly wrong by electrodacus is assumung a balance of power to decide if a vehicle can move. The seems to be his starting point leeding to lots of stupid mistakes and claims.
The more relevant point is the balance of forces.
fourfathom:
--- Quote from: electrodacus on November 28, 2022, 06:39:23 pm ---You do realize you try to describe an overunity device ? If vehicle is direct down wind aboe wind speed.
--- End quote ---
But I didn't describe an over-unity device. If I had, the device would accelerate with zero (ground-referenced) wind speed. Nobody is claiming this, and no experiments have shown this, because this *ISN'T* over-unity.
Your cherished equation for "wind power available to a wind powered vehicle" applies to an isolated wind-generator mounted on a moving vehicle, or mounted on the ground, assuming the equation is correct, which may be the case. Yes, of course, at zero relative windspeed that wind-generator will deliver no power. But that's not the situation, and your equation does not apply.
Stop thinking of the propeller as a wind-generator. That's not how it's operating in the DDWFFTW situation. The propeller provides the propulsion -- it's a "pusher prop".
Please go back to the wheels, gears, and two surfaces model. Try to understand how the gears are not locked. Otherwise you are doomed.
electrodacus:
--- Quote from: Kleinstein on November 28, 2022, 08:19:55 pm ---The equation / expression alone does not prove anything: one has to use the formulas in the right context. Just because a formula applies to one context does not mean it is also correct in another context.
An important point constantly wrong by electrodacus is assumung a balance of power to decide if a vehicle can move. The seems to be his starting point leeding to lots of stupid mistakes and claims.
The more relevant point is the balance of forces.
--- End quote ---
Yes you are correct in the fact that formula needs to be applied in the correct context also.
So here is the simplest test that shows what I'm claiming.
Have an electric vehicle (simpler that way) even an ebike drive directly upwind within a known wind speed and see what power is needed to start moving.
All of you seems to be saying this below is power needed by the motor (correct me if you think something else) when vehicle wants to move upwind:
P = Fd * vehicle speed
I (and others) say:
P = Fd * (wind speed - (-vehicle speed)) = Fd * (wind speed + vehicle speed).
The difference is so large that it will be easy to measure.
This calculator can be used for the equation I agree with https://www.electromotive.eu/?page_id=12&lang=de
A day with 30km/h winds should be easy to find (right now at my location is 50km/h with 70km/h gust)
Anyone that biked with a 30km/h wind gust will know what a struggle that is so with my prediction power needed to be provided to drive at 5km/h is
We use 0.827 as effective projected area as default in that calculator
Fd = 0.5 * 1.2 * 0.827 * (35/3.6)2 = 46.9N (we all agree with this)
Then power needed for propulsion I say
P = 46.9 * (35/3.6) = 456W a bit less than what that calculator will show 554W as that takes rolling resistance and drivetrain efficiency in to account.
But you say
P = 46.9 * (5/3.6) = 65W and that is such a large difference that is very easy to verify.
Circlotron:
--- Quote from: electrodacus on November 28, 2022, 05:07:21 pm ---
--- Quote from: Circlotron on November 28, 2022, 11:03:15 am ---If you are staying at zero speed then no work is being expended. A force maybe, but if there is no movement then no power is being used.
--- End quote ---
Have you ever had an electric motor providing a force / torque without using energy ? Or even simpler an electromagnet providing a force without using energy.
Even you using your arm to provide a force will be using energy even tho there is no movement and yes that energy will be dissipated as heat.
--- End quote ---
A superconducting motor or electromagnet would continue to supply a non-moving force without any ongoing power being consumed once initially energised. For that matter, so would a permanent magnet. I have seen that “supplying continuous power without movement” argument used by the free energy from magnets crowd.
As far as your arm needing energy to produce a non moving force, that’s true, but you could also get that force by for example driving a wedge into a narrow space. Once you have stopped pressing it in you are no longer putting energy into it but it continues to exert a force against an object. Are we to assume it continues to dissipate energy into this object indefinitely? A force, certainly, but there is no movement, so where is the energy?
Kleinstein:
Driving 5 km/h against a 30 km/h head wind is relatively easy, if you have suitable low gears (e.g. montain bike). The difficult point is more to keep the balance at slow speed, if gusty.
5 km/h is about walking speed and when walking 30 km/h head wind is not hard either.
Providing 460 W is in contrary only possible for a short time for a top athlet.
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