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Mess with your minds: A wind powered craft going faster than a tail wind speed.
bdunham7:
--- Quote from: electrodacus on December 16, 2021, 06:13:58 pm ---So this boat with a wind turbine and an energy storage device can finish the race much faster than 1h
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Oh my! What a load of.....????
What was said was that a sailboat without any energy storage can sail the 20 miles in less than an hour. In fact, give enough distance and a steady 20mph wind from the east, it could sail west indefinitely at more than 20mph.
electrodacus:
--- Quote from: Kleinstein on December 16, 2021, 06:22:50 pm ---
The air resistance is a force. So the force is the same, however that does not mean that the power is the same.
Is there any problem understanding the very basic equation for mechanical power being force time velocity ?
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That force will multiply with air speed to get the power available. If you transfer all that force to ground through a bake then vehicle is part of the earth.
As soon as you want the vehicle to move relative to earth vehicle is on his own and it needs that specific power to be able to move against the wind direction.
I'm not going to provide the same examples again but look at the posts I made yesterday to find examples.
Answer this for an ideal case no friction.
How long will it take for a 100kg vehicle to be accelerated from 0m/s to 5m/s with vehicle powered by wind directly downwind and the sail of the vehicle is 1m^2 Wind speed is 10m/s and air density 1.2kg/m^3
Use whatever equations you want answer will be in seconds.
electrodacus:
--- Quote from: bdunham7 on December 16, 2021, 06:32:11 pm ---Oh my! What a load of.....????
What was said was that a sailboat without any energy storage can sail the 20 miles in less than an hour. In fact, give enough distance and a steady 20mph wind from the east, it could sail west indefinitely at more than 20mph.
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I do not get your reaction. I proved to you that a vehicle that has an energy storage device can get there in less than an hour.
Blackbird has an energy storage device (pressure differential created by the propeller).
Kleinstein:
--- Quote from: electrodacus on December 16, 2021, 06:28:33 pm ---And yes I fully agree that you should know what to expect as a result.
There is no problem in using force instead of power but in that case you need to know what is the correct speed you need to multiply with to get the correct result.
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"Knowing" (or better some intuition) what to expect as the result, however does not mean to put you intuition for the result over the calculation. A carefully done calculation is way more reliable than the intuition. There are some effects in physics that confuse a simple mind and some effects even surprise an expert (though less often). The vehicle in the video is such a case that may cause irritation, though this still a relatively simple one.
Of cause there is not problem using forces and it is the easier way here. Trying to use only power and avoid to use forces at all costs can be quite error prone. It can work in some cases, but gets really tricky with elements like a sail or prop that have less than 100% efficiency.
Of cause one needs to use the right speed and here there seems so be a controversy between electrodacus and essentially the rest of the world, on which speed to use.
--- Quote from: electrodacus on December 16, 2021, 06:43:38 pm ---
That force will multiply with air speed to get the power available. If you transfer all that force to ground through a bake then vehicle is part of the earth.
As soon as you want the vehicle to move relative to earth vehicle is on his own and it needs that specific power to be able to move against the wind direction.
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The power actually transfered to the vehincle or needed to more the vehicle is not the same as the power theoretical available from the wind. For the power transfered to the vehicle or needed to move it against the wind, the relevant speed is the speed of the vehicle relative to the ground (the reference system you transfer the force to or calculate the kinetic energy in).
Using the wind speed would need the addition seprate treatment of moving with or against the wind or standing still. Equations in physics are rarely so unsteady, especially not basic mechanics. A high power already at near zero speed would also mean diverging (near infinite) acceleration to store all that power in kinetic energy. So using the full wind speed for the power transferred to the vehicle is obviously wrong.
IanB:
--- Quote from: electrodacus on December 16, 2021, 06:45:31 pm ---I do not get your reaction. I proved to you that a vehicle that has an energy storage device can get there in less than an hour.
Blackbird has an energy storage device (pressure differential created by the propeller).
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That's irrelevant. A sailboat, with no turbine, and no energy storage, can sail 20 miles downwind in less than an hour with a 20 mph following wind. It could do this indefinitely.
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