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

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bdunham7:

--- Quote from: gnuarm on December 27, 2021, 08:30:58 pm ---Consider the worst case where the speed of the vehicle has dropped infinitesimally toward zero.  In that case vo has approximated zero and so the power required to move the car has approximated zero. 

--- End quote ---

I'm afraid reductio ad absurdum isn't going to work here.  You can't convincingly reduce an argument to an absurdity when it already is an absurdity.

gnuarm:

--- Quote from: electrodacus on December 27, 2021, 08:38:05 pm ---
--- Quote from: gnuarm on December 27, 2021, 08:30:58 pm ---
Consider the worst case where the speed of the vehicle has dropped infinitesimally toward zero.  In that case vo has approximated zero and so the power required to move the car has approximated zero. 


How much power does that formula say is required for a zero bike speed?   The correct answer is zero.  I think anyone who understand work and power understand there is no work if there is no motion, so no power also.  Otherwise there are a lot of buildings doing a lot of work which we should be able to tap into.  We could make windmills without any movement!  That would be a big plus!!!

--- End quote ---

That is not the correct answer. You assume there is brake. Remove the brake and think again what will be the power needed to maintain around zero speed while you have a headwind.
You will need to remove the brakes in order to move so at very low speed you have no brakes to anchor you to the ground.

--- End quote ---

I didn't say anything about a brake.  I said zero motion.  Work is force over a distance.  You can push as hard as you want against an object and if it does not move you have done no work on it.  You may have sweated up a storm, but that's internal inefficiencies.  Actually, I can lean into a wall without any real effort on my part.  I'm just using my weight and gravity to provide the force.

I would construct some examples to show you there is no work being done, but it really gets tiresome that you just can't understand the basic concepts. 

So you think that holding an object in a fixed position against a force requires performing work on the object?  Do you understand what is meant by "performing work" in the physics sense?  If the wind is blowing, and something is pushing against the object, but not moving it against the wind, is the something doing work? 

electrodacus:

--- Quote from: gnuarm on December 27, 2021, 08:49:17 pm ---I didn't say anything about a brake.  I said zero motion.  Work is force over a distance.  You can push as hard as you want against an object and if it does not move you have done no work on it.  You may have sweated up a storm, but that's internal inefficiencies.  Actually, I can lean into a wall without any real effort on my part.  I'm just using my weight and gravity to provide the force.

I would construct some examples to show you there is no work being done, but it really gets tiresome that you just can't understand the basic concepts. 

So you think that holding an object in a fixed position against a force requires performing work on the object?  Do you understand what is meant by "performing work" in the physics sense?  If the wind is blowing, and something is pushing against the object, but not moving it against the wind, is the something doing work?

--- End quote ---

I remove the brakes from a bicycle you can sit on that and then in a 230km/h head wind and see if you can have a low speed close to zero relative to ground. In fact you can keep the brakes and you will still have no change against 230km/h wind.
A wall is anchored to ground.

Kleinstein:
If a sail would be more efficient than the wind turbine, there would be more sails and less turbines. :-DD

To show than an equation is correct, one can not use that same equation as an argument. That's not how logic works - sorry.


--- Quote from: electrodacus on December 27, 2021, 08:13:05 pm ---My earlier example is perfectly correct and for a bicycle to move at 1km/h against a 230km/h head wind it will require a minimum of 66kW and gears can not help with anything as a cyclist may be able to provide 300W for minutes and maybe peak around 1kW it will never be able to provide 66kW and so never be able to move against a 230km/h head wind let a lone the possibility that 300W is enough to do that as the wrong formula and that calculator will imply.

--- End quote ---

I don't hink the high power makes sense: We all know mechanical power is force times speed.

With a constant power at a low speed this would need an awful lot of force - more force the slower you go. With this logic it would be impossible the slowly walk against an even weak head-wind, as it would need near inifite force at a very slow speed.

electrodacus:

--- Quote from: Kleinstein on December 27, 2021, 08:58:05 pm ---
I don't hink the high power makes sense: We all know mechanical power is force times speed.

With a constant power at a low speed this would need an awful lot of force - more force the slower you go. With this logic it would be impossible the slowly walk against an even weak head-wind, as it would need near inifite force at a very slow speed.

--- End quote ---

What if the bicycle traveled at 231km/h ? Will you then agree that it needed 66kW to do so ?
If you agree with the above why do you think there is a difference between vehicle moving or air moving ?
Yes the force needed is ridiculous that is why in real world it will not happen the bike will just slip and be pushed in the wind direction.

You can slow walk with low head wind speed but not against a 230km/h wind that will not be possible.
Against a weak wind you can.  For a bicycle you can slowly bike against 35 to 40km/h depending on fit you are and how long you need to do that.
Every time the head wind doubles in speed you need 8x more power so 80km/h (8x more power than at 40km/h) is already impossible even for best most fit cyclist.

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