General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
<< < (191/285) > >>
electrodacus:

--- Quote from: msuffidy on December 21, 2021, 05:40:17 am ---No really, I am not sure what is going on here,but in general, this shape somehow slows down other air around the craft and thus converts this energy into a greater speed for the craft. So conservation is retained as the other air is slowed down. This could have to do with a strong negative energy source against the positive one, caused by the positive one.

--- End quote ---

What happens is that energy is stored as pressure differential (propeller uses some of the wind power to increase pressure differential).
This stored energy while vehicle is below wind speed is then used to accelerate for some limited amount of time above wind speed.
gnuarm:

--- Quote from: electrodacus on December 21, 2021, 04:57:43 am ---
--- Quote from: bdunham7 on December 21, 2021, 04:40:22 am ---
--- Quote from: electrodacus on December 21, 2021, 04:33:44 am ---There is nothing wrong with that definition if you understand what it means.

--- End quote ---

What definition?  ::)   Where have you 'defined' power other than to say that it has no time dimension?

--- End quote ---

I need to say that since you think it has a time dimension.  I was refereeing to the definition you provided.
Power is just the rate at which energy is converted to another form so it is not containing time. The thing that contains time is energy.

So when I say 10W of braking power and I do not mention any time interval for witch this power is applied then time is not involved at all and no work it is being done.
You may know speed as in case A 2m/s and you can calculate force 5N but that is about all you can know. You can not say about any changes to vehicle speed or kinetic energy as time is not mentioned anywhere.
As soon as I say the 10W is applied for 1ms then you have the time and also the energy 0.01Ws and thus you can calculate what happens to vehicle over that time period including change in kinetic energy and speed.
To me it just seems you do not see power for what it is and it just seems you confuse that with Energy.

--- End quote ---

Power is in units of work per time, J/s.  The 10W already includes the time element as it is joules per unit time.  Joules is not defined with any regard to time.  It can be 10 W for one second or 1 W for ten seconds.  10 W is 10 J/s.  So that could be 10 joules in 1 second, or 100 joules in 10 seconds, but it is 10 joules per second. 

Interesting that neither work nor power are considered SI base units, but that's not important.  I don't know if this matters either, but NIST shows work (energy) as derived from force and distance.  They show power as defined by work per unit time (J/s). 

NIST Diagram

Does that help?
gnuarm:
What brought it home for me was when the video talked about the difference in ground speed and air speed.  The wheels are working at ground speed while the propeller is dealing with the wind speed.  The higher ground speed means the "gearing" (including the propeller pitch, et. al.) allows the faster ground speed to create less resistance than the force generated by the prop. 

As others have pointed out the propeller blows air backwards from the car reducing the wind speed and lowering the wind's energy content conserving energy.

That should cover it, no?
electrodacus:

--- Quote from: gnuarm on December 21, 2021, 05:49:25 am ---
Power is in units of work per time, J/s.  The 10W already includes the time element as it is joules per unit time.  Joules is not defined with any regard to time.  It can be 10 W for one second or 1 W for ten seconds.  10 W is 10 J/s.  So that could be 10 joules in 1 second, or 100 joules in 10 seconds, but it is 10 joules per second. 

Interesting that neither work nor power are considered SI base units, but that's not important.  I don't know if this matters either, but NIST shows work (energy) as derived from force and distance.  They show power as defined by work per unit time (J/s). 

NIST Diagram

Does that help?

--- End quote ---

Sorry not sure where you have this wrong definition of what power and energy is.
Power is measured in Watt
Energy measured in Joules
Applying a power of 10W for one second will result in 10J or what I prefer 10Ws.
So the 10W includes not time is just a rate of work nothing to do with time while Energy includes time thus why I prefer to use Ws instead of Joules.
One Joule is power of 1W applied for 1 second but can also be 10W applied for 100ms or 1kW applied for 1ms. You get the idea that power has nothing to do with time it is the rate at witch work is done.

Not sure why Power and Energy are so misunderstood and the way they are defined by NIST is juts super bad and understandably confusing.
Why will you ever define Power using Energy ?

No wonder why so many are confused about how blackbird works.
gnuarm:
Here's a thought experiment.  Instead of wind and a propeller, what if the wind were a conveyor belt and the prop were another wheel on that conveyor belt? 

Now the wheels can be geared so the ground movement applies a force to the conveyor belt.  Can the car move forward faster than the conveyor belt? 

Yes, as the conveyor ramps up speed the car would be propelled faster than the conveyor from the start.  Give the wheels a 2:1 gear ratio and the car will move at twice the conveyor speed.  Like with the propeller, the ground wheels turn faster, so the conveyor wheels can exert more torque resulting in a net forward force. 

This is stirring a memory of some toy I had years and years ago.  It also is making me wonder if that could be used for something practical.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod