"Rick Cavallaro" wrote in message
...

Energy is a particularly nasty little item. Very few people seem to
stop and realize that it's not an intrinsic property that something
has. The amount of kinetic energy something has depends entirely on
the frame of reference from which you measure it. Any frame will do
to give you the same results, but you sort of have to pick a frame and
stick with it.


For once I can agree with rick (shock + horror), use the same frame of
reference and do the sums.


try this example..

assume a wind of -10 m/s at the cart
assume the ground is doing -20 m/s


What does this mean you ask..

well its a cart going at twice the wind speed down wind.
Its the same as the wind being 10 m/s and the cart travelling down wind at
20 m/s which I will use for the example as it makes the maths easier. You
can use any other figures you like, the conclusions are the same.

This gives a kinetic energy to the relative wind passing the cart of 100
units (-10 x -10) but do notice it is going backwards and is being generated
by ricks prop.

Ah look at the energy needed, we are expending energy to make the wind go
backwards.

However we are not slowing the real wind at all (it is still -10 m/s) so it
is not losing any energy to anywhere

This is impossible.
There is no energy input.

Now rick will say the energy comes from the wheels and drives the prop..

so in this steady state the wheels must be providing the 100 units of energy
but its not slowing the real wind so there can't be any energy coming from
the wind. In actual fact the energy is going into the prop and none is
coming back.

Opps that's impossible too you can't just create energy from nowhere ( not
even using wheels and props).



so lets increase the speed of the prop to throw more air back and slow the
wind to extract energy as rick says he does..

So we increase the prop wind to -11 m/s , the energy used has gone up from
100 to 121 = 21 extra units (using the energy = constant x mass x velocity
squared formula and remembering two negatives multiplied are a positive).


So what has happened to the real wind? Yes it has gone from -10 m/s to -11
m/s and its energy has changed by 121-100=21 extra units.

Remember we are using a single frame of reference, the cart.

Do I see a problem here? is 21 greater than 21? no so we have no extra
energy extracted from the wind by the cart.

What does it say..

well even with perfect conditions there are no circumstances using any
device (even a 100% efficient prop) where you can extract more energy by
slowing the wind when the device is moving faster than the wind in the
direction of the wind than the energy it takes to do it.
(This isn't a surprise to a physicist as its just conservation of energy.)


Well is this what really happens? probably not, after all this is the
perfect case, lets look at some of the problems rick appears to avoid in the
real world..

To stop the physical impossibility of the air piling up behind the the cart
would have to increase speed to maintain the steady state of the wind speed
from the prop being the same as the wind, if it doesn't turbulence will
occur even with a 100% efficient prop. (The excess air has to go somewhere
and the prop has no control over it) there is no energy being input so this
can't happen.

To actually overcome losses the prop would have to accelerate the air to a
higher speed than in the steady state this results in the pressure build up
behind the prop and that causes turbulence as the air trys to even its self
out, this loses energy so more energy is needed by the prop, its impossible
to get this energy back as it was shown above.
there are friction losses,
etc.

Are there any values of speed where it works and you can actually extract
energy?
well yes, if the cart is going slower than the wind you can get energy from
the wind, this is not really surprising.