dennis@home wrote:

Now put E=mv2 into the picture

... the energy of a moving object is
mass x velocity squared.

Two problems with that Dennis -- "wrong" and "wrong".

The energy of a moving object is NOT "mass x velocity squared", and
that isn't even what you expressed in your equation. Your equation
says that energy is mass x velocity x 2.

Before you go claiming all sorts of wrongness on others parts at least
you should learn the actual equation for energy and then also learn
how equations are written.

http://en.wikipedia.org/wiki/Kinetic_energy














Say you have a cart travelling at 20 m/s with a wind behind it blowing at
10 m/s

The relative wind at the cart is 10 m/s to the rear.

Yes

Now to stop the wind behind you have to accelerate the air going past the
cart to 20 m/s.

Yes

Now put E=mv2 into the picture

Near enough. *Kinetic energy isn't emm vee squared, it's a half emm vee
squared.

and work out how much energy it takes to
accelerate the air going past from 10 m/s to 20 m/s so it stops the wind
from behind.

OK

My simple maths tells me its 4 times the energy you get from stopping the
wind .

Yes, near enough. *There's 50 J of energy in 1kg of air moving at 10 m/s,
and 200 J (four times as much), in 1kg of air moving at 20 m/s.
The difference, the energy you have to add to the 10 m/s air to accelerate
it to 20 m/s, is 150 J, which is *three* times the 50 J you can get by
stopping 1kg of 10 m/s air.

Now if anyone can tell me how it stopping enough wind to generate four
times the energy it releases I would be interested.
I would like TNP to explain but he won't.

So let me try.

And while we are at it can anyone explain what the coupling is between
the
energy in the air that is stopped and the cart.
Its all very well saying the energy is lost by the wind but there is no
obvious coupling to the cart which is travelling faster than the wind.

Correct. *There is no obvious coupling. *That doesn't mean there is no
coupling, it just means the coupling which is there is non-obvious.
I don't know where it is either, but it's in there somewhere.

I expect that it goes into turbulence and is just lost. Lets face it an
airplane capable of doing 100 knots doesn't suddenly go 10 knots faster
flying downwind in a 10 knot wind so none of the energy released by
stopping the wind is absorbed by the plane.

Correct, this is because the airplane, unlike the car, is not on wheels
which are in contact with the ground and which it could couple to a
generator to provide power for its propellers. *The airplane only sees
the air and just moves at 100kn relative to it. *Obviously if someone
were to accelerate the air all around the plane (e.g. if the weather
system causes a 10kn breeze to start up out of nothing), then the
ground speed of the plane goes up to 110kn, and the plane's kinetic
energy relative to the ground has to go up by 21%. *The way this is
achieved is that the plane's headwind temporarily drops from 100kn to
90kn, and there's less air resistance at that speed, so if the engines
are running at the same power, the spare surplus power will accelerate
the plane until the headwind reaches 100kn again.

The key to understanding how the car works lies in the fact that the
car has access both to the ground and to the air. *But first, imagine that
the car has no propeller, and that its front axle is coupled to an
electric
motor, and its back axle is coupled to a generator, and the generator
powers the electric motor. *If there are no losses in the system, we can
tune it so that the forces match (i.e. generator drag matches motor
thrust)
and then the powers will also match (the generator would supply exactly
the
same amount of energy per second as the motor consumes). *The car would
then
behave in exactly the same way as if the generator and motor were just
sitting decoupled from the axles, doing nothing. *You could tow the car up
to any speed you want using an auxiliary vehicle, or you could use a
battery
to run the motor initially, and as soon as you cut the tow rope or
switched
the battery out, the car would just gradually slow down due to friction,
air
resistance, etc.

In other words, the system is energy-neutral. *While the car is running,
the energy coming into the car from the generator goes out again through
the motor, and it just loops round and round. *In isolation, the motor is
providing real thrust and doing real work, and the generator is extracting
real energy from the moving road, and applying real drag. *But there is no
free lunch. *The car is essentially running on incestuous power, the
motor's
thrust is balancing the generator's drag so the system if force-neutral,
and the energy coming in via the generator is "free", so long as we don't
do
anything with it except essentially give it back to where it came from.

Now we add the propeller, and uncouple the motor from the axle and couple
it to the prop instead. *We can use an auxiliary vehicle to tow the car to
a certain speed, or a battery to power the prop, and then switch on the
generator-to-motor coupling and switch off the battery or cut the tow
rope.

Same as with the motor driven axle, you should be able to get an energy
neutral system, in which free energy from the moving road is used to
power the propeller, to give the thrust to keep the road moving. *Now you
simply need to do the energy budgeting to see at what wind speeds this
will
be capable of being self-sustaining.

In other words some of the energy we need to accelerate the air from
10 to 20 m/s is "free" because it comes back in via the wheels. *So
long as the free proportion is big enough that the rest is not more
than we get from slowing down of the wind, we should be in business.

No it isn't.
To slow the wind you have to accelerate the air passing the prop.
At the same speed as the wind, say 10 m/s there is zero airflow and ignoring
losses it will go forever.

Now go to 11 m/s for the car you have to accelerate the air (using the prop)
by 21% to even match the wind speed.

If you don't accelerate the air to the same speed as the cart or faster it
will just pile up in front of the car's prop and we know that can't happen
if its going to keep going.

You have to accelerate it by more if you want to slow the wind and extract
energy.

When you do the maths you find that you always have to extract more energy
from the air to speed up the air going through the prop than you get from
slowing the wind.

It doesn't matter if you find an unknown coupling mechanism it still doesn't
work as the energy equation is busted.

You can't claim the wheels provide extra energy as there is nowhere for the
energy to come from other than the wind and given the above there isn't any,
i.e. it can only be extracted by slowing the wheels down. That by definition
means slowing the car.

As with a sailing boat which pulls energy out of the relative motion
between two different media (air and water), the car also has the benefit
of being on the interface between two media (air and ground), and can
exploit their relative motion to extract energy.

That only works when sailing across wind, where the speed of the boat
downwind is less than the wind speed, that is not the case with this claim.



An airplane cannot
do that because it's entirely surrounded by just one medium and has
nothing else to grab on to.- Hide quoted text -

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