"Ronald Raygun" wrote in message
...
dennis@home wrote:

"Ronald Raygun" wrote in message
...

Read what I said in the last post you replied to.
I just showed that you never get any energy back from slowing the wind
when you are travelling down wind even with perfect conditions and a
perfect prop.

You strung a lot of words together, but in a way which didn't make
clear what you were trying to say. I'm unable to follow your argument
because your words don't convey a clear statement of what facts you're
establishing and what conclusions you're drawing from them.


That's because i am trying to convey simple physics to people that obviously
have no understanding of physics.
If they did understand physics I wouldn't need to do it.


The main thing is that you need to get the energy budget to balance.
It doesn't matter which frame you do it in, but for each balancing
exercise you have to stay in the same frame.

Rubbish.
Stick to one frame, you obviously have trouble when you change frames just
like rick does.


It would help if you used terminology which is obvious to understand
and appropriate to the frame you're using. For example "real wind" or
"true wind" is something which really only makes sense in the ground
frame. The corresponding term for the cart frame would be "apparent
wind" or "relative wind".

If you have a cart going at 30mph in a 20mph wind, i.e. the cart is
10mph faster than the wind, then the real wind is 20mph and the
relative wind is -10mph. OK?

Just like I said in mine except I chose 20 and 10.
That is -10 and -10 referenced to the cart and hence to the prop.
See the problem now? they are both the same, you are travelling at twice the
wind speed and are not slowing the wind.
You are not extracting any energy.
This is possible with a perfect prop, there are no losses.


Now if you speed up the prop and the cart (I don't care how it doesn't
matter) and you add 1 to the speed what has happened?


When you were saying the wind was not being slowed, did you mean the
prop was just freewheeling? So that it was not changing the speed
of the air at all? Then it would be consuming no power. That's not
interesting.

Of course its interesting, you are travelling at twice the wind speed and
not consuming energy to do it, this is what a perfect prop does no losses.
Its the equivalent of a machine that just accelerates air through a tube
creating no turbulence, etc. (They don't exist but lets not stop reality
getting in the way of physics).


What we want the prop to do is to provide thrust.

No we want the prop to extract energy from the wind, we don't care how it
does this.

To do this it has
to slow down the wind, for example from its incoming -10mph to an
outgoing -15 or even -20mph. You see that -20 is less than -10,
which is why I call it "slowing down". You may prefer to think of
it as speeding up from 10 to 20 (both backwards). That's fine, because
after all for kinetic energy purposes we square the velocity so any
minus sign will disappear.

It is clear that in this situation, and in the cart frame of reference,
you are not reducing the relative wind's kinetic energy, but increasing
it. So you can't harvest any energy from it. To make it work, the
balance
of kinetic energy has to come from somewhere else, and as I explained
elsewhere, it comes from slowing down the ground.

So you feel its OK to just switch frames of reference to get the answer you
want then?
Do you go by the name rick or thin air when things are going bad then?

That's why it's easier to work in the ground frame, where the ground
does not move, but the real wind is slowed down (for example from 20mph
to 15mph, or even to 10mph, or even to zero).


Well if it doesn't work in the frame relative to the cart we can always
create a new one where it does.
Shame that that is breaking the laws of physics.

Anyway now we have established that to do it you have to change frames of
reference I think we can just forget it and find some amusement elsewhere.