dennis@home wrote:

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

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.

Why are saying stuff which makes no sense? To my objection that
you are not being clear, you respond that this is because you want to
make it simple. But the best way to make it simple is to be clear!

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.

Again you are saying stuff that makes no sense. I say stay in one frame,
and you say "Rubbish, stick to one frame". If you're agreeing with me,
why do you say "rubbish"?

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.

What do you mean -10 and -10? Whose speed is the first -10 and whose is
the second?

If the cart is going at 30 and the wind at 20, then in the cart frame
the wind is indeed going at -10. That's the wind coming from the front
and going into the propeller from the front. What else is going at -10?
Do you mean the wind coming out of the back of 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.

If I may assume that you are indeed saying that air is coming out
of the prop at the same speed as it's going in, then the prop is
freewheeling (spinning at just the right speed to let the air through
without either speeding it up or slowing it down), it is neither
extracting energy (which would be acting as a turbine) nor expending
energy (which would be acting as a propeller). This consumes zero
power from the electric motor, but gives no thrust, assuming it's
not infinite in size.

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?

No, no, no. We don't want to speed up the cart. We want to analyse
the situation of the cart going *and staying* at twice the air speed
to see if it can self-sustain this speed.

The whole idea, which I don't think you've grasped yet, is that the
prop is really acting as a propeller and not as a turbine. It is
putting energy *in* to the relative wind, not taking it *out*, it is
*speeding up* the relative wind, *not* slowing it down. The prop
on the cart is operating in exactly the same way as that on an airplane
in flight, it is pushing backwards on the air in order to get forward
thrust on the plane.

Our cart wants the propeller to provide the thrust to keep going
at its existing speed, without getting faster. The need for the thrust
comes from the need to balance the drag caused by the generator coupled
to its wheels, and also the air drag caused by the headwind acting on
the chassis.

So if you analyse the energy budget in the cart frame, you lose energy
by speeding up the wind, but gain energy by slowing down the ground.

If instead you analyse the energy budget in the ground frame, you gain
energy by slowing down the wind.

On 07/10/2010 10:44, Ronald Raygun wrote:
dennis@home wrote:

"Ronald 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.

When did Denis ever do anything else? ;-)

snip

On 07/10/2010 05:40, Gib Bogle wrote:

I can't believe Drivel is as stupid as he appears. Therefore I think
he's pulling everyone's legs.

Unfortunately his behaviour over the 10 years or so he has been
infesting this ng indicates that if anything he is even more stupid than
his recent activities would indicate.

"Ronald Raygun" wrote in message
...


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.

What do you mean -10 and -10? Whose speed is the first -10 and whose is
the second?

See what I mean about you not understanding relative frames.
this is really simple and I have no idea how to describe it in a less simple
way.
You leave me no choice but to give up.

On Oct 7, 6:01*am, "dennis@home"
wrote:

You leave me no choice but to give up.

I've been trying to post for 2 days. I think it's still broke for me.

Rick Cavallaro wrote:

On Oct 7, 6:01 am, "dennis@home"
wrote:

You leave me no choice but to give up.

I've been trying to post for 2 days. I think it's still broke for me.

Well this post of yours has managed to get through, at least.
I've been looking forward to your assessment of my analyses.

Since my analyses confirmed that I was wrong before, and, I hope,
showed where I went wrong, I presume your assessment will be approving.
I'd welcome any additional comments you'd care to make.

I think one of the things which confused me initially was the notion
of propeller efficiency. I thought what was meant was something
similar to the efficiency of any other device or machine intended to
convert one form of energy into another, like the generator, the motor,
and any associated bits and pieces of couplings.

Any inefficiency would mean a "loss" of energy, a conversion into
undesirable form, usually heat. I assumed that in the same sense
a less than 100% efficient propeller would also be losing energy
somehow.

That's why working in terms of the skater analogy helped. By leaving
behind any mystery associated with propellers, I could just pretend
that our hero's arms were 100% efficient machines, and yet he suffers
a kind of inefficiency, but it's not really an "engineering" kind,
it's more "strategic".

Our hero can *choose* to take 100% of the kinetic energy of each pleb
he pushes, or he can choose to take a smaller fraction of each pleb's
KE, but from more plebs each second, while getting the same thrust,
and in so doing he can convert (harvest) almost twice as much energy.

The inefficiency is not due to a conversion loss into the wrong form of
energy, it's due to failing to convert as much KE as in available to be
converted.

On Oct 9, 3:15*am, Ronald Raygun wrote:

I've *been trying to post for 2 days. *I think it's still broke for me.

Well this post of yours has managed to get through, at least.
I've been looking forward to your assessment of my analyses.

Well, let's see if this gets through.

On Oct 9, 3:15*am, Ronald Raygun wrote:

I've been looking forward to your assessment of my analyses.

Alrighty then... it looks like I'm back. Yes, I agreed with each part
of your analysis.

I'd welcome any additional comments you'd care to make.

Unfortunately, I don't recall what comments I did attempt to make over
the past few days. Both JB and I were shut out from posting for some
reason.


I think one of the things which confused me initially was the notion
of propeller efficiency. *I thought what was meant was something
similar to the efficiency of any other device or machine intended to
convert one form of energy into another, like the generator, the motor,
and any associated bits and pieces of couplings.

But that is exactly what is meant by propeller efficiency.

Any inefficiency would mean a "loss" of energy, a conversion into
undesirable form, usually heat. *I assumed that in the same sense
a less than 100% efficient propeller would also be losing energy
somehow.

That's right. Consider a Cessna sitting still in the run-up area with
brakes locked and prop spinning at full revs. It's blowing a lot of
air, but the plane is not moving forward. All of that air motion
ultimately becomes waste heat.

Rick Cavallaro wrote:

On Oct 9, 3:15 am, Ronald Raygun wrote:

I've been looking forward to your assessment of my analyses.

Alrighty then... it looks like I'm back. Yes, I agreed with each part
of your analysis.

I'd welcome any additional comments you'd care to make.

Unfortunately, I don't recall what comments I did attempt to make over
the past few days. Both JB and I were shut out from posting for some
reason.

I think one of the things which confused me initially was the notion
of propeller efficiency. I thought what was meant was something
similar to the efficiency of any other device or machine intended to
convert one form of energy into another, like the generator, the motor,
and any associated bits and pieces of couplings.

But that is exactly what is meant by propeller efficiency.

But not in the sense of what I said below, that energy is lost to
heat *during* the actual conversion.

Any inefficiency would mean a "loss" of energy, a conversion into
undesirable form, usually heat. I assumed that in the same sense
a less than 100% efficient propeller would also be losing energy
somehow.

That's right. Consider a Cessna sitting still in the run-up area with
brakes locked and prop spinning at full revs. It's blowing a lot of
air, but the plane is not moving forward. All of that air motion
ultimately becomes waste heat.

Yes, ultimately, but we're not really interested in what happens ultimately,
we're interested in what happens initially. Perhaps I should say "I" rather
than "we", because I think I was interpreting efficiency differently from
the way you were, and are. Remember, what I'm trying to do now is explain
how I think my understanding of prop efficiency differed from yours, so if
I say "I thought it was X" it confuses me more if you then say "but that's
what it does mean".

Let's change the skater analogy to match your Cessna. Suppose our hero
has a rope tied around his waist which is tied to a stake in the ground
behind him. This time the plebs are coming towards him at 1 m/s from the
front, and he pushes back on one of them for a second at the same 60 N as
before, accelerating him to 2 m/s. During the second of contact the pleb
travels 1.5 m, so our hero does 90 J of work. The 60 kg pleb's kinetic
energy increases from 30 J to 120 J.

The hero's arm may conceivably have been 100% efficient at converting 90 J
of chemical energy into a 90 J kinetic energy increase for the pleb. In
the same way, the Cessna's prop could conceivably be 100% efficient at
converting engine power into kinetic air power. We don't care what happens
to the accelerated pleb after he's left the grasp of the hero's arm, in
terms of how that pleb might vaporize when it collides with another pleb.
Likewise we shouldn't need to care whether the accelerated air gets hot
as it slows down a few tens of yards downstream.

I was thinking about the prop's efficiency as how its kinetic output power
relates to the mechanical input power. I think you're considering prop
efficiency in terms of what the plane gets out of the deal. In your Cessna
with the brakes on, I presume you are considering the prop as running at 0%
efficiency because its thrust is doing no work *on the plane*. I guess you
don't care whether the loss ends up as heat or not, you simply consider any
work done on the air as wasted. But that won't help you calculate how much
fuel the engine's going to need.

If I cut the skater's rope so that he can accelerate too (corresponding to
your Cessna releasing the brakes), he will travel forward 0.5 m during the
second, and so he will have done 120 J of work. 90 J of that is still going
into the pleb, and the remaining 30 J will have accelerated himself. In my
sense his arm is still 100% efficient, in another sense he could be thought
of as 25% efficient because, out of 120 J expended, 30 J have gone into
useful KE, and 90 J into useless KE. But in your sense I think he's being
50% efficient because 30 J/s (30 W) is 50% of the best he could get by
exerting 60 N at 1 m/s (60 W). Am I right?

On 10/10/2010 02:12, Doctor Drivel wrote:

"Roger Chapman" wrote in message
...
On 07/10/2010 05:40, Gib Bogle wrote:

I can't believe Drivel is as stupid as he appears. Therefore I think
he's pulling everyone's legs.

Unfortunately his behaviour over the 10 years or so he has been infesting this ng indicates that if anything he is even more stupid than his recent activities would indicate.

Censored information reinstated.

Roger is an idiot. Sad but true.

Dribble isn't clever enough to come to that conclusion about anyone but
himself.

Some 10 years ago Dribble (then masquerading as Adam) asked on another
ng for an explanation of latent heat and at much the same time on this
ng asked if metres cubed was a measure of speed. Such a display of
ignorance made his parallel claim to be a heating engineer absolutely
preposterous while his inability to understand that constantly posting a
mixture of garbage and insults reinforces the impression that he is
terminally stupid.

Doctor Drivel wrote:

"Ronald Raygun" wrote in message
...

So if defies gravity.

Something doesn't defy gravity just by going up.

A helium filled balloon goes up because gravity is pulling on the
air around and above it more strongly than on the balloon itself.

The balloon defies gravity. It goes up against the pull of gravity.

Drivel!

Take a playground see-saw. Small child on one end. Along comes big
child, sits on other end. Big child's end goes down, small child's
end goes up.

Would you say the small child is defying gravity? No, of course
you wouldn't. The fact is that gravity is *making* the small
child go up as a side effect of pulling the big child down.

It's the same with the balloon. Gravity pulls down on the heavier
air around it, into the space underneath, and this is what forces
the balloon up.

Without gravity the balloon would not go up.

Ronald Raygun wrote:
Doctor Drivel wrote:

"Ronald Raygun" wrote in message
...

So if defies gravity.
Something doesn't defy gravity just by going up.
A helium filled balloon goes up because gravity is pulling on the
air around and above it more strongly than on the balloon itself.
The balloon defies gravity. It goes up against the pull of gravity.

Drivel!

Take a playground see-saw. Small child on one end. Along comes big
child, sits on other end. Big child's end goes down, small child's
end goes up.

Would you say the small child is defying gravity? No, of course
you wouldn't. The fact is that gravity is *making* the small
child go up as a side effect of pulling the big child down.

It's the same with the balloon. Gravity pulls down on the heavier
air around it, into the space underneath, and this is what forces
the balloon up.

Without gravity the balloon would not go up.

I never like getting involved in these 'does the mortar hold the bricks
together, or keep them apart' arguments..

;-)

On 10/10/2010 13:49, Doctor Drivel wrote:

"Roger Chapman" wrote in message
...

Roger is a babbling idiot. Sad but true.


Dribble isn't clever enough to come to that conclusion about anyone but
himself.

Some 10 years ago Dribble (then masquerading as Adam) asked on another
ng for an explanation of latent heat and at much the same time on this
ng asked if metres cubed was a measure of speed. Such a display of
ignorance made his parallel claim to be a heating engineer absolutely
preposterous while his inability to understand that constantly posting a
mixture of garbage and insults reinforces the impression that he is
terminally stupid.

Doctor Drivel wrote:

Take a playground see-saw. Small child on one end. Along comes big
child, sits on other end. Big child's end goes down, small child's
end goes up.

Yep. One end defies gravity and goes up.

I think you misunderstand the meaning of "defy".
If you have access to a decent dictionary, try looking it up.

On 10/10/2010 19:03, Tim Streater wrote:

Quite right. If the earth suddenly vanished and left the air behind, a
balloon would no longer rise in the air. [1]

Objects in a gravitational field not in free fall (e.g objects on the
surface of the earth) can be said to be *resisting* gravity, by doing so
they experience weight. Something in free fall, such as a satellite or a
person on the ISS, is not resisting gravity and so experiences no
weight. It still has mass, of course. The word "defy" does not apply in
any relevant context.

[1] Actually this is not quite true. There would be a tiny residual
gravitational field due to the mass of the atmosphere, in a thin shell
(until it dispersed of course). The field would act as if all the mass
were concentrated at the centre of the shell.


Surely if the air stayed still the balloon would still rise, because of
the (gravitationally induced) pressure difference across its height?
And in fact would rise faster, as there would now be no gravity holding
it back?

Of course the pressure difference would vanish as the air scattered
rather rapidly...

Andy.

On Oct 10, 4:27*pm, "Doctor Drivel" wrote:

If men did not put gas in the balloon it would not resist gravity,the fabric
of the balloon would go the other way - DOWN towards the pull of gravity. So
it is an anti-gravity machine.

No need to put gas in the balloon to make it rise. I once made a hot
air balloon by simply taping 6 big trash bags together into a big
tetrahedron. I put nothing but air inside. The sun warms it up, and
it goes thousands of feet into the air. You could call it "defying"
gravity if you like, but it defies gravity to pretty much the same
extent that you defy gravity when you get out of bed.

On 7/10/2010 5:40 p.m., Gib Bogle wrote:

OK, I stand corrected.

On 11/10/2010 00:22, Doctor Drivel wrote:

"Roger Chapman" wrote in message
...
On

Roger is a babbling idiot. Sad but true.


Dribble isn't clever enough to come to that conclusion about anyone but
himself.

Some 10 years ago Dribble (then masquerading as Adam) asked on another
ng for an explanation of latent heat and at much the same time on this
ng asked if metres cubed was a measure of speed. Such a display of
ignorance made his parallel claim to be a heating engineer absolutely
preposterous while his inability to understand that constantly posting a
mixture of garbage and insults reinforces the impression that he is
terminally stupid.

Dribble seems under the impression that if he edits out of his replies
facts he is unable to dispute they disappear from the public record but
all the potentially endless repetition merely gives extra publicity to
the original facts.

I don't intend to bother reinstating my original comments again unless
Dribble adds something new.

Andy Champ wrote:

On 10/10/2010 19:03, Tim Streater wrote:

Quite right. If the earth suddenly vanished and left the air behind, a
balloon would no longer rise in the air. [1]

Objects in a gravitational field not in free fall (e.g objects on the
surface of the earth) can be said to be *resisting* gravity, by doing so
they experience weight. Something in free fall, such as a satellite or a
person on the ISS, is not resisting gravity and so experiences no
weight. It still has mass, of course. The word "defy" does not apply in
any relevant context.

[1] Actually this is not quite true. There would be a tiny residual
gravitational field due to the mass of the atmosphere, in a thin shell
(until it dispersed of course). The field would act as if all the mass
were concentrated at the centre of the shell.

Surely if the air stayed still the balloon would still rise,

Agreed.

because of
the (gravitationally induced) pressure difference across its height?

What pressure difference? If you're saying the air pressure outside
the balloon is lower near the top of the balloon than near the bottom
of the balloon, isn't this compensated for by the gas pressure inside
the balloon also being higher at the bottom than the top? The motivating
force would continue to be entirely due to density difference between gas
and air.

And in fact would rise faster, as there would now be no gravity holding
it back?

Eh? Gravity never did hold it back, it was what *caused* it to go up!
It would rise slower because there would be less gravity.

Of course the pressure difference would vanish as the air scattered
rather rapidly...

The pressure of both air and gas would decrease so much, that the balloon
would soon burst. But if the balloon were strong enough not to burst, the
tension in its material would at a certain size prevent the gas pressure
inside from dropping further. In due course the gas's density would exceed
that of the air outside, which is still expanding, and then the balloon
would "sink" to the middle of the air cloud.

On Oct 11, 3:24*am, Ronald Raygun wrote:

*In due course the gas's density would exceed
that of the air outside, which is still expanding, and then the balloon
would "sink" to the middle of the air cloud.

If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. There's no gravity inside a
uniform shell of any thickness.

I still owe you a response on your most recent analysis. Sorry for
the delay.

On 11/10/2010 15:51, Rick Cavallaro wrote:
If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. There's no gravity inside a
uniform shell of any thickness.

You sure about that?

In article , Roger Chapman wrote:
On 11/10/2010 15:51, Rick Cavallaro wrote:
If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. There's no gravity inside a
uniform shell of any thickness.

You sure about that?

He ought to be, it's a standard result:
http://en.wikipedia.org/wiki/Shell_theorem

Whether a spinning atmosphere with weather patterns will remain as
approximately a spherical shell for long when the earth disappears is
another question, but he did say "If we're still imagining ..."

On 11/10/2010 18:23, Alan Braggins wrote:
In , Roger Chapman wrote:
On 11/10/2010 15:51, Rick Cavallaro wrote:
If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. There's no gravity inside a
uniform shell of any thickness.

You sure about that?

He ought to be, it's a standard result:
http://en.wikipedia.org/wiki/Shell_theorem

Whether a spinning atmosphere with weather patterns will remain as
approximately a spherical shell for long when the earth disappears is
another question, but he did say "If we're still imagining ..."

Thanks for the pointer. It is something you either know or don't know as
it is not exactly obvious that the sum of the gravitational forces on a
body within a planetary shell is zero regardless of position, but I take
a crumb of comfort in the fact that the thought that first came to mind
was not of a planetary shell at all but of a man sized sphere with
Dribble weightless within it 'defying gravity'.

That lack of knowledge reminds me of a put-down by a climbing
acquaintance some 44 years ago, when discussing the dynamic waist belay:
some thing like "call yourself an engineer and you don't know that the
capstan effect is exponential" and also a few years later, when the boot
was on the other foot my less than complimentary remarks to another
acquaintance who was a geologist and sadly lacking in any knowledge of
Radon and the danger it posed to householders in certain parts of the
country. However expert you are in a subject you can't possibly know it all.

Roger Chapman wrote:
On 11/10/2010 15:51, Rick Cavallaro wrote:
If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. There's no gravity inside a
uniform shell of any thickness.

You sure about that?

Hmm. there's a nasty integral to be done there..Or some tensor calculus..

I would not be surprised if it were true.

On Oct 11, 12:48*pm, The Natural Philosopher
wrote:
Roger Chapman wrote:
On 11/10/2010 15:51, Rick Cavallaro wrote:
If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. *There's no gravity inside a
uniform shell of any thickness.

You sure about that?

Hmm. there's a nasty integral to be done there..Or some tensor calculus..

I would not be surprised if it were true.

It's true, and the integral is not pretty. It's more easily proved by
simulation (in my opinion).

Roger Chapman wrote:

I don't intend to bother reinstating my original comments again unless
Dribble adds something new.

It'll never 'appen.

There was also Drivel's attempt to sell "Cataclean" in another
newsgroup. He claimed it was approved by the Lancashire police garage
who had stated that it improved vehicle performance and repaired vehicle
catalysts. At the time I did work from time to time for Lancashire
police at Hutton Hall, Preston which is where the vehicle repair and
maintenance section is based. So I popped around to the garage with a
printout of Drivel's (Adam's) claims.

They declared that the claim was "a load of old ********" and wanted to
know who the loony was who abused their name.

And then we get back to Drivel and dowsing, magnetic water softeners and
indeed any old load of pseudo-scientific malarkey.

On Oct 9, 9:18*am, Ronald Raygun wrote:

....about heroes and plebs. Sorry for the shamefully delayed reply.

Let's change the skater analogy to match your Cessna. *Suppose our hero
has a rope tied around his waist which is tied to a stake in the ground
behind him. *This time the plebs are coming towards him at 1 m/s from the
front, and he pushes back on one of them for a second at the same 60 N as
before, accelerating him to 2 m/s. *During the second of contact the pleb
travels 1.5 m, so our hero does 90 J of work. *The 60 kg pleb's kinetic
energy increases from 30 J to 120 J.

Okey dokey.

The hero's arm may conceivably have been 100% efficient at converting 90 J
of chemical energy into a 90 J kinetic energy increase for the pleb. *In
the same way, the Cessna's prop could conceivably be 100% efficient at
converting engine power into kinetic air power. *We don't care what happens
to the accelerated pleb after he's left the grasp of the hero's arm, in
terms of how that pleb might vaporize when it collides with another pleb.
Likewise we shouldn't need to care whether the accelerated air gets hot
as it slows down a few tens of yards downstream.

Well, yes and no (have I got that one covered?). You're right that
the prop might have been 100% efficient in converting its power to
kinetic energy of the air. But several things should be said about
that...

1) The prop *will be* 100% efficient in transferring its energy to the
air - there's no other choice.
2) This is why the efficiency for an airplane propeller is measured
differently than that of a house fan - they have two different jobs.
3) The above statement is more true than you might guess. You might
say - "but no - they both have the job of accelerating air". But this
isn't the case. The airplane prop has the job of pulling the plane
forward through an airmass while the house fan has the job of
accelerating air. For a given thrust and airspeed a given propeller
can not be more efficient than some amount that's easily calculated -
because it's only able to develop its thrust by moving an amount of
air that can be encountered by a prop of its specific diameter. So
it's less an issue of not caring what happened to the air afterwards,
and more an issue of considering that you have a prop that can *only*
generate the desired thrust by accelerating the air in an inefficient
manner (namely accelerating a small amount of air by a relatively
large delta-V).

The above is not intended to be argumentative. It's intended to help
you understand another perspective. The measure of efficiency of an
airplane prop is not as arbitrary as it might first seem (imo).


I was thinking about the prop's efficiency as how its kinetic output power
relates to the mechanical input power. *I think you're considering prop
efficiency in terms of what the plane gets out of the deal.

That's right. But keep in mind, its output power *must* equal it's
input power. So if you don't like the definition given, you have to
think of another somewhat arbitrary definition. For example, you
could say that only the velocity added along the axis of the prop
counts as being approved output energy - while the swirl velocity is
wasted energy. But then we could have this same debate about how much
energy the prop "really" added to the air. The point being - without
some sort of "arbitrary" definition of prop efficiency, the prop will
always be 100% efficient.

In your Cessna
with the brakes on, I presume you are considering the prop as running at 0%
efficiency because its thrust is doing no work *on the plane*.

Correct. But it's not "my" definition.

If I cut the skater's rope so that he can accelerate too (corresponding to
your Cessna releasing the brakes), he will travel forward 0.5 m during the
second, and so he will have done 120 J of work. *90 J of that is still going
into the pleb, and the remaining 30 J will have accelerated himself.

O.K.

*In my sense his arm is still 100% efficient, in another sense he could be thought
of as 25% efficient because, out of 120 J expended, 30 J have gone into
useful KE, and 90 J into useless KE.

Agreed. I think you'll find that in your sense everything is always
100% efficient. That's why efficiency formulas are always somewhat
arbitrary.

*But in your sense I think he's being
50% efficient because 30 J/s (30 W) is 50% of the best he could get by
exerting 60 N at 1 m/s (60 W). *Am I right?

I don't really have a particular sense. In the sense of the airplane
propeller his efficiency would be the energy he gained divided by the
energy he expended. Yes - it's arbitrary - but only sort of. First
because efficiency relates to the desired result, and second because
any "non arbitrary" definition of efficiency will always be 100%
(because mass/energy isn't created nor destroyed).

On Mon, 11 Oct 2010 07:51:19 -0700 (PDT), Rick Cavallaro
wrote:

If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. There's no gravity inside a
uniform shell of any thickness.

But wouldn't the stuff outside the shell move down, er, midpointwards, under the
influinece of gravity, and then continue on to the middle, i.e. the shell fill
up eventually?

Thomas Prufer

On Oct 11, 11:52*pm, Thomas Prufer prufer.pub...@mnet-
online.de.invalid wrote:

But wouldn't the stuff outside the shell move down, er, midpointwards, under the
influinece of gravity, and then continue on to the middle, i.e. the shell fill
up eventually?

I think all sort of things would happen. The shell of atmosphere
would most likely disperse pretty quickly as it wouldn't have anywhere
near enough gravity to sustain itself as an atmosphere. If it could
be magically held in place, things outside the shell would be drawn
toward the center of the sphere - but once inside the shell, they'd
float freely to the other side of the shell where they'd go flying
back out at the same speed they came in at (minus any frictional
losses they encountered passing through the shell.

In article , Roger Chapman wrote:
On 11/10/2010 18:23, Alan Braggins wrote:
In , Roger Chapman wrote:
On 11/10/2010 15:51, Rick Cavallaro wrote:
If we're still imagining the air cloud to be a shell with nothing in
the middle (i.e. the place where the earth is missing), objects would
not sink to the middle of that void. There's no gravity inside a
uniform shell of any thickness.
[...]
Thanks for the pointer. It is something you either know or don't know as
it is not exactly obvious that the sum of the gravitational forces on a
body within a planetary shell is zero regardless of position, but I take
a crumb of comfort in the fact that the thought that first came to mind
was not of a planetary shell at all but of a man sized sphere with
Dribble weightless within it 'defying gravity'.

A man size sphere isn't going to have significant gravity outside either,
unless it's made of something very exotic. "no gravity inside" means
"no overall gravity inside _due to the shell_", obviously, no-one is claiming
that the shell will block the influence of masses outside that aren't
uniformly distributed. But let's assume the moon vanished with the earth,
so we don't have to worry how its orbit about the sun changes.

On Tue, 12 Oct 2010 00:37:57 -0700 (PDT), Rick Cavallaro
wrote:

I think all sort of things would happen. The shell of atmosphere
would most likely disperse pretty quickly as it wouldn't have anywhere
near enough gravity to sustain itself as an atmosphere.

Ah, ok, you're thinking the Earth vanishes, and the planets and stuff are still
around. I was thinking of a hollow shell of gas in a theoretical void -- the
same void used for electrons next an infinite plane with a charge... In such a
void, I'd think it would pull itself into a cloud without a hole in the middle,
slowly.

but once inside the shell, they'd
float freely to the other side of the shell where they'd go flying
back out at the same speed they came in at (minus any frictional
losses they encountered passing through the shell.

A "subsurface orbit", perhaps?

An engineer friend saw a perpetual motion machine once, at a trade show.
Low-friction clockwork gears and weights going up and down, and ran for many
days in a sealed glass box. Bunch of engineers standing around trying to figure
out what kept it running as it was well-made, and not obvious. He never did find
out what made it work -- but it served it's purpose of attracting people to the
stand very well.


Thomas Prufer

On Oct 12, 1:42*am, Thomas Prufer wrote:

Ah, ok, you're thinking the Earth vanishes, and the planets and stuff are still
around. I was thinking of a hollow shell of gas in a theoretical void -- the
same void used for electrons next an infinite plane with a charge... In such a
void, I'd think it would pull itself into a cloud without a hole in the middle,
slowly.

I'm pretty sure the cloud would not coalesce. Each molecule of the
cloud has a velocity. And that velocity will be greater than the
escape velocity for a body with the gravitational field of that cloud
itself. This is why the moon has no atmosphere - and it has far more
gravitational pull than the cloud we're describing would.

An engineer friend saw a perpetual motion machine once...

Are you sure? : )

On 12 Oct, 13:48, Rick Cavallaro wrote:
On Oct 12, 1:42*am, Thomas Prufer wrote:



Ah, ok, you're thinking the Earth vanishes, and the planets and stuff are still
around. I was thinking of a hollow shell of gas in a theoretical void -- the
same void used for electrons next an infinite plane with a charge... In such a
void, I'd think it would pull itself into a cloud without a hole in the middle,
slowly.

I'm pretty sure the cloud would not coalesce. *Each molecule of the
cloud has a velocity. *And that velocity will be greater than the
escape velocity for a body with the gravitational field of that cloud
itself. *This is why the moon has no atmosphere - and it has far more
gravitational pull than the cloud we're describing would.

An engineer friend saw a perpetual motion machine once...

Are you sure? *: )

The nearest thing I saw to perpetual motion was a little turbine in a
glass bulb that just rotated. It was suspended on a diamond resting
on a needle, ie very low friction. It was actuallypowered by light.
The turbine blades were shiny on one side and black on the other.
There was a vacuum in the glass bulb

Just a minute. The moon is in perptual motion round the Earth. Or as
near as dammit.

On 12/10/2010 16:55, harry wrote:

Just a minute. The moon is in perptual motion round the Earth. Or as
near as dammit.

The tides slow it down - it's measurable.

Running for a long time != perpetual motion.

harry wrote:

The nearest thing I saw to perpetual motion was a little turbine in a
glass bulb that just rotated. It was suspended on a diamond resting on a
needle, ie very low friction.

No, that's not perpetual motion, it a heat engine. It's no more
perpetual motion than the internal combustion engine. And somehow I
doubt that a diamond was involved, a glass bearing is good enough.

It was actuallypowered by light.

No, it was actually powered by heat.

In article , harry wrote:
The nearest thing I saw to perpetual motion was a little turbine in a
glass bulb that just rotated. It was suspended on a diamond resting
on a needle, ie very low friction. It was actuallypowered by light.
The turbine blades were shiny on one side and black on the other.
There was a vacuum in the glass bulb

Actually it relies on there being a very low gas pressure but not a
perfect vacuum, if you make the vacuum too good it stops working:
http://en.wikipedia.org/wiki/Crookes_radiometer

On 12/10/2010 07:36, Rick Cavallaro wrote:

1) The prop *will be* 100% efficient in transferring its energy to the
air - there's no other choice.

ummm... yes... but... and I hate to point this out to someone who knows
_lots_ more about this that I do...

snip
That's right. But keep in mind, its output power *must* equal it's
input power. So if you don't like the definition given, you have to
think of another somewhat arbitrary definition. For example, you
could say that only the velocity added along the axis of the prop
counts as being approved output energy - while the swirl velocity is
wasted energy. But then we could have this same debate about how much
energy the prop "really" added to the air. The point being - without
some sort of "arbitrary" definition of prop efficiency, the prop will
always be 100% efficient.


Add to the swirl, the localised turbulence around the blades, especially
the tip - and heat from plain old skin friction. Now I'm pretty sure
_you_ know all that, but others won't.

Andy

On Oct 12, 11:01*am, (Steve Firth) wrote:

*It was actuallypowered by light.

No, it was actually powered by heat.

As I understand, it was intended by the inventor to be powered by
light - but it ended up going the wrong direction. This is because
the black side got hot and caused the air molecules to bounce off with
more energy (obviously not a perfect vacuum).

But I'm guessing that's what the link says. Or maybe it says I'm
completely wrong(?)

On Oct 12, 3:51*pm, Tim Streater wrote:
Judging by what I read, the answer is a bit complex.

Yeah, I went and looked at that. Definitely more complicated than I
remembered. I didn't have time to make complete sense of it, but it
seemed like they were also talking about a number of transient
effects.

Rick Cavallaro wrote:

On Oct 12, 1:42 am, Thomas Prufer wrote:

In such a void, I'd think it would pull itself into a cloud
without a hole in the middle, slowly.

I'm pretty sure the cloud would not coalesce. Each molecule of the
cloud has a velocity. And that velocity will be greater than the
escape velocity for a body with the gravitational field of that cloud
itself. This is why the moon has no atmosphere - and it has far more
gravitational pull than the cloud we're describing would.

Aren't stars the result of large populations of gas molecules saying
to themselves "Look, we can't go on like this, let's pull ourselves
together!"? :-)

I guess there needs to be a critical mass for that to work, though.
At the sizes we were talking about that won't happen.

If we did start with a finite thickness shell of air, with vacuum both
on the inside and on the outside, it would just expand in all directions,
both inwards and outwards. Soon the inside would be "full" and the only
way to expand further is outwards.

If the balloon stops expanding with the air's decreasing pressure, and does
not burst, its size will remain finite while the air perimeter gradually
becomes infinite. Thus the balloon will find itself nearer and nearer the
centre of the air cloud (in the sense that the ratio of its distance from
the centre to cloud's radius approaches zero) without even needing to move
towards it.

But I think it nevertheless will move towards it:

If the expanding sphere of air has a well-defined centre, and the balloon
is not at it, then by the shell theorem (I didn't know about this, it
looks cool) all those air molecules which are further away than the balloon
from this centre will have no gravitational effect upon the balloon, but all
those in the inner sphere (nearer the centre than the ballon is) will have
an effect. But this effect will decrease as the air pressure and hence
density diminishes, and will also decrease if and when the balloon moves,
because both of these things reduce the mass of the inner sphere.

If and when the balloon reaches the centre it might overshoot and oscillate.
The oscillations may be damped by air resistance, but with diminishing
air density that resistance will decrease, so the oscillations may go
on for a long time.