Re: OT Here is an example of pseudo science.

[Tim Streater]

Yes, that is actually quite true, and once upon a time I could probably have proved it. Not these days, though!

[Steve Firth]

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 bollocks" 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.

[Rick Cavallaro]

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

Okey dokey.

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).

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.

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

Agreed. I think you'll find that in your sense everything is always

100% efficient. That's why efficiency formulas are always somewhat arbitrary.

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).

[Thomas Prufer]

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

[Rick Cavallaro]

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.

[Alan Braggins]

[...]

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.

[Thomas Prufer]

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.

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

[Rick Cavallaro]

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.

Are you sure? : )

[Tim Streater]

The tides on earth extract energy from the system, as do probably the tides on the moon. So the moon spins further away.

After some large number of years, IIRC, our moon will be far enough away ...

Ah, we have the following authoritative entry for the Moon:

Bold text The moon is mad of cheese

... I was going to say that if the moon is too distant, it will no longer act to prevent chaotic changes to the direction of the Earth's spin axis.

[Steve Firth]

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.

No, it was actually powered by heat.

[Alan Braggins]

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:

[Andy Champ]

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

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

[Rick Cavallaro]

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(?)

[Tim Streater]

Judging by what I read, the answer is a bit complex.

[Rick Cavallaro]

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.

[Tim Streater]

Mmmm, dunno about transient. Looked more like there's a couple of reasons it works, but it's unclear which dominates.

[Ronald Raygun]

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.

[Doctor Drivel]

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.

When I get out of bed I fall down on the floor.

[Doctor Drivel]

Roger is a babbling idiot. Sad but true.

[Doctor Drivel]

This man is a pervo. He should be tagged.