Plane on a coneyor belt
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I thought we'd moved on to the second question now?
Alan, referring to the first question i agree with you, the plane can take off. The second question is a different matter entirely.
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Sorry Remus, I didn't see the second part.
To be honest, I don't think it would make much difference. Think of a ski-plane on a teflon-coated conveyor. By that I mean that the rotational speed of the wheels is largely irrelevant...as is the speed of the surface they are rolling over. Obviously, in the real world, they'd eventually overheat, seize up and then we're into a whole new calculation; but this isn't the real world.
Certainly the plane is slightly handicapped by whatever resistance is offered by its undercarriage, but the conveyor is equally handicapped by the inertia (if not forward momentum) of the plane. This would get more acute the faster it moved. It's the old tablecloth trick. -
It does make quite a bit of difference, actually.
Re-read the my post with the numbers in.
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Alan, you mentioned it: Overheating of the tyres.
and this overheating is literally lost forwart movement, transferred into another form of energy - heat.again I take the example of pav on his treadmill. this time pav took his old skateboard. now he is standing on is board on the treadmill. when he turns on the training device, he is suddenly transported backwards. to prevent this, he grasps the handrail in front of him.
now we have a similar situation to the plane on the conveyor. the conveyor is moving backwards but the plane is keeping it's position by applying a forward thrust that is not dependent on the conveyor's surface.
back to pav. he really enjoys this experience, turning up the treadmill to its maximum speed. there he is racing faster than he ever did before on his skate board - without even moving. cool!
but after a minute or so his arms start to ache! at the same time the wheels of his board get quite warm..
why do his arms ache? because the frictional resistance of the wheels transferred some of the backwards-movement of the treadmill to the board and therefore to pav. he had to compensate this backwards-movement with the power of his arms. it was not much - but after a while it was exhausting nevertheless.
and now pav thinks: "what would have happened, if my treadmill could run with 10 times the speed? or even a 100 times?"
I hope this little anecdote out of pavs life helps to understand this little experiment
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he he he, feel like i'm famous!
pav
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you ARE, pav!
whe should start producing a tv-series, where some blokes discuss a problem (like this plane-conveyor-thing), use loads of examples out of PAV's every day life, and finally don't come to a conclusion!
that could be a true success!
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that would be awesome!
but what could we call it?
plot would you like to be co-presenter, or executive producer? ha ha
pav
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my embarrasingly German english and little vocabulary stops me from being co-presenter. on the other hand, that could be a laugh.
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i think that would be a wicked combo.
i speak exceptionally quickly, and slip in and out of "market slang" and "impress the client" lingo.
it's like talking to someon with dual personality who is heavily dosed up on MDMA
so i'm told.
pav
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sounds marvellous! and we film all this in front of a green screen and replace the background with a new SketchUp interior design for every episode!
and we use the "proper animation" tool to move chairs and such! and the face/edge-styles change during the show... can't wait to start with it.
you don't happen to have a hd-camera and a big green screen at home,do you?
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i do have a HD camera as it goes.
as far a a big screen goes, i only have the one for my home cinema, not sure it's gonna be big enough (only 2.5m x 2m)
good for the screening of the pilot episode though!
pav
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Im sure a big green tarp would be fine
When does the pilot episode air btw? wouldnt want to miss your debut... -
we will definitely announce the pilot, of course.
the problem is not so much the screen itself - you only need a (preferably non reflective) unicoloured face. a bright green is very unlikely to turn up in the scene (unless pav insists to wear a bright green shirt - then we have to get a blue screen. but I tend to wear blue jeans...
).the big problem is the lighting. every bit of the screen has to recieve exactly the same amount of lighting. otherwise you get a gradient, where the colour difference is too big. so you have to give the tool that deletes the chosen colour (in premiere or avid for example), a too big range.
and for this lighting we need big lighting screens (point a light source not directly at the green screen, but at a white rectangle, that reflects the light to the green screen to create a diffuse light (without shadows and highlights).
so we need several of those and then we need a luxmeter to precisely measure the amount of light at every point of the green screen. and such a luxmeter is quite expensive as far as I know...but it would be sooooo great to have "in SketchUp environment" scenes...
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we could just use my projector, put it behind us, and project sketchup scenes onto the screen behind us.
then when we go national and eventually worldwide with the show, we can get all that other gizmometry you mentioned earlier.
pav
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then we have this "old-superman-flies-in-front-of-projected-environment look"
I just googled it... a cheap lux meter for 22 "
http://www.digital-meters.com/CEM-DT-1300-Light-Meter-p-16524.html
Chromakey Green Screen Backdrop 10' X 12' for 50 £
shit, that is much cheaper than the last time I looked for it...
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that is reasonable!
just over charge a few clients, and jobs a gooden.
we need to sit down, storyboard and decide when to start filming.
my god we're gonna be famous.
pav
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Brain storming session ?? Fellows !
On the same level: a small monkey is climbing a rope, which other side around a pulley, is attached to a stone having exactly the same weight than the monkey ( ouf it's not so easy to explain )
the pulley is supposed to be perfect, the rope's weight very light.
question : does the stone lift up when the monkey climbs up ?
MALAISE
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hold on, im not willing to let the other one go, as it seems to be still wrong. Plot-paris, you were so close with your skateboard example, but I'm wondering if you read my skateboard example earlier?
So Pav's on his skateboard. As soon as the conveyor moves backwards, he goes with it. But if he grabs on to the rail with his hand, he hold still. No matter how fast the conveyor goes, he holds still. All he has to do is pull forward with his arms to go forward. This forward motion will then make the conveyor go faster, which makes his wheels spin faster, but it doesn't make him hold stationary. His arms are pulling him forward, and they are not controlled by the ground. The conveyor doesn't make him lose hold with his hands. He can still pull himself forward.
So a plane sits on the conveyor and it moves backwards, so it grabs on to the air which is sitting still. It pulls itself forward through the air. Even though the conveyor begins to spin faster, and the wheeels spin faster, they do not keep the plane from moving forward. I'm sorry, they just don't. You can't assume that a conveyor can go infintely fast, and then argue that you have to take in to account friction on the wheels. If you go with one perfect case scenario, then you go with all.
Why this doesnt work for a car.
Pav is back on his skateboard, but this time there are no handrails. Its just him kicking his foot on the ground propelling him forward. Now the conveyor can keep him from moving forwards, because he has no other means of propulsion. he is only moving by exerting force directly on the ground below him, which is moving.
And so a car would be the same. A converoy belt will keep it in place, because it only moves by pushing on the ground. A plane doesn't move by doing anything to the ground, it pulls itself through the air. Its wheels will spin as fast as needed to stay fixed to the aircraft.
Are we getting there yet?
Chris
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I agree with Chris.
It's not a good idea to start introducing real world problems of overheating etc into a theoretical argument. In any case, why apply mechanical failure only to the plane?...it's the conveyor that really has to do the work. If anything is going to burn out it's the conveyor motor.In the second scenario:
The plane moves forward at 10 mph. The conveyor attempts to counteract this by moving backward at 10 mph. However, the plane will still move forward at 10 mph...I'm sorry, it just will.
However, its wheels are now spinning at 20 mph. The conveyor adjusts its speed to 20 mph...now spinning the wheels at 30 mph, (assuming no acceleration on the part of the aircraft). So the conveyor matches that at 30 mph, spinning the wheels at 40 mph; and so on up to light speed.I think it's clear that even if the aircraft only travels at a constant speed...however small...not even accelerating...then, if the feedback to the conveyor is instant, the conveyor will instantly accelerate to a phenomenal speed. You have a looping function (conveyor speed = conveyor speed + 10mph) that has everything to do with the conveyor and next to nothing to do with the aircraft. In effect, you'll pull the tablecloth from under the plane....and it won't move back with it any more than the china on the table does.
Therefore, the plane will just move forward pretty much as normal. Or I guess you could say that the small amount of resistance in the wheel bearings would be equivalent to it taxiing slightly uphill. If it chose to accelerate rather than just taxi there would be nothing to stop it getting airborne.
Any idea that the conveyor will somehow transport the plane backwards is largely fallacious. There will be some degree of that, but nothing like as much as most people intuitively suspect.
I suggest taking a piece of paper out of the printer, resting it on the table, then standing a roll of Sellotape on top and pulling the paper backwards. You'll find that the Sellotape is not nearly as willing to travel back with the paper as you might imagine.
It might...if you pull the paper extremely slowly...but we aren't talking slow, we're talking a plane accelerating to takeoff speed. -
but Chris, you are allways completely disregarding the friction. and that is the important thing.
the FRICTIONAL RESISTANCE is the only thing, that can work against the plane's forward thrust.
surely we both agree, that if the brakes of the wheels were locked, the plane would be transported backwards. see the native frictional resistance of a wheel as a tiny brake-effect.
and you don't need to have the conveyor spin with indefinite speed. if we asume that only 1/1000 of the conveyor's backwards movement is transferred into heat-energy, meaning that this amount of energy in form of movement was transferred to the plane, then the conveyor simply has to spin one thousand times faster than the plane.
simply try it out. sit onto a bicycle and hold onto a car that is driving forward. you will need energy to holt onto the car. part of this energy is lost by wind resistance, part of it by keeping your ballance - and part of it by the friction of the wheels!
FRICTION, FRICTION, FRICTION!!!
if you find a wheel that does not have a frictional resistance, give me a shout and henceforth I will cycle to work
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