Plane on a coneyor belt
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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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jakob, i reckon you need to get a skateboard and go down to the gym, after your TV show with pav, of course.
Alan, you cant just disregard wheels spinning infinitely fast, if one part of the system doesnt work then the system as a whole cant work.
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True Remus, but considering practicalities like that brings the whole argument crashing down, because the bearings on the conveyor motor would fail long before those on the aircraft wheels...they are carrying a much heavier load with a massive amount of torque. There is virtually no torque on the aircraft wheels...they're essentially freewheeling. If the conveyor motor fails, the plane takes off....end of argument. My point was that you can't start contemplating actual shortcoming of the plane's mechanics while allowing the conveyor mechanism to have unlimited magical powers.
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Its not a shortcoming in the physicalities though, its a problem with the theoretical physics/mathematics behind the problem.
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Any volunteer to make the model in Sketchup Physics and see what happens?
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There isn't any problem with the physics/mathematics behind the problem. The physics say that the plane will take off. The calculations really aren't that difficult. It's the seemingly persistent...and entirely intuitive...notion that the conveyor will have any significant effect on the plane that's the problem.
The friction would be F=uR, where F= friction force, u is friction coefficient, and R= weight. The friction is constant (other than the plane getting lighter as it approaches takeoff speed); therefore the speed of the conveyor is totally irrelevant....nowhere does its velocty (v) appear in the frictional calculation.
This frictional force is not great. There are YouTube videos of people pulling Lear jets with their teeth. They are overcoming not only the friction, but also the inertia.
If the speed of the conveyor is irrelevant...and it is...it doesn't matter whether it's matching the speed of the plane or the rotational speed of the wheels. It's irrelevant in both cases.
As long as the forward thrust of the plane is greater than the friction (not exactly difficult) it will accelerate and take off as normal. It wouldn't surprise me to discover that a fighter jet could take off even with its brakes on. All it has to do is burn rubber for a few seconds. In such a case, of course, the conveyor would be stationary as the wheels wouldn't be rotating at all.
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I was refering to the fact that for the wheels to be moving forward they must spin infinitely fast+the speed of the plane.
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For any matter to reach an infinite speed would require all the energy in the Universe.
Perhaps the second part of the question ought to have asked whether such a model was possible at all rather than asking something more specific like whether or not the plane could take off. If the model isn't valid then whether the plane can take off or not ceases to have any meaning -
But the model does work (to an extent) and the plane cant take off.
Th reasoning: if the plane has any forward velocity this sets the wheels off in to a cycle whereby the wheels keep rotating faster and faster forever, this is impossible so the plane cant have a forward velocity.
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@alan fraser said:
As long as the forward thrust of the plane is greater than the friction (not exactly difficult) it will accelerate and take off as normal.
but the question was what happens, if the conveyor prevents exactly this. and for this question is theoretical we can asume that the conveyor can keep up to the speed (we could create a huge one with incredible speed after all).
is it right, if we say:
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theoretically the conveyor could stop the plane from taking off, because there is no windpressure on it's wings to take off
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practically the speed of the conveyor necessary to stop the plane from moving forward is impossible to reach with todays technology
(I think that is a satisfying solution, because it shows, that we all where right - only approaching the problem from differend ends.
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