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Old 23 November 2007, 09:36   #51
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Originally Posted by J S View Post
It is not correct. The plane still has airflow over its wings.

James
read the statement again!! I dont imply that there is airflow. The statement is correct.
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Old 23 November 2007, 10:04   #52
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Originally Posted by Nos4r2 View Post
Therefore based on the numbers,the treadmill speed has to increase by 100metres per second every second to equal the forces exerted by the plane-whereas the plane is only capable of 10 metres per second per second of acceleration.
The question stated that the imaginary conveyor can match the plane.

You can't ignore the friction, rolling resistance etc. otherwise the contact with the ground could be considered not to exist and then you'd just be accelerating a body in free air.

Whether the forces would be ginormous has no bearing on in.
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Old 23 November 2007, 10:54   #53
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If the plane is stationary, then according to the questions rules, the conveyor belt must also be stationary.
So you can not have the conveyor moving at a speed which keeps the plane in a static position or the rule is broken. That means the plane must move forward so that the conveyor belt can move at the same speed as the plane.

edit: thought about it a bit more and infact it would be impossible for the conveyor to be moving at the same speed as the plane without the plane stopping which means the conveyor would also have to stop. Therefore the questions premise that the conveyor would accelerate at the same speed as the aircraft would be impossible to achieve under the rules of the question. It's a dumb question.
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Old 23 November 2007, 12:26   #54
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It's a dumb question.
Thatís the conclusion I come too.

If you take the real world scenario and place a model plane on a treadmill it would easily take off (until you get to the point where the treadmills going soooo fast the wheel/bearing friction is greater than the engine can overcome, or the wheels fall off).
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Old 23 November 2007, 14:33   #55
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Quote:
Originally Posted by jwalker View Post
The question stated that the imaginary conveyor can match the plane.

You can't ignore the friction, rolling resistance etc. otherwise the contact with the ground could be considered not to exist and then you'd just be accelerating a body in free air.

Whether the forces would be ginormous has no bearing on in.

You're missing the point.

What I'm saying (and I know I'm right-it's schoolboy physics) is that for the plane to stay stationary then the conveyer has to be accelerating a magnitude faster than the plane would be-therefore the conveyer can't be matching the plane's speed...and that's not the question.

QED- plane will take off.
The wheels won't be moving at some sort of infinite speed either.

--plane@150mph-over-the-ground------>
<--conveyer@150mph-over-the-ground---

give a difference of 300 mph (or 300mph closing speed if you want to imagine there being an object or a line drawn on the conveyer).The speeds are matching...

<edit-where's AndyB007 when you need him?> (and yes, I know mass is in KG not Newtons but the result is still the same at 1G)
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Old 23 November 2007, 17:24   #56
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Well it seems fairly simple to me

Despite the rolling resitence on the tread mill which the engines will overcome (or whatever) for the plane to stand still, no matter how fast the tread mill moves, the planes engines will overcome this whilst standing still ok ?.. The belt speed could go on for ever, but the plane will never take off, until it moves forward relative to the airspace which surrounds it, since the air space is static compared to the belt speed, once the plane moves forward, to an extent, the belt speed is irrelevant, as its friction is minimal compared to the forward air speed needed to lift off.. the plane will need to exceed the load on the belt for sure, and derive enough forward speed to lift off so..static air is what it is pushing against.. with its thrust, (less the drag from the belt which is minimal).. so yes ..it will take off once its gained enough speed from the static air around it .. be damned those physicists
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Old 23 November 2007, 17:50   #57
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interesting

ok lets put this in 2 boating,,if a boat is in a river and water is running at x speed,,the boats motor has to be kept at x revs to keep it stationary,,,if the speed of water in creases so does the boat revs to keep stationary,,,question,,,will the boat sit on the plane at a stationary speed,,,to equal the water passing under it if the water was at a theoretical speed to the hull plane speed,,,u have all the forces ,,thrust equaling the friction created by the water,,but no wind,,
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Old 23 November 2007, 17:55   #58
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ok lets put this in 2 boating,,if a boat is in a river and water is running at x speed,,the boats motor has to be kept at x revs to keep it stationary,,,if the speed of water in creases so does the boat revs to keep stationary,,,question,,,will the boat sit on the plane at a stationary speed,,,to equal the water passing under it if the water was at a theoretical speed to the hull plane speed,,,u have all the forces ,,thrust equaling the friction created by the water,,but no wind,,

That analogy is wrong as the propeller is pushing against the water rather than the air.

Try it with a jet engined boat (bluebird for example) and the analogy works.
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Old 23 November 2007, 19:23   #59
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This will get em arguing at work.

Imagine a plane is sitting on a massive hypothetical conveyor belt, as wide and as long as a runway. The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction.
To the letter of the question this statement could only have two answers. Think about it.

It doesn't state that the belt is revolving at the same speed as the aircraft is traveling (surface speed), the question reads "the conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction" it actually implies the whole belt assembly is moving not the surface speed of the thing That means if take off speed is 200mph then the whole belt assembly would move in the opposite direction to the plane during acceleration to 200mph (or 400mph in relation to the plane) the effective result being that the plane would crash off the end of the conveyor belt well before take off speed was acheived assuming that the plane needed all of the runway to take off in the first place. Of course if the plane could take off in less than half the distance of the runway length, then the plane would acheive take off speed and would fly away.

So yes it's a crap question, one that's not specific enough for a definitive result.

By the way, for what it's worth!

If the question was different and stated the surface speed of the belt matched the speed of the plane then I would say that speed is a measure of distance over time, yes? Miles per Hour!

So the plane would have to be moving for the conveyor to move, yes? Agreed!

So if the plane was moving at 200mph (which is speed over ground) then it would take off, the conveyor surface speed would be moving at 200mph in the opposite direction and the wheels of the plane would be revolving at twice the normal take off speed.

But that's a load of rubbish as the question doesn't state the surface speed of the belt as being the same take off speed.
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Old 23 November 2007, 20:06   #60
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A good bevvy tonight was it, Andy?
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