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Discussion in 'General Chat' started by imported_Nigma, Mar 21, 2006.
god not this again, lmao
I hope you havent been searching for this thread looking for the answer as i have read through it again and im none the wiser
WTF this was like when I still had my Audi, who dug this little gem up then!
I needed to find the question for work but when they ask what the answer is i'll just have to say that i have no idea whatso ever
Physics do not allow the conveyor to stop the plane moving forward. The thrust of the jets is pushing the plane forward, not the wheels. The plane would take off.
But does it not say that the conveyor belts speed matches the jets speed exactly.
I am sending this thread to Mythbusters!!!
Check the pop-up video on the MythBusters site i found when trying to find the palce to send this stuff to them...http://dsc.discovery.com/fansites/mythbusters/mythbusters.html
No, it says that the conveyor belt matches the speed of the wheels, so if they were free wheeling, as the jet requires thrust from it's engines to move forwards, the conveyor belt shouldn't have any effect on the plane to move forwards with thrust.
no the previous post is right - and the conveyor belt speed is academic and irrelevant - the plane's wheels are freewheeling, spinning free (at whatever speed the belt determines) - but meanwhile the body of the aircraft is being pushed forward through the air by its own engines (regardless of what speed or direction the belt and its freewheeling wheels are doing) and the plane will take off in the normal way - from the point of view of a distant bystander it will be no different to a normal take off on a normal runway.. no myth to this at all, other than the mistaken belief amongst some that the conveyor could somehow hold the plane in place by acting on its freewheeling wheels. If the planes 'brakes' were on then there would be a coupling between the runway and the aircraft, but they are not. There would be a miniscule effect from the friction of the wheels' bearings, but on quality aircraft engineering this is a) miniscule and b) nothing at all compared to the thrust of the engines pushing the craft forward. The wheels will spin faster than they would on a static runway - but that is all! It definitely will take off.
For those who think the plane won't take off:
Under your thought process, if you reverse the direction of the conveyor belt, the plane could effectively take off horizontally.
No. It cant.
I know i've promised this a few times but i will video my lab set up as i have nothing better to do before christmas, that clearly shows the plane cannot fly.
excellent thread guys!!!
the plane would take off...
think of it this way...you have a big downhill conveyor belt. leave the car on neutral and set it free...since the wheels are turning freely, the car would start to roll downhill, although the wheels will spin much faster than they normally would.
since the plane isn't driven by its wheels, the only thing required is to provide enough power to overcome the added wheel/bearing friction.
just read outlore s post...excellent point, totally correct.
thinking on it again, when the wheels reach around 230mph (if i remember correctly) the tyres will burst and the plane will probably never tak off
It won't reach 230mph... According to Emmet Brown... As soon as the plane reaches precisely 88mph, there will be a fluctuation in the space time continuum, and the plane will find it self in the past.
Ha ha, I remember one of my university lecturers giving us this one Friday as a "thought for the weekend".
After a short discussion, we went to the pub, decided he was a sod and forgot all about the question
But on Monday he told us that it wouldn't take off. So there
If you apply science/physics to the question, then the wheels and conveyorbelt would go straight to infinity the moment it moves forwards - that's a fact! Wether it would effect the forward motion of the plane is another question haha, as the wheels would most probably drop off, leaving the plane on it's undercarriage, so it wouldn't take off!
Looks like we are back to step 1 again folks, where can I get me a conveyorbelt that will goto infinity?
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Don't agree with you there jojo - look: 'the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction' so..
as the plane moves forwards along the runway and gathers speed (note for all those who think it stays put, it is moving forward through the air as normal.. if it stayed still there would be no need for the belt to move at all) the wheels are turning, and distance is being covered. The belt, moving in the opposite direction, at the same speed (note that the belt motion merely 'matches' the wheel speed, but in the opposite direction) means that the wheels would rotate twice as fast as normal. If the plane moves up to 150mph to take off , then the belt would be doing 150mph the opposite direction.. wheels turning at a 300mph rotational speed. no suggestion of infinity at all... The plane still is moving forward through the air, along the runway, and still takes off..
What about if someone had stuck NO MORE NAILS to the planes wheel.
HA HA i have you now suckers, victory is mine
Bascule, I know exactly where you are coming from, because that's what I thought, but if you use maths(I can picture the equation, but not put it in text-sorry!), the belt WOULD go straight to infinity as soon as the plane moves forward a millimetre.
If the drive was to the wheels, then yes, the conveyorbelt would counteract and keep the plane still, but as it's most probably thrust pushing the plane forwards, once the wheels rotate, the conveyorbelt would activate and essentially keep accellerating to infinity...
Is that infinity and beyond as my mate Buzz will be able to tell you the equation
hmmm.... Im sure weve been through this.... many times over..... who dug this up?
Think of a jogger on a running machine his legs are doing the work, which are touching the conveyor. If you put the jogger on roller skates on the conveyor and let his hands (i.e. engine) pull him forward by holding a rail he will go forward! No matter how fast the conveyor is going.
It will fly!
I dont get this. But if he was wearing roller skates and just held himself steady he wouldnt go anywhere! The wee wheels on his wee skates would just turn and he would stay still altough a bit wobbly.
He is not holding himself steady he is pulling himself forward with his hands on the bar!
Could you push a toy car against the flow of a tesco checkout conveyer belt? Of course you could. It wouldnt matter how fast the belt was going.. it could be going really fast, some phenomenal 'scale' speed for the car.. the little wheels would be spinning like mad.. but would it be fundamentally any harder for you to push that toy car against the direction, than if the belt was stationary? No, not significantly at all, those little wheels are freewheeling, they would just turn faster.. Now substitute a toy plane.. its little wheels spinning.. now push that plane forward just like a normal plane taking off..
Its just a question of scale. The planes engines are pushing it's body along and forwards, just like your hand pushing the body of the toy. the wheels, and the speed of the belt, are irrelevant for anything other than the slight friction of their bearings. the plane's engines will propel the body forwards, along the runway.. it will take off..
Bascule, for your toy car to have a nett forward movement across your tesco belt, as per your example, the wheels must be moving faster than the belt below them. This is not the case in the question, the speeds remain equal.
The question is about wheel rotational speed not body speed, ie the wheels cover 5ms-1 and the belt moves -5ms-1 so nett = 0ms-1, no flight.
I reckon that some top boffins from NWR are on here now posting, now i admire your abilities to explain the plane and the conveyor belt, but if any of you are from planning stop giving me T3's on my night shift you swines
Right, i'm going to try and make sense of this. For the purposes of this explanation the plane is a Jet. The jet is sitting on the treadmill runway, the engines fire up anf the belt starts to turn, when the engines are providing thrust, they are moving the body and the landing gear forwards, doesn't matter how fast the wheels are turning, the fuselage will start to move forward as the thrust is acting upon it, the plane is moving in relation to the air around it, not the ground beneath it, if the plane were in a vacuum then things may be different.
Yes, a plane in a vacuum definitely will not take off
If the plane stays static, sitting on the runway.... what happens thousands of pounds of thrust the engine is producing? Energy does not just "disappear"....
like I said, the plane would take off because it would be moving in relation to the air around it not the ground under its tyres.
But it still doesn't change the fact that the rotation of the wheels/bearings and the conveyor belt will go straight to infinity the moment the plane twitches lol.
i'm with jojo...it'll take off...
From a guy at work who sent me this text:
"Here's the original question: "A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?" (The Straight Dope: 060203.)
The implicit assumption is that if the conveyor belt's speed backward exactly counteracts the airplane's "speed" (whatever that means) forward, the plane remains stationary relative to the earth and, more importantly, to the air. (We assume the winds are calm.) With no wind moving past its wings, the plane generates no lift and can't take off.
But the assumption is false. While the conveyor does exert some modest backward force on the plane, that force is easily overcome by the thrust of the engines pulling the plane ahead. The plane moves forward at roughly its usual speed relative to the ground and air, generates lift, and takes off. Many people have a hard time grasping this (although it can be easily demonstrated in the lab), but eventually they do, smack their foreheads, and move on. We'll call this Basic Realization #1.
Message-board discussions of this question tend to feature a lot of posters who haven't yet arrived at BR #1 talking right past those who have, insisting more and more loudly that the plane won't take off. Then there's a whole other breed of disputants who, whether or not they've cracked the riddle as originally posed, prefer to reframe it by proposing progressively more esoteric assumptions, refinements, analogies, etc. Often they arrive at a separate question entirely: Is there a way to set up the conveyor so that it overcomes the thrust of the engines and the plane remains stationary and doesn't take off?
The answer is yes. Understanding why is Basic Realization #2.
The conveyor doesn't exert much backward force on the plane, but it does exert some. Everyone intuitively understands this. To return to the analogy in my original column, if you're standing on a treadmill wearing rollerblades while holding a rope attached to the wall in front of you, and the treadmill is switched on, your feet will initially be tugged backwards. Partly this is due to friction in the rollerblade wheel bearings, but partly--this is key--it's because the treadmill is accelerating the rollerblade wheels and in the process imparting some angular (rotary) but some linear (backward) momentum to them. You experience the latter as backward force. Eventually the treadmill reaches a constant speed and the rollerblade wheels cease to accelerate. At this point you can easily haul in the rope and pull yourself forward.
But what if the treadmill continues to accelerate? Different story. In principle it's possible to accelerate the treadmill at a rate that will exactly counteract any forward force you care to apply. (This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force.) The only mathematics needed to demonstrate this is the well-known physics axiom F = ma--that is, force equals mass times acceleration. Given that the conveyor exerts some backward force F on the plane, we simply crank up the acceleration as much as necessary to equal any forward force F generated by its engines. Result: The plane stands still and doesn't take off. Welcome to BR #2.
You may say it's impossible to build a constantly accelerating treadmill, that eventually we run into the limitation imposed by the speed of light, etc. True but irrelevant--BR #2 has an intrinsic elegance that transcends such practical concerns. Why didn't I bring it up in the first place then? You've got to be kidding. It took an entire column to get BR #1 across, and a second one to convey (I hope) BR #2. One fricking thing at a time."
Exactly. So, er will it take off?
Having said that - why dont they use conveyor belts on aircraft carriers? That way they could put them in reverse, and the plane could take of vertically, then smash back into the ship and explode. Good stuff.