This argument is going on on another forum I visit:
If there was a Travellator and whatever you placed on there it would match speed.
So running at 10mph would make the thing go at 10mph so you would be standing still.
A car at 80mph would make the thing move at 80mph so the car would appear still.
Put a plane on there for arguements sake a relatively small jet like a Boeing 737, would the plane be able to move/take off?
I am arguing that the plane would take off, since the speed of the wheels is entirely irrelevant, and what matters is the speed relative to the air so that lift may be generated. Since the plane is powered by engines that push air backwards, invoking an equal and opposite reaction in a forward direction, the plane will move forwards relative to the air, therefore gaining lift, as normal, with the only differance being that the wheels would be turning at twice the normal speed (twice the airspeed of the aircraft).
Everyone else is convinced that the plane will stay stationary. Am I right?
Absolutely nothing would happen. If the plane remains standing still then there is nearly zero airspeed (wind only) which means it ain't going nowhere.
Surely the plane would not remain standing still, since the engines would be pushing it forewards relative to the air (which is what is important for it to gain lift) - The wheels would be moving at twice the speed of the travelator, but the plane would still be moving forward? The wheels only serve as a point of contact, rather than anything else...
Pezzoni wrote:Surely the plane would not remain standing still, since the engines would be pushing it forewards relative to the air (which is what is important for it to gain lift) - The wheels would be moving at twice the speed of the travelator, but the plane would still be moving forward? The wheels only serve as a point of contact, rather than anything else...
This is correct. I've seen this question before and it always seems to confuse people althugh when you think about it the answer is obvious.
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weemadando wrote:Absolutely nothing would happen. If the plane remains standing still then there is nearly zero airspeed (wind only) which means it ain't going nowhere.
It's a double trick question, where you think you see the underlying point but that's the trap. The plane doesn't remain standing still because its thrust isn't coming from traction against the travelator (which is true for the person and car). Although depending on how the travelator matches speeds, the plane's wheels might end up spinning fast enough to fail, obviously resulting in a wanktastic fireball.
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If there was a Travellator and whatever you placed on there it would match speed.
So running at 10mph would make the thing go at 10mph so you would be standing still.
A car at 80mph would make the thing move at 80mph so the car would appear still.
Put a plane on there for arguements sake a relatively small jet like a Boeing 737, would the plane be able to move/take off?
The phrasing of the question quite clearly states: MATCH SPEED.
Would this thought experiment make more sense with a glider on tow from a car?
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Would enough air be going through the engines and around the wings for it to take off, though? I mean, I understand that the engines work by sucking air through, but if the plane is stationary, would there be enough air to actually provide enough lift for it to take off? Exempting strong winds and stuff.
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I don't think that the plane would remain stationary, as the wheels are free spinning: The plane is actually moved forwards through the air by the engines.
Presumably this would also depend on how much friction there was between the wheel bearings and wheels, and the tyres and conveyor belt? (If the belt can move freely under the plane, then it stays in one place, even without power?)
This question is really getting on my tits now, it's distracted me from revising Physics for about 3 hours
In my eyes, it implies that the speed that the wheels are rotating at will be matched, meaning that the plane, though it may end up pumping out full thrust, will keep on standing still.
Consider if instead of the plane being powered by an engine, there was a winch attached to the nose which dragged it forward.
This makes it obvious that it would take off, as the plane would be moved forwards and generate lift etc.
The plane cannot 'tell' what is dragging it forwards, whether it is tension in a cable from a winch or thrust from the propellor. This means that when the plane tries to take off, it will just move forwards with the wheels spinning lots.
Try to think what would happen if the plane tried to land on the treadmill, it wouldnt just stop instantly would it?
And the plane will take off. The conveyor belt is matching the plane's speed, not the thrust from its engines. Since the speed, which the wheels will be rotating at, will be twice the speed they would have against a stationary surface, I don't think the friction is going to be nearly enough to affect the plane noticably (unless the wheels are crap).
Basically, the net force acting on the plane is not affected by the surface other than through friction, and the only friction in this case is the friction from when wheels are rotating around their axes (the only friction that will be affected by the speed of the wheels, that is).
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Matching velocity of the threadmill and the wheels will achieve nothing since the wheels are free-wheeling and are on planes solely for the purpose of giving them a low friction means of moving while on land.
The only way that threadmill is stopping the plane is by imparting enough force in the opposite direction to equal the thrust of the engines, and this would mean the threadmill has to continously ACCELERATE (at a ludicrously high velocity, at that). Merely 'matching speed' will NOT do this.
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AniThyng wrote:The only way that threadmill is stopping the plane is by imparting enough force in the opposite direction to equal the thrust of the engines, and this would mean the threadmill has to continously ACCELERATE (at a ludicrously high velocity, at that). Merely 'matching speed' will NOT do this.
Actually the treadmill does not have to accelerate, as the force imparted on the wheel to counteract friction in the bearings has an effect on the centre of mass, as well as its effect as a couple (ie it is a torque). If you put a bike on a treadmill running at constant speed and hold the handlebars you will feel a (small) force.
The whole question is just stupid, its only point is to trick people who don't realize that plane wheels don't work like car wheels. Last time I saw this posted (on another form), there were countless pages of people assuming the belt can actually transfer force to the plane, because the plane gets its forward speed like a car does.
The belt has no way to transfer an effective amount of force to the plane. Friction in the wheels will slow its acceleration down a bit, but it's a trivial amount compared to full engine power. The plane takes off, assuming two things:
1) The runway is long enough and the slightly slower acceleration doesn't extend its takeoff distance too far.
2) The wheels can handle the extra stress from spinning twice as fast as normal, and don't fail.
Of course neither of these were mentioned in the question, so I seriously doubt they're the answer.
Steel wrote:Consider if instead of the plane being powered by an engine, there was a winch attached to the nose which dragged it forward.
This makes it obvious that it would take off, as the plane would be moved forwards and generate lift etc.
The plane cannot 'tell' what is dragging it forwards, whether it is tension in a cable from a winch or thrust from the propellor. This means that when the plane tries to take off, it will just move forwards with the wheels spinning lots.
Indeed, and this is the best way to word the response, I think.
The engines are pulling the plane forward by slinging air back, just as surely as if it were being towed by a rope that wasn't on the treadmill. The plane's tires may be damaged, but it takes off with no problem.
