Anyone good at physics, please clear this mystery

[ant75]

Ok, this has been going on BuF OT. I'm not all for that linking other forums' threads usually, but this has been eating me up inside . I refrained from giving my opinion there because i don't have the sufficient knowledge in physics to formulate what's just an intuition. I can't help to think most people there are wrong. If some people here have a strong background in physics (like sublime ) i would really appreciate if you enlightened me, and those poor folks as well, on this specific riddle.

 

[-Snakebite-]

I would say no too, the plane is not packing air under its wings, its static. Just wasting a lot of fuel. It would have exactly the same result if a plane with no wings was running the experiment.

 

[Psychomorph]

The wings need to cut the air, that requires the plane to move fast to get off.
If you place a giant propeller in front of the plain, than it can get off, but when it leaves the airflow it will get down again. It needs constant airflow to fly.
As -Snakebite- said, remove the wings and you get the same result.

 

[-Snakebite-]

Actually - I'm wrong.

I was assuming the drive is coming from the wheels, not the engines. The belt would delay the take off, but the plane would still be gathering speed and moving forwards.

If the plane was not moving forwards it wouldnt take off. Assuming that it is moving fowards, and gets up to the correct speed, it would take off.

 

[geogob]

airspeed airspeed airspeed. Nothing else matters.

What ever happens with "ground speed" is not important for airplane and is only use to calculate travel time between waypoints.

If the plane doesn't move relative to the ground (e.g. ground moving bellow the plane) is not affecting the speed of the air around the wings. Thus, no lift will be produced.

A similar idea is could a plane take off without moving relatively to the ground... e.g you park an airplane and lock it in place so it could only move up or down relative to the ground. If you get a head wind strong enough to get the V2 speed in the current plane configuration and load factor, the plane will take off, without moving forward at all relatively to the ground.

EDIT: OT Wow. someone is using MY avatar. burn him.

 

[geogob]

Ok, I was going to reply on BUF OT, but these people are a waste

the *only* thing that matters is the speed of air around the wings. The speed of the plane relative to the ground (what people usually call speed) is irrelevent. So if a plane is on a moving conveyor belt, it would achieve nothing. The plane would move and take off normally like if there was no belt. Only, the wheel would turn twice as fast. The friction of the wheels is negligable if you compare it to the force it takes to accelerate 300 tones of airplane and fight friction of the air.

This is especially true since the wheels on a plane are free running... the speed at which the plane moves through the air and the speed at which the wheel turns are not linked. It's the the wheel, when turning, that move the plane forward, but the contrary. When the plane moves forward, it makes the wheels turn. If the plane doesn't move the wheel don't move either.

 

[ant75]

OK i think i know what part of the riddle confused me. This guy summed up my confusion :

Of course, this means that the tires will move forward as well (as they are fixed to the plane). But, in order for the tires to move forward, they must have some relative difference in 'velocity' (sure, its rotational) compared to the linear velocity of the treadmill. But this magical treadmill will instantly match any velocity the tires have...

So what would really happen?

I argue that as soon as you flip the switch on the engines, both the treadmill and the tires will accelerate to infinity almost instantly, until they both explode in a gigantic violent fireball that is visible from space. The plane is destroyed in the carnage and is unable to take off.

 

[Rostam]

The plane won't be able to take off. This is because even though the engines are providing power, the acceleration still comes from the wheels. Thus the plane won't accelerate, won't get enough air under the wings and therefor won't lift.

 

[Olethros]

No, no and fucking NO! Geo is right and a depressingly large amount of people on BUF are fundamentally and inexcusably WRONG. We're not talking "half-full, half-empty"-wrong here, but "the Earth is flat because our invisible sky pixie "God" hath declared it so"-wrong.

At the risk of restating the obvious: Because the wheels are free-rolling, the conveyor exercises no horizontal force on the plane. And therefore, the only thing that matters is the push from the engines on the surrounding air.
If we assume there is no wind, the plane's speed relative to the ground will be the same as that through the air. Assuming a take-off speed of 200 km/h, when the plane reaches this speed relative to the ground and air the conveyor will be running in the opposite direction at 200 km/h as well. The plane's speed relative to the surface of the conveyor will be 400 km/h, but this will only make the wheels turn faster, not slow it down in relation to the ground or air.

QED, fuckers.

 

[Rostam]

It's still too early to really think about it. But Olethros, wouldn't the conveyer just move in the oposite direction even faster, keeping the plane still?

 

[ant75]

No, no and fucking NO! Geo is right and a depressingly large amount of people on BUF are fundamentally and inexcusably WRONG. We're not talking "half-full, half-empty"-wrong here, but "the Earth is flat because our invisible sky pixie "God" hath declared it so"-wrong.

At the risk of restating the obvious: Because the wheels are free-rolling, the conveyor exercises no horizontal force on the plane. And therefore, the only thing that matters is the push from the engines on the surrounding air.
If we assume there is no wind, the plane's speed relative to the ground will be the same as that through the air. Assuming a take-off speed of 200 km/h, when the plane reaches this speed relative to the ground and air the conveyor will be running in the opposite direction at 200 km/h as well. The plane's speed relative to the surface of the conveyor will be 400 km/h, but this will only make the wheels turn faster, not slow it down in relation to the ground or air.

QED, fuckers.

Not being a math guy, it took me a bit more time than everyone else to figure it out . But i do have another (possibly very stupid and badly worded) question :
The wheels are free rolling ok. But isn't there still a very small friction with the ground, therefore exercising a very slight resistance ? (stop me where i'm wrong). Now would it be possible for the treadmill to run at an incredible speed (possibly exponential acceleration) that would prevent the plane from moving ? Is that theoritically possible ?

 

[Hadmar]

Put on some roller skates. Stand on a treadmill. Grab the handle with your hands. Put the threadmill on the highest setting. Pull yourself forward. Do you move forward?

 

[Rostam]

Put on some roller skates. Stand on a treadmill. Grab the handle with your hands. Put the threadmill on the highest setting. Pull yourself forward. Do you move forward?

Yes you would, but that is not the same question (duh). The threadmill in that thread will move in the oposite direction at the same speed as the wheels. In other words, I think the force the other way (friction) can move up to infinity.

But Hadmar you do prove an interesting thought, maybe we should turn it around. You stand on a threadmill that is moving at top speed (20km/h) and you are standing on it, holding the front so you don't fall off. When somebody pushes you, does it require the wheels to move at a greater speed than 20 km/h to make you move? The answer is yes. So therefor I don't think the plane can lift off (since it stands still).

 

[Hadmar]

Well, the hands are the engine, the handle is the air. The belt dosn't affect the air (handle) which the engine (hands) use to create movement.

Unless the belt moves at Ludicrous Speed(TM) which makes the resistance from the wheels so high that it realy slows down the plane I don't see how the plane has much problems taking off.

 

[ant75]

Put on some roller skates. Stand on a treadmill. Grab the handle with your hands. Put the threadmill on the highest setting. Pull yourself forward. Do you move forward?

If that was a reply to my last question, the treadmill i was imagining doesn't have a constant speed like the one in your example. In my theoritical example, i was wondering if the treadmill could have a constant acceleration so that it prevents the plane from gaining speed.
Yeah, not possible IRL, i know. But theoritically ?

edit : instaposted, i got my answer. Ludicrous speed(tm), that is.

 

[Rostam]

Ludicrous Speed(TM), that's exactly the right word for it. But yeah, I assumed that was part of the question. That's why I still don't see how a plane could take off (except if it would FIRST lift, like a harier).

 

[Olethros]

Yes you would, but that is not the same question (duh). The threadmill in that thread will move in the oposite direction at the same speed as the wheels. In other words, I think the force the other way (friction) can move up to infinity.

But Hadmar you do prove an interesting thought, maybe we should turn it around. You stand on a threadmill that is moving at top speed (20km/h) and you are standing on it, holding the front so you don't fall off. When somebody pushes you, does it require the wheels to move at a greater speed than 20 km/h to make you move? The answer is yes. So therefor I don't think the plane can lift off (since it stands still).

Your basic misunderstanding is that the plane's speed relative to the conveyer will approach infinity. This is not the case. The conveyer will only move as fast as the plane relative to the ground factored by -1. The increased friction from the wheels would be negligible in a practical experiment and isn't even a factor given the parameters of the theoretical equation.

What you (and a lot of others) seem to be thinking is that the conveyer will accellerate opposite of its own speed relative to the plane, which is utter bollocks. Then you'd have F(v) = vplane+F(v) or F(v) = 2*F(v) neither of which make any mathematical sense. The real equation is F(v) = 2*v which will only approach 400 and not infinity when v approaches 200 (using the speeds in my example).

 

[Harrm]

There no air moving under the wings, therefore no lift. So no.

--Harrm

EDIT:

airspeed

I dont read threads anymore.

 

[Psychomorph]

I dont read threads anymore.

I see you don't, because I said airflow.

 

[cracwhore]

And that's why I will always have this sig.