Something to ponder... and I don't know the right answer

[flintlock]

A guy I work with just pulled this of another forum and asked me. I said no, but I'm no physicist and it seems I'm in the minority. Thoughts?


A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

The question is:

Will the plane (747 passenger jet) take off or not?

[Death_ADDER]

no because its not really moving and its not getting enough air to lift the wings.

Steve

[phaeton]

Everythings relative.  Good 'ole Einstein. 

Think about the speed of the airplanes wing relative to wind speed. Since the plane is not moving relative to the ground, and hence, not relative to the wind, the wind speed over the wings would equal (or approximately) zero. Therefore, with zero wind speed, there would be zero lift, and the plane would not lift off.

If this worked, they wouldnt need aircraft carriers to launch jets at sea.  =)

.phaeton

[Cole]

Absolutely not.  Like adder said.  The wing give the plane lift via fluid dynamics.  There are two schools of thought there.  One has to do with deflection of air causing phyical lift, i.e. a normal force, and the other has to do with differences in fluid (air) velocity causing higher pressure under the wing, lower pressure above the wing causeing the material between the two local pressure zones (the wing) to lift up. 

The forward motion of the plane starts the process of creating relative air movement.  Once you're in the air, the forward thrust helps maintain altitude which is why pitch is important for fuel efficiency and airspeed.

[Kaneda]

Depends.... are any of the passangers morbid? 

[flintlock]

no because its not really moving and its not getting enough air to lift the wings.

Steve

That's what I was thinking, but then I get another e-mail this morning because I demanded an answer and below is what I got. The guy giving the explanation seem like a bit of a dick, nontheless, his explanation seems logical. I'm bold typing what I think is the key point. Maybe the question is flawed becuase it uses the term "speed." I know jets use thrust to move, who doesn't, but I interpret speed as a way to measure thrust. Thoughts?



When I read the first page of this thread I was amazed that there would even be a disagreement on this topic. The answer is so obvious I thought it wasn't even worthy of a discussion, much less a debate.

First, off let me preface my statement on this as follows: I served on the faculty at both Ga. Tech and Washburn University teaching Physics and Mechanical Engineering and Engineering Science and Mechanics. Besides working as an Engineering Scientist in the private sector, I was also a senior consultant at NASA JSC Mission Control.

I am 100% absolutely beyond any and every question totally confident that my answer is correct. Further, for those who think differently but know a little about how airplanes work, I can prove it with a very simple mental exercise.

So, without further delay, let me finally and definatively answer the question:

THE PLANE WILL TAKE OFF.

Why? Because I'm also an attorney who know how to READ CAREFULLY!!!

Let's carefully consider the question:

"A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction)."

I see nothing in this statement that says the plane remains stationary relative to the ground. (As many assume. And in which case the plane would not take off. This is why this is a trick question!!!) To the contrary, the statements says the motion of the converyor is relative to the "planes speed", not its thrust. Thus, if the plane isn't moving, neither is the conveyor.

If in fact you could create such a runway, the plane would accelerate and move forward just as it normally would in a normal take off (with a minimal reduction in acceleration caused by the increased resistence of having to spin the wheels and tires up to twice the normal speed. And when it got to the normal take off speed relative to the ground (and twice that speed ralative to the conveyor) it would take off normally.

Talking about physics consider the following: Force = Mass * Acelleration

If those jet engines are at max thrust, the mass of the plane is going to accellerate forward unless there is an opposing force in the opposite direction of equal strength to oppose it. The force of the conveyor on the wheels would not supply such a force. Thus the plane WILL move forward, even as the conveyor accelerates in the opposite direction at the same rate. And since the plane accelerates almost as quickly as it normally would, it will reach and exceed the stall speed and be able to lift off.

[Death_ADDER]

if you do that you'd have a lot of busted tires... good luck on the new landing.

Steve

[Cole]

ok flint.  That guy is obviously a liar about his background.  Thrust is a word for work done by jet engine.  That's all.  Put an engine on a car, does it fly?  no.


He's talking about acceleration.  what is acceleration?  it is instantaneous velocity.  Velocity per unit time.  Rate of increase in velocity.  He is calling thrust accelleration in his post.  However, it the conveyor is to match speed (velocity) at all moments, it must match accelleration too.  Thus, the mass cannot move.  There is no relative momentum transfer.   the belt has "thrust" in the opposite direction of the plane.  net zero.

Now, this asshole thinks by calling himself the king of england, he's gonna get free tea.  well, fuck him. 

He obvously doesn't understand how a plane flys let alone the laws of Newtonian physics.  I'd bet he was an english major in high school.  Now he just sits around his mom's house eating pop tarts.

[Death_ADDER]

well if his theory was to work, the tires are already rolling backwards... now the weight of the plane moving forward would make the wheel move forward which means them tires are gone from all the friction.

Steve

[Cole]

look, the tires will not be damaged at all.  not even the bearings, well eventually they will breakdown.  When planes land, they're putting shitloads of force on the gear, and they don't blow up, always..

Rolling friction is the only friction the whells will exerience, which is extremely minimal.  The forces are translated into angular forces since the wheels are round.  It's tangential, and the rubber has enough of a coefficient of frictrion to go pretty damn fast before it completely wears out.  It's more likely to blow out due to angular momentum (centripital force) pulling it off the rim. 

That guy doesn't have a theory.  All he is ... is a pathetic excuse for a psuedointellectual.  Oh yeah, we believe you lawyer man!  lol.  I mean 3 inch dick man.  ;p

Now, someone tell vergas to stop posting that crap.

[Kaneda]

To begin my explanation, I must state my background to install an element of leetness. I am currently attending AP physics.... so I've got props on any fag with a Ph. D in physics. Also if all of my calculations are correct, I'm almost to the point where I've figured out how to rip space time. Also I am President of three different countries, so my answer to this question must be absolutely, 100% correct.

But anyway, I don't know about you guys, but this shit fucks with your head. The guy is a douche bag yes... but i think he's right. I'm not going to go into a drawn out physics answer and what not, because I have much better things to invest my time into, such as finishing my time travel theorems - instead I'll just use some logic. The plane is powered by jet engines, not the wheels. Is it possible that the jet engine will move the plane regardless of how fast the ground is moving opposite? Wouldn't it just make the wheels spin twice as fast as if it were taking off normally? I think the plane would still have a forward motion because the net force is created by the simple action/reaction caused by the jet engines, and not by the friction on the ground. Like i said, not totally sure if my thinking is correct... but i think the plane would still be moving forward regardless of the surface.

[Cole]

to answer you shortly, no.

do a block diagram for this one. 

remember.  the plane flys due to phenomenon of fluid dynamics.  movement of air past the wings.
when the plane is on the runway, how is it held up when it's not moving.  it's gear.

so, when the jets are fired up, and you get some forward movement, where is the energy from the jets being transfered.

That's right.  the ground.  the energy will eventually be transfered to the wings, but not until you hit that critical point to where you start creating lift.

so, if the runway starts moving backwards, it opposed, or absorbs, the energy transfer of the engine. 

fix the plane to the grond so it can't move.  turn on full thrust.  what happens.

one step further.  make it so the plane is fixed by a giant shear pin some three-ten feet of the ground, that is it will break free at some give force on the pin.  Make that force the maximum force that the plane can do.  Obviously the plane can fly when its at full power.  So, the pin breaks, the plane is ten feet off the ground at full engine on.  what happens?  the plane pitches down and crashes.  The guy from flints post is on the ground cause he believes it will fly, but you know what...now he's dead. 

[Kaneda]

Instead of replying to that completely erroneous answer, which still falls short of my answer... I shall show you this: this is what the reaction of my family was when they tried to undersand your answer.... because its totally false

[Kaneda]

J/K THATS NOT MY FAMILY GOT YOU GOOD BITCHES.....



....its just the average Kenyan of year 2005








still though, you're wrong 

[WolbEm]

Kaneda -A+

Cole, I expected so much more out of you man.  Put down the bong.  Your physics professor want to see you.  He said something about tearing up your diploma.

Riddle me this batman - how do rockets/thrusters work in space if there is no ground to transfer energy to???

Rockets, jet engines and cole all work off the same principle... expanding hot gas. For every action,  there is an equal and opposite reaction.  The expanding hot gas from the jet engines will THRUST the plane  (Thrust - v. to push or drive with force) relative to the rest of the earth and atmosphere (which is not moving with the conveyor). This force will greatly exceed any drag imposed by the conveyor and wheels.  Therefore, the plane will move forward, accelerate, and when it reaches sufficient speed, be able to take off.

Put a jet engine on a car -does it fly? no.  Does it move? yes.

Cannot access offset of type string on string