Physics Question

[imported_electron]

Originally posted by: letdown427
i'd prefer proof over simplistic thought chains any day. As with your way, maths n00bs will think, yeh but infinity plus 1 man, and treat infinity as a number. at least with the proof, you can walk them through it and suddenly it happens, and they can tangibly see it. also, i think it's easier for 'them' to imagine an infinite series of 9s, tha an infnite series of 0s, with a 1 on the end. lol. meh.

There is no 1 at the end.

Proofs are fine if they understand it. But if they don't? I'm just offering an alternative to those who can't follow the mathematical proof. If they could, the discussion would be over.

 

[Armitage]

Originally posted by: letdown427
"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?"

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels are moving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

like, imagine a strong fan, on wheels, blowing air backwards on one of these conveyers. say it blows air out at a rate sufficient to get it to move forward at 10mph. now, assuming there is no resistance as we always do in simple physics, it will move forward at 10mph whether it is in thea air, on the ground on its wheels, or even in the sea (sticking to NO RESISTANCE here). if it was on this conveyer, all that would happen is that it would still be moved forward by the fan at 10mph, but the wheels would be traveeling at 20mph, ass they conveyer would be moving opposite to the wheels under it.

OMG, I can't believe there are 3 pages of posts on this. No I haven't read them all, but letdown got it right on the first page.

The speed of the ground/conveyer belt under the plane is completely irrelevant. As everybody says, it's the speed of the plane through the air that matters. Well, the plane moves forward by thrust from the engines - it is accelerating forward regardless of what the ground is doing underneath it. Assuming the brakes are off of course

The wheels will be spinning at twice the forward velocity of the plane of course, but the plane will be moving forward relative to the air.

Draw a free body diagram of the plane. You have thrust from the engines. What is counteracting that thrust to keep the plane motionless?

Not the conveyer belt - there will be some rolling resistance of the wheels across the belt of course - just like a normal takeoff. Though a bit higher due to the higher wheel speed.

But it is moving and will takeoff.

 

[911paramedic]

OMG, I ask for a simple answer. The answer is NO, the plane will NOT take off. The net velocity is ZERO.

HELLO, ZERO, anybody out there???

 

[myusername]

Originally posted by: 911paramedic
OMG, I ask for a simple answer. The answer is NO, the plane will NOT take off. The net velocity is ZERO.

HELLO, ZERO, anybody out there???

Yeah, that may be the simple answer, but that's the WRONG answer

 

[Armitage]

Originally posted by: 911paramedic
OMG, I ask for a simple answer. The answer is NO, the plane will NOT take off. The net velocity is ZERO.

HELLO, ZERO, anybody out there???

You have thrust from the engines. What is keeping the plane from moving forward relative to the air?

 

[Armitage]

Originally posted by: KillerCharlie
The question is worded very poorly. Given proper wording (such as giving the velocities in terms of a coordinate system), I am sure I will be right on the answer.

I am an aerospace engineer getting my MS in aerodynamics.

You need to go back and have another look at freshman dynamics.

 

[imported_electron]

Originally posted by: 911paramedic
OMG, I ask for a simple answer. The answer is NO, the plane will NOT take off. The net velocity is ZERO.

HELLO, ZERO, anybody out there???

Are you serious or just joking now? Your question has been answered 8 ways from sunday. Did you not read the answers? Not follow the explanations, not believe the posters or are you just kidding that you think it doesn't take off?

have you seen this?: http://www.straightdope.com/columns/060203.html

 

[cirthix]

HELLO EVERYBODY, The question is worded wrong and cannot be anwswered in it's current state. It does not specify wether the monitoring system is measuring velocity relative to the conveyor or the ground.

 

[Armitage]

Originally posted by: cirthix
HELLO EVERYBODY, The question is worded wrong and cannot be anwswered in it's current state. It does not specify wether the monitoring system is measuring velocity relative to the conveyor or the ground.

It doesn't matter unless you're suggesting that the landing gear can be spun fast enough that its rolling resistance is equal to the thrust of the engines. On a normal aircraft that won't be true - the rolling resistance on a takeoff run is far smaller then the aerodynamic drag that the engines have to overcome. Lots of other things on that aircraft will break before you generate enough rolling resistance from the freely spinning gear to equal the engines thrust.

 

[letdown427]

Originally posted by: Armitage

Originally posted by: letdown427
"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?"

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels are moving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

like, imagine a strong fan, on wheels, blowing air backwards on one of these conveyers. say it blows air out at a rate sufficient to get it to move forward at 10mph. now, assuming there is no resistance as we always do in simple physics, it will move forward at 10mph whether it is in thea air, on the ground on its wheels, or even in the sea (sticking to NO RESISTANCE here). if it was on this conveyer, all that would happen is that it would still be moved forward by the fan at 10mph, but the wheels would be traveeling at 20mph, ass they conveyer would be moving opposite to the wheels under it.

OMG, I can't believe there are 3 pages of posts on this. No I haven't read them all, but letdown got it right on the first page.

The speed of the ground/conveyer belt under the plane is completely irrelevant. As everybody says, it's the speed of the plane through the air that matters. Well, the plane moves forward by thrust from the engines - it is accelerating forward regardless of what the ground is doing underneath it. Assuming the brakes are off of course

The wheels will be spinning at twice the forward velocity of the plane of course, but the plane will be moving forward relative to the air.

Draw a free body diagram of the plane. You have thrust from the engines. What is counteracting that thrust to keep the plane motionless?

Not the conveyer belt - there will be some rolling resistance of the wheels across the belt of course - just like a normal takeoff. Though a bit higher due to the higher wheel speed.

But it is moving and will takeoff.

Thank You. lol.

 

[firewall]

Originally posted by: 911paramedic
OK, a friend of mine frequently posts brainteasers for us on our site. (400+ members, all veterans or active duty) This is his latest question and we are deeply divided on the answer, just about down the middle. I would like to put this out there so I can get a very simple answer to explain it to him. (I am sure of my answer, but dont want to post it for fear of contamination) Here is his 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?"

Thanks in advance. (no, its not homework, you guys know me...duh)

No.

There is forward thrust, there is an equivalent drag (assuming conveyor provides the drag) but there is no lift cause there is no air flowing around the wings to generate the lift. Hence, the forward thrust is rendered ineffective. It stays there. My 2 cents.

Edit: Wow! Was that wrong......

 

[eits]

the plane can't take off without lift. lift is dependent on air speed over and under the wings. since the plane is stationary, it's got no lift. lift is not dependent on the speed of the wheels of the plane.

 

[eits]

Originally posted by: letdown427
"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?"

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels are moving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

like, imagine a strong fan, on wheels, blowing air backwards on one of these conveyers. say it blows air out at a rate sufficient to get it to move forward at 10mph. now, assuming there is no resistance as we always do in simple physics, it will move forward at 10mph whether it is in thea air, on the ground on its wheels, or even in the sea (sticking to NO RESISTANCE here). if it was on this conveyer, all that would happen is that it would still be moved forward by the fan at 10mph, but the wheels would be traveeling at 20mph, ass they conveyer would be moving opposite to the wheels under it.

still creates no lift.... only thrust.

 

[eber]

Seems like something like this should be sent to "Myth Busters".

The real problem is the wording of the problem (as has been stated many a times).

If the plane is stationary (i.e. the wings are stationary) to a fixed reference point there will be no lift (or pressure differences between the top and bottom of the wings).

 

[Acanthus]

No, it wont be able to take off, you need to be moving in order to get lift.

 

[mugs]

Read the thread people. The plane is not stationary, the converyor belt cannot keep it stationary because it is not propelled forward by its wheels. And even if the conveyor belt could keep the plane's groundspeed at 0 it could still take off with a large enough gust of wind.

 

[Acanthus]

Originally posted by: mugs
Read the thread people. The plane is not stationary, the converyor belt cannot keep it stationary because it is not propelled forward by its wheels. And even if the conveyor belt could keep the plane's groundspeed at 0 it could still take off with a large enough gust of wind.

Then its just an unclear question, and has nothing to do with physics.

 

[MikeMike]

http://www.brandonobrien.com/random/nolift.gif

lol

 

[Eeezee]

The plane most definitely can not take off, the air around the plane is not moving fast enough to create lift

The plane could be moving forward very fast, but the conveyor is pulling it back equally fast. In turn, the plane is not moving relative to the air.

Here is an equal situation; can a plan take off if it has infinitely powerful brakes that are always active? No, since the plane can never move.

 

[eber]

Originally posted by: MIKEMIKE
http://www.brandonobrien.com/random/nolift.gif

lol

:laugh:

 

[mugs]

Originally posted by: Acanthus

Originally posted by: mugs
Read the thread people. The plane is not stationary, the converyor belt cannot keep it stationary because it is not propelled forward by its wheels. And even if the conveyor belt could keep the plane's groundspeed at 0 it could still take off with a large enough gust of wind.

Then its just an unclear question, and has nothing to do with physics.

It absolutely has to do with physics, you can't explain why the plane would take off without physics. And it's not unclear it's just a tricky question.

 

[MrDudeMan]

Originally posted by: letdown427

Originally posted by: Armitage

Originally posted by: letdown427
"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?"

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels are moving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

like, imagine a strong fan, on wheels, blowing air backwards on one of these conveyers. say it blows air out at a rate sufficient to get it to move forward at 10mph. now, assuming there is no resistance as we always do in simple physics, it will move forward at 10mph whether it is in thea air, on the ground on its wheels, or even in the sea (sticking to NO RESISTANCE here). if it was on this conveyer, all that would happen is that it would still be moved forward by the fan at 10mph, but the wheels would be traveeling at 20mph, ass they conveyer would be moving opposite to the wheels under it.

OMG, I can't believe there are 3 pages of posts on this. No I haven't read them all, but letdown got it right on the first page.

The speed of the ground/conveyer belt under the plane is completely irrelevant. As everybody says, it's the speed of the plane through the air that matters. Well, the plane moves forward by thrust from the engines - it is accelerating forward regardless of what the ground is doing underneath it. Assuming the brakes are off of course

The wheels will be spinning at twice the forward velocity of the plane of course, but the plane will be moving forward relative to the air.

Draw a free body diagram of the plane. You have thrust from the engines. What is counteracting that thrust to keep the plane motionless?

Not the conveyer belt - there will be some rolling resistance of the wheels across the belt of course - just like a normal takeoff. Though a bit higher due to the higher wheel speed.

But it is moving and will takeoff.

Thank You. lol.

you arent right and neither is armitage. this is a very simple question with a very simple answer. it has been said many times.

the thurst created by the engines is counter-acted by the friction of the wheels. that is the whole point of this hypothetical problem. the "station wagon on the treadmill" scenario is a perfect example of this. the "thurst" is the same thing as the car moving the wheels with an engine. you guys are very wrong by stating the plane would lift off because it is still stationary with respect to the ground. that is how the question is asked so the answer is no, it doesnt lift.

 

[mugs]

Originally posted by: Bigsm00th

the thurst created by the engines is counter-acted by the friction of the wheels. that is the whole point of this hypothetical problem. the "station wagon on the treadmill" scenario is a perfect example of this. the "thurst" is the same thing as the car moving the wheels with an engine. you guys are very wrong by stating the plane would lift off because it is still stationary with respect to the ground. that is how the question is asked so the answer is no, it doesnt lift.

The question said that the treadmill moves at the same speed as the plane in the opposite direction, it did not say the plane remains stationary. There'd have to be a hell of a lot of friction in those wheels to counteract the thrust of the jet engines. The wheels would simply be rotating twice as fast as the plane is moving.

 

[MrDudeMan]

Originally posted by: mugs

Originally posted by: Bigsm00th

the thurst created by the engines is counter-acted by the friction of the wheels. that is the whole point of this hypothetical problem. the "station wagon on the treadmill" scenario is a perfect example of this. the "thurst" is the same thing as the car moving the wheels with an engine. you guys are very wrong by stating the plane would lift off because it is still stationary with respect to the ground. that is how the question is asked so the answer is no, it doesnt lift.

The question said that the treadmill moves at the same speed as the plane in the opposite direction, it did not say the plane remains stationary. There'd have to be a hell of a lot of friction in those wheels to counteract the thrust of the jet engines. The wheels would simply be rotating twice as fast as the plane is moving.

i see your point. that is a tricky question.

 

[eber]

Originally posted by: Bigsm00th

Originally posted by: letdown427

Originally posted by: Armitage

Originally posted by: letdown427
"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?"

waaait a minute. a plane is not a car right? Right. A plane is not moved forward by it's wheels right? Right. For the plane to move forwards, air is sucked into the engines (i'm not thinking of a propeller driven plane here, but like, a commerical airliner, irrelevant though) Neway, for it to move forward, this must happen, it mustbe sucking the air into the engine, generating thrust n stuff, fine i don't know for sure the exact technical details, but i actually think it can take off. The plane actually moves forward independent of the ground. its wheels stop it from dragging on the floor nothing more. so if the plane is moving forward at say 140mph (to take off) yes, the conveyer is moving at 140 opposite direction, but this just means that the wheels are moving at 280mph. the planes speed is created in the air if u will, by the engines, not by the wheels. therefore, i think it can take off. contrary to my initial assumption of this is a duuuuumb post. do i win? lol

like, imagine a strong fan, on wheels, blowing air backwards on one of these conveyers. say it blows air out at a rate sufficient to get it to move forward at 10mph. now, assuming there is no resistance as we always do in simple physics, it will move forward at 10mph whether it is in thea air, on the ground on its wheels, or even in the sea (sticking to NO RESISTANCE here). if it was on this conveyer, all that would happen is that it would still be moved forward by the fan at 10mph, but the wheels would be traveeling at 20mph, ass they conveyer would be moving opposite to the wheels under it.

OMG, I can't believe there are 3 pages of posts on this. No I haven't read them all, but letdown got it right on the first page.

The speed of the ground/conveyer belt under the plane is completely irrelevant. As everybody says, it's the speed of the plane through the air that matters. Well, the plane moves forward by thrust from the engines - it is accelerating forward regardless of what the ground is doing underneath it. Assuming the brakes are off of course

The wheels will be spinning at twice the forward velocity of the plane of course, but the plane will be moving forward relative to the air.

Draw a free body diagram of the plane. You have thrust from the engines. What is counteracting that thrust to keep the plane motionless?

Not the conveyer belt - there will be some rolling resistance of the wheels across the belt of course - just like a normal takeoff. Though a bit higher due to the higher wheel speed.

But it is moving and will takeoff.

Thank You. lol.

you arent right and neither is armitage. this is a very simple question with a very simple answer. it has been said many times.

the thurst created by the engines is counter-acted by the friction of the wheels. that is the whole point of this hypothetical problem. the "station wagon on the treadmill" scenario is a perfect example of this. the "thurst" is the same thing as the car moving the wheels with an engine. you guys are very wrong by stating the plane would lift off because it is still stationary with respect to the ground. that is how the question is asked so the answer is no, it doesnt lift.

Agree. Shouldn't the real question be, "Can the thrust of the plane counter-act the friction of the wheels so that it might take off?". Then you'd need to get all nerdy and know the specs of the engine, weight of the plane, coefficients of friction, etc... The question isn't precise enough.