Physics Question

[letdown427]

i know that the wheel speed isn't the point of the OP, i just answered youre question about why i added the wheel speeds.

swtethan, we all understand what you mean, and it is valid, but this is probably a thought exercise, it's not meant to be realstic.

 

[CoveX]

giantbunny,
I agree that the wording of the problem is probably 90% of the confusion here. My understanding of it was that if we stipulate at any time that the plane is moving at 10mph relative to an absolute orange grove by the runway, the conveyor is running at 10mph in the opposite direction. NOT that the conveyor is rigged to keep the plane stationary, whereby it could not take off.

Thereby the conveyor is simply introducing a modest amount of drag by spinning the wheels faster.

 

[911paramedic]

Originally posted by: CoveX
giantbunny,
I agree that the wording of the problem is probably 90% of the confusion here. My understanding of it was that if we stipulate at any time that the plane is moving at 10mph relative to an absolute orange grove by the runway, the conveyor is running at 10mph in the opposite direction. NOT that the conveyor is rigged to keep the plane stationary, whereby it could not take off.

Thereby the conveyor is simply introducing a modest amount of drag by spinning the wheels faster.

OMG, thank you. This has to do with TAKE OFF, not flight. People are confusing apples and oranges.

 

[letdown427]

Originally posted by: CoveX
giantbunny,
I agree that the wording of the problem is probably 90% of the confusion here. My understanding of it was that if we stipulate at any time that the plane is moving at 10mph relative to an absolute orange grove by the runway, the conveyor is running at 10mph in the opposite direction. NOT that the conveyor is rigged to keep the plane stationary, whereby it could not take off.

Thereby the conveyor is simply introducing a modest amount of drag by spinning the wheels faster.

actually, good point, the very words of the question say the plane moves in one direction. therefore, it moves...problem solved. lol.


oranges? they're the green things right?

 

[giantpinkbunnyhead]

Holy crap... It just hit me... the plane will take off! Boy do I feel like a moron! Haha, I"m going to run this by the other pilots at work and see who I can fool with it.

I should have realized it sooner... I mean, flying in Alaska puts me in situations where we do takeoffs on very icy runways. Situations where a car would spin its tires yet we apply takeoff power and we accelerate at the same exact rate as we would on dry asphalt. G'awd! I've been owned by physics again. Heh, that was a fun discussion!

 

[imported_electron]

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)

You've got to be kidding me. That is the dumbest question I've ever heard.


Of course the plane can take off.

 

[letdown427]

Originally posted by: giantpinkbunnyhead
Holy crap... It just hit me... the plane will take off! Boy do I feel like a moron! Haha, I"m going to run this by the other pilots at work and see who I can fool with it.

I should have realized it sooner... I mean, flying in Alaska puts me in situations where we do takeoffs on very icy runways. Situations where a car would spin its tires yet we apply takeoff power and we accelerate at the same exact rate as we would on dry asphalt. G'awd! I've been owned by physics again. Heh, that was a fun discussion!

it takes a very decent guy to admit his first thoughts weren't accurate. kudos to you!

 

[CoveX]

Originally posted by: 911paramedic

Originally posted by: CoveX
giantbunny,
I agree that the wording of the problem is probably 90% of the confusion here. My understanding of it was that if we stipulate at any time that the plane is moving at 10mph relative to an absolute orange grove by the runway, the conveyor is running at 10mph in the opposite direction. NOT that the conveyor is rigged to keep the plane stationary, whereby it could not take off.

Thereby the conveyor is simply introducing a modest amount of drag by spinning the wheels faster.

OMG, thank you. This has to do with TAKE OFF, not flight. People are confusing apples and oranges.

Umm, I'm pretty sure we disagree about the problem.

My position is that the conveyor can only apply a force on the plane through friction by doubling the speed of the spinning wheels, and that this is not enough to counter the thrust of the engines.

 

[911paramedic]

Originally posted by: giantpinkbunnyhead
Holy crap... It just hit me... the plane will take off! Boy do I feel like a moron! Haha, I"m going to run this by the other pilots at work and see who I can fool with it.

I should have realized it sooner... I mean, flying in Alaska puts me in situations where we do takeoffs on very icy runways. Situations where a car would spin its tires yet we apply takeoff power and we accelerate at the same exact rate as we would on dry asphalt. G'awd! I've been owned by physics again. Heh, that was a fun discussion!

You said it yourself..."you accelerate"...according to this you dont. Your scenario calls for the outside world to be moving in relation to your plane, hence air over the airfoils. This question leaves the plane motionless with no air movement but plenty of wheel spin.

 

[imported_electron]

Originally posted by: 911paramedic

Originally posted by: giantpinkbunnyhead
Holy crap... It just hit me... the plane will take off! Boy do I feel like a moron! Haha, I"m going to run this by the other pilots at work and see who I can fool with it.

I should have realized it sooner... I mean, flying in Alaska puts me in situations where we do takeoffs on very icy runways. Situations where a car would spin its tires yet we apply takeoff power and we accelerate at the same exact rate as we would on dry asphalt. G'awd! I've been owned by physics again. Heh, that was a fun discussion!

You said it yourself..."you accelerate"...according to this you dont. Your scenario calls for the outside world to be moving in relation to your plane, hence air over the airfoils. This question leaves the plane motionless with no air movement but plenty of wheel spin.

Let me guess you think it can't take off according to your original question?

 

[911paramedic]

good gawd, how about some lifers answering this question for me please. PM me if you need to...

These noobs are probably in early grades just guessing...

 

[letdown427]

there is no wheelspin. the wheels only move if the plane actually moves.

the engines thrust will make the plane move forward, even if it's only at 0.00000001mph relative to the dog by the runway. then, the planes body will be moving forward at 0.000001mph, and the conveyer moving backwards at 0.0000001mph, therefore wheels will be otating as if they're doing 0.0000002mph (the 0000's are figurative, please don't ask me to check the numbers)

the wheels do not move the plane forwards. they are like the rear wheels on a front wheel drive car. they keep the metal bits off the ground(brake schmake )

 

[pray4mojo]

If you dont understand this, draw a damn diagram. Suppose velocity in the positive x direction is 100mph given by the plane. Velocity in the negative direction is -100mph given by the belt. Add these together and you get 0mph! As for take off and flight, it will do neither assuming thrust is only applied in the x direction. Therefore sum of all the forces in the y direction is the weight of the airplane.

Basic physics.

 

[imported_electron]

Originally posted by: 911paramedic
good gawd, how about some lifers answering this question for me please. PM me if you need to...

These noobs are probably in early grades just guessing...

good gawd. You judge a person's intellect based on post count of some internet forum? You have no idea whom you are talking to do you? I can answer this question once and for all but you seem to be afraid to converse with me lest you be "owned" by a "noob". Why can't you just have a rational discussion without bringing post count into it. It's actually a VERY simple question from my point of view. But then I guess I'm not a dope like people who LIVE on an internet forum and think their post count means anything.

 

[letdown427]

the y direction has no relevance really. (at the moement, the y thrust will come once the plane (ignore the wheels) is moving enough to create lift)

thrust is applied by the engines to move the body of the plane <--

yes the wheels rotate round, but this is just a side effect. the conveyer moves -->, but this does not effect the <-- thrust of the engines, that still moves the body of the plane forwards, the --> of the conveyer rotates the wheels a bit quicker, that's it.

 

[giantpinkbunnyhead]

Originally posted by: 911paramedic

Originally posted by: giantpinkbunnyhead
Holy crap... It just hit me... the plane will take off! Boy do I feel like a moron! Haha, I"m going to run this by the other pilots at work and see who I can fool with it.

I should have realized it sooner... I mean, flying in Alaska puts me in situations where we do takeoffs on very icy runways. Situations where a car would spin its tires yet we apply takeoff power and we accelerate at the same exact rate as we would on dry asphalt. G'awd! I've been owned by physics again. Heh, that was a fun discussion!

You said it yourself..."you accelerate"...according to this you dont. Your scenario calls for the outside world to be moving in relation to your plane, hence air over the airfoils. This question leaves the plane motionless with no air movement but plenty of wheel spin.

The outside world WILL move in relation to the plane, regardless of if it's on conveyor belts, ice, or asphalt. This is because all the conveyor belt can possibly to is spin the wheels of the plane at XXX speed. This represents very little backwards force and the thrust of the plane would easily overcome it.

Initially the plane is at rest; as is the belt. Now the plane applies thrust and moves forward. (relative to everything around it; not the belt). THe belt instantaneously moves backwards at the same speed the plane moves forward.But what happens now? The belt can only act on the wheels of the plane, and it does... spins them faster. But this does very little, if anything at all, to arrest the forward force generated by the engines.

 

[letdown427]

Originally posted by: giantpinkbunnyhead

Originally posted by: 911paramedic

Originally posted by: giantpinkbunnyhead
Holy crap... It just hit me... the plane will take off! Boy do I feel like a moron! Haha, I"m going to run this by the other pilots at work and see who I can fool with it.

I should have realized it sooner... I mean, flying in Alaska puts me in situations where we do takeoffs on very icy runways. Situations where a car would spin its tires yet we apply takeoff power and we accelerate at the same exact rate as we would on dry asphalt. G'awd! I've been owned by physics again. Heh, that was a fun discussion!

You said it yourself..."you accelerate"...according to this you dont. Your scenario calls for the outside world to be moving in relation to your plane, hence air over the airfoils. This question leaves the plane motionless with no air movement but plenty of wheel spin.

The outside world WILL move in relation to the plane, regardless of if it's on conveyor belts, ice, or asphalt. This is because all the conveyor belt can possibly to is spin the wheels of the plane at XXX speed. This represents very little backwards force and the thrust of the plane would easily overcome it.

Initially the plane is at rest; as is the belt. Now the plane applies thrust and moves forward. (relative to everything around it; not the belt). THe belt instantaneously moves backwards at the same speed the plane moves forward.But what happens now? The belt can only act on the wheels of the plane, and it does... spins them faster. But this does very little, if anything at all, to arrest the forward force generated by the engines.

QFT
FTW

lol and yes, that whole post count thing is a stunning insight into the workings of your mind.

 

[TuxDave]

Imagine a treadmill with a wagon sitting on it and you're standing off the treadmill. You turn on the treadmill to like 10mph and you keep your hand on the back of the wagon to prevent it from falling off. The force that your hand exerts is equivalent to the thrust of the airplane. So it's possible to create a system where a plane is exerting thrust and isn't moving relative to air.

This is of course with the assumption that you have to exert some force to prevent the wagon from falling off. Otherwise if the necessary force to keep the wagon on the track equals zero, then such a system can't exist.

 

[potam]

http://www.fohguild.org/forums/showthread.php?p=385233

Such a dumb question.

 

[CoveX]

Originally posted by: TuxDave
Imagine a treadmill with a wagon sitting on it and you're standing off the treadmill. You turn on the treadmill to like 10mph and you keep your hand on the back of the wagon to prevent it from falling off. The force that your hand exerts is equivalent to the thrust of the airplane. So it's possible to create a system where a plane is exerting thrust and isn't moving relative to air.

Yes.
But that's not the system the OP's problem describes.

 

[letdown427]

do you have a wheely office chair? push yourself away from the desk. now, imagine you were on a conveyer belt, moving at 10mph towards ur desk. u hold urself still against the desk, and then push with the same forece as you did before. youll move back from the desk again, not as fast, because the friction from the conveyer belt will use up some of your initial energy, but you will move backwards. just like that plane. except forwards. d'oh.

 

[imported_electron]

Also this should have been in highly technical and then you might have gotten more serious responses with less "noise" from people that don't know enough physics to answer the question and just take a guess at it.

 

[giantpinkbunnyhead]

Originally posted by: TuxDave
Imagine a treadmill with a wagon sitting on it and you're standing off the treadmill. You turn on the treadmill to like 10mph and you keep your hand on the back of the wagon to prevent it from falling off. The force that your hand exerts is equivalent to the thrust of the airplane. So it's possible to create a system where a plane is exerting thrust and isn't moving relative to air.

This is of course with the assumption that you have to exert some force to prevent the wagon from falling off. Otherwise if the necessary force to keep the wagon on the track equals zero, then such a system can't exist.

Faulty logic. If your hand's force merely keeps the wagon stationary on the treadmill, you are exerting the same force against the wagon as the treadmill exerts against it in the opposite direction. But the thrust from a jet would be far stronger than the force the conveyor belt could exert, just like you can apply more force to your hand and "push" the wagon further upstream.

 

[letdown427]

think of it as conservation of energy then.

the engines use fuel to effectively make thrust (lets just simplify it) and that can just be counted as energy making the plane go forwards. the conveyer can go as fast as it likes backwards, but the only way that can have an input on the plane, is friction energy on the wheels. this will not ever match that of the thrust energy. and if it did, a lot of that plane and conveyer would melt.


ok this is not a good application of the theory of coservation of energy/momentum. it isn't even conservatin of energy or anything, that was just a buzzword that sprung into my mind. i'm sorry, it's 1:30 for me, and iv been up since 7. This has been fun though, thank you all.

 

[imported_electron]

Originally posted by: giantpinkbunnyhead

Originally posted by: TuxDave
Imagine a treadmill with a wagon sitting on it and you're standing off the treadmill. You turn on the treadmill to like 10mph and you keep your hand on the back of the wagon to prevent it from falling off. The force that your hand exerts is equivalent to the thrust of the airplane. So it's possible to create a system where a plane is exerting thrust and isn't moving relative to air.

This is of course with the assumption that you have to exert some force to prevent the wagon from falling off. Otherwise if the necessary force to keep the wagon on the track equals zero, then such a system can't exist.

Faulty logic. If your hand's force merely keeps the wagon stationary on the treadmill, you are exerting the same force against the wagon as the treadmill exerts against it in the opposite direction. But the thrust from a jet would be far stronger than the force the conveyor belt could exert, just like you can apply more force to your hand and "push" the wagon further upstream.

But what if you had a "magic" treadmill, that could provide just as much force as the jet engines even at full thrust? What do you think would happen then? Actually that wouldn't be magic, just one incredibly large powerful treadmill.