Physics Question

[TuxDave]

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.

If you plot treadmill speed versus necessary force to keep it stationary and if you're telling me that force will limit to some asymptotic value as treadmill speed goes to infinity, then you are correct that if you exert a force greater than that asymptotic value then the conveyor can never go fast enough. If that's not true, then you are not correct.

 

[giantpinkbunnyhead]

Originally posted by: 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.

I think for a treadmill to provide an equal amount of thrust as the jet engines, considering the huge amount of loss due to wheelspin, it would have to be spinning so damn fast that it would probably self destruct itself, not to mention the plane's entire landing gear assemblies.

 

[imported_electron]

Originally posted by: TuxDave

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.

If you plot treadmill speed versus necessary force to keep it stationary and if you're telling me that force will limit to some asymptotic value as treadmill speed goes to infinity, then you are correct that if you exert a force greater than that asymptotic value then the conveyor can never go fast enough. If that's not true, then you are not correct.

Actually he could still be correct There is something you might be forgetting.

 

[letdown427]

engines exert forward force on planes body -> moves plane forward -> wheels are connected to pane, and get dragged along too -> treadmill could be travelling at 100mph in the opposite direction, but that is just producing a tiny amount of force against the huge thrust of the engines, which is independent of the ground. the wheels will spin more quickly than usual, but the plane will take off.




in all fairness, if the plane didn't take off, the question would be as simple as it first seems to the casual observer, and it would not have been created and widely spread in the first place under the title of brainteaser.

 

[TuxDave]

Originally posted by: electron
Actually he could still be correct There is something you might be forgetting.

A little help here.

 

[mugs]

Yes, I believe it can. A plane is not like a car, it does not move by exerting force against the ground. The wheels are freespinning, so the plane will move from the force it exerts against the air. It's wheels will be spinning as though it's moving twice as fast as it is. The only thing the conveyor belt would do is make the wheel spin faster.

And if you don't believe that, even if the plane had a ground speed of 0 it could still take off - it would just need a gust of several hundred mile per hour wind.

 

[TuxDave]

Originally posted by: mugs
Yes, I believe it can. A plane is not like a car, it does not move by exerting force against the ground. The wheels are freespinning, so the plane will move. It's wheels will be spinning as though it's moving twice as fast as it is.

And if you don't believe that, even if the plane had a ground speed of 0 it could still take off - it would just need a gust of several hundred mile per hour wind.

Yeah so.. the question is, are the wheels freespinning?

 

[Powermoloch]

no wind, no go

 

[letdown427]

i wouldn't say freespinning, they're forced into spinning by that nasty conveyer exerting a small force on them through friction as it pointlessly tries to stop the plane from moving. hehe

 

[letdown427]

Originally posted by: Powermoloch
no wind, no go

nope, sorry, you're wrong. lol. i just had to say it.

 

[imported_electron]

Originally posted by: TuxDave

Originally posted by: electron
Actually he could still be correct There is something you might be forgetting.

A little help here.

Mugs beat me to it. Gust of wind. Even if you held the plane back it would still take off all it needs is a tornado. Well i know that complicates matters a bit but it is a brainteaser after all and the original question did not take account of everything that could happen. The question is too vague. We call that sort of thing a loaded question

 

[Powermoloch]

Originally posted by: letdown427

Originally posted by: Powermoloch
no wind, no go

nope, sorry, you're wrong. lol. i just had to say it.

lol :roll: lift?

 

[KillerCharlie]

Originally posted by: DrPizza
i.e. when you turn on the engines, the plane moves relative to the air;.

This is incorrect. Just because your engines are on does not mean there is a freestream velocity. Engines produce thrust at zero velocity - though the freestream velocity is zero, the streamtube capture area is infinite and thrust is produced. It is entirely possible that the airplane is moving at the opposite velocity of the conveyor in the fixed reference frame, resulting in a plane that is not moving relative to that fixed reference frame (air). In this trimmed condition, a balance is struck between the propulsive force (which is NOT creating a freestream velocity) and the rolling resistance of the gear.

 

[letdown427]

lol, read the thread.
lol, physics.
lol, ask electron to e-school you.


anyway,

respect again to giantpinkbunnyhead for being a man about this whole thing, having a discussion not a shouting match and so on.

congratulations to electron for disproving 911emergency's ridiculous theory that post count is directly correlated to IQ.

thanks to 911emergency for introducing us to this great little question. lol. night. (it's as good as 2 here, i've got a dog to walk)

 

[dawks]

Originally posted by: TuxDave

Originally posted by: mugs
Yes, I believe it can. A plane is not like a car, it does not move by exerting force against the ground. The wheels are freespinning, so the plane will move. It's wheels will be spinning as though it's moving twice as fast as it is.

And if you don't believe that, even if the plane had a ground speed of 0 it could still take off - it would just need a gust of several hundred mile per hour wind.

Yeah so.. the question is, are the wheels freespinning?

Nearly. Pick up a bike and spin the wheel.. does it spin? Can you do the same with an airplane? Yes, might take some more force to spin the wheel, but a jet engine can provide more then enough thrust for that no doubt.

 

[mugs]

The equivalent situation with a car would be if the drive wheels were sitting on solid ground and the free wheels were sitting on the conveyor belt. It doesn't matter how fast the conveyor belt pushes against the free wheels, the drive wheels can still make the car go. Same situation. The wheels on the plane don't make it go, thrust does.

Now if you changed to so that the plane had two sets of engines, one facing forward and the other facing backward, and the ones facing backwards automatically put out as much thrust as the ones facing forward, THEN the plane would sit there on the runway and only a huge gust of wind could get it off the ground. But by that time the wings would have probably fallen off.

 

[Jon855]

Originally posted by: LoKe
No, the plane can not take off. Even though it's moving at the proper speed, the plane is not actually moving so it can not manipulate the air.

Of course, I'm probably wrong.

DONE /thread

 

[mugs]

The real question is though, does .999... = 1?

 

[zoiks]

So you're saying the plane is on a treadmill.

 

[letdown427]

it can take off.

here's another way of looking at it.

imagine the engines are seperate for a sec. they move forward, as they produce a huge force, thrust.

now, attach those to the plane.

the conveyer, it produces a pathetic little force, on the wheels. this force does oppose the huge thrust generated by the engines, but there sure isn't no balance to it. the force,thrust, will easily, and comprehensively overpower the pathetic little frictional force created by the conveyer moving at whatever speed you like.

In this diagram, size of arrow represents relative size of the force exerted.


<--------------------------- Thrust
__________->______conveyer friction crap, exeted on the wheels


that tiny little -> aint going to stop that <----------------------------- of the engines.

 

[Powermoloch]

Originally posted by: Jon855

Originally posted by: LoKe
No, the plane can not take off. Even though it's moving at the proper speed, the plane is not actually moving so it can not manipulate the air.

Of course, I'm probably wrong.

DONE /thread

 

[zoiks]

I just hate airline food.

 

[pray4mojo]

Originally posted by: letdown427
it can take off.

here's another way of looking at it.

imagine the engines are seperate for a sec. they move forward, as they produce a huge force, thrust.

now, attach those to the plane.

the conveyer, it produces a pathetic little force, on the wheels. this force does oppose the huge thrust generated by the engines, but there sure isn't no balance to it. the force,thrust, will easily, and comprehensively overpower the pathetic little frictional force created by the conveyer moving at whatever speed you like.

In this diagram, size of arrow represents relative size of the force exerted.


<--------------------------- Thrust
__________->______conveyer friction crap, exeted on the wheels


that tiny little -> aint going to stop that <----------------------------- of the engines.

the assumption made in the problem is that the belt has enough force to counter that of the engines.

 

[letdown427]

Originally posted by: mugs
The real question is though, does .999... = 1?

yes.

The way I, and many oter people probably see it

Let:

0.99999999.... = x

Multiply both sides by one hundred.

99.999999999..... = 100x


Subtract x from both sides (remember x = 0.9999.....)


=> 99 = 99x

Diveide both sides by 99.

1 = x = 0.999999999......


So yes it does.


jon855, you're wrong too. go back to school.

 

[letdown427]

pray4mojo, no it's not. the assumption made is that the conveyer can match the speed of the plane in the opposite direction. force is not meniotned. in fact, the very wording of the question implies the plane can move in the first place.