Ultimate brain teaser......

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Dragon_Cooler
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Ultimate brain teaser......

Post by Dragon_Cooler » Mon Apr 16, 2007 1:38 am

I havent heard this one before, so if you have please debate....


A plane is standing on a runway that can move (like a giant conveyor belt). 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) instantly.

Will the plane be able to take off?









I think this will happen to be honest... LOLOLOLOLOLLOLl

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Post by DMB2000uk » Mon Apr 16, 2007 4:41 am

You see where people always get hung up on this one is just technicalities.

And the fact that most people think of the plane like a car...

It does take off, see if you can work out why though from what I've just said.

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Post by KnightRid » Mon Apr 16, 2007 5:35 am

I say it will takeoff because it still is getting lift from the air flowing over its wings.

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Post by bubba » Mon Apr 16, 2007 5:54 am

If the plane is standing on the conveyor that matches its speed instantly in the opposite direction; then the forces cancel each other out and the plane will go no where. but like the animation the max speed limit could ultimately be reached but that is an unknown

Imagine yourself on a tread mill, everything being equal you go no where. As soon as you slow down or speed up and the treadmill does not compensate then you move.
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Post by Bwall » Mon Apr 16, 2007 6:52 am

Exactly, if the plane isn't moving anywhere then no air is moving to make it lift off.
If it was a Harrier there would be no problem.
Last edited by Bwall on Mon Apr 16, 2007 6:59 am, edited 1 time in total.
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Post by DMB2000uk » Mon Apr 16, 2007 6:56 am

But the wheels aren't providing force, the engines are. Its not getting its forward drive by pushing back at the ever increasing-in-speed treadmill its getting its drive from the thrust of the engines against the air.

So the wheels aren't doing anything except keeping the plane off of the runway. Now you could argue that the speed will get to infinity pretty quick, which would mean either the wheel bearings/tire rubber or treadmill would, give out, which is what I meant earlier about technicalities. Razz

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Post by Illuminati » Mon Apr 16, 2007 7:18 am

ya, the wheels are not the source of the forward force in this scenario. the jets on the wings moving air in the atmosphere are the forward force. The wheels just provide a means of moving forward on the surface until enough speed is picked up to provide lift.

So assuming the bearings in the wheels hold out long enough during takeoff on the treadmill, the plane is going to overpower the treadmill and take off as if it weren't even there because it is doing nothing to slow down the jets moving air.
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Post by skier » Mon Apr 16, 2007 8:05 am

i do not think the plane can take off, how can it get lift if it is not moving, air needs to be flowing under the wings at a certain speed, not through the engines, when the plane is standing still, how can wind be provided?(probably not even if it was in a hurricane)
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Post by TristanM » Mon Apr 16, 2007 8:57 am

The answer has already be given, but I've seen this question before and the simplest explanation is that the wheels are irrelevent - an airplane's wheels just spin (imagine wearing roller skates on a treadmill while holding on to the bar - you won't go anywhere, but the wheels will spin faster as you increase the speed. If you provide "thrust" by pulling yourself forward, the wheels will spin even faster, but you will still move forward.) Since the thrust being provided is from the engines not the wheels, the plane will still move forward, the wheels will just spin faster as it does so. The treadmill is only adjusting its speed to match the speed of the plane in the opposite direction, so the wheels will be spinning at twice the normal speed for takeoff, but the plane will still fly. It can be safely assumed that the bearings on airplane wheels will easily do twice the normal takeoff spinning speed since they also take an even bigger hit when the plane lands - in other words they're fairly tough.

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Post by bubba » Mon Apr 16, 2007 9:27 am

:? :evil:

Well, now that my caffeine has kicked in and I have thought about it. It can take off and in the same as if it was on a runway.

As the plane goes forward the conveyor (runway) goes in the opposite direction at matching speeds. The wheels will roll in the direction the plane is going and (this will blow your mind) the same direction as the conveyor (runway).

Pick up a pen, hold between your hands. Plane is the left hand, pen the wheels, and the conveyor (runway) your right hand. push the left hand away and pull the right towards you.

thats what I get for not fully waking up before answering a trick question.
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Post by Illuminati » Mon Apr 16, 2007 10:28 am

skierkid450 wrote:i do not think the plane can take off, how can it get lift if it is not moving, air needs to be flowing under the wings at a certain speed, not through the engines, when the plane is standing still, how can wind be provided?(probably not even if it was in a hurricane)
Given the scenario described, the trick is that the plane is moving just as if it were on a normal runway. The conveyor belt is not really effecting anything (other than the wheel speed) since its the jets providing the force, not the free-rolling wheels.
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Post by bigblockmatt » Mon Apr 16, 2007 12:11 pm

i dont see how the plane could take off...

im not an expert on flight and lift but here goes

why do planes fly?
They have specially shaped wings to produce lift.
Air moves over the wings (at a certain speed) lift is produced.
the plane moves the flap things on the wings to adjust how the plane lifts.

in the situation, i dont see how air is moving over the wings of the plane. you dont need jets for something to fly. the jets provide the power to move the plane which moves air over/under the wings. a glider flies, it just needs to get pulled by something until the whole "lift" thing occurs.

so, it the plane is standing still, no air is moving over the wings, no lift is produced, no flying plane - just wasted jet fuel.
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Post by DMB2000uk » Mon Apr 16, 2007 12:20 pm

Tristans answer pretty much explained how it does move forward and then will takeoff.

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Post by stopthekilling77 » Mon Apr 16, 2007 12:35 pm

jets push the wind through them, eventually overtaking the lack of movement from the wheels/conveyor belt combo

might make for a jarring or inaccurate takeoff?
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Re: Ultimate brain teaser......

Post by Alathald » Mon Apr 16, 2007 1:29 pm

Dragon_Cooler wrote:A plane is standing on a runway that can move (like a giant conveyor belt). 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) instantly.
OK the key here is in the wording, it talks about the SPEED of the plane and conveyor belt being equal and opposite, not the forces. If the forces were equal and opposite, the plane would never move. A good way to visualize this is to imagine a cable attached to the front of the plane thats attached to a wall; if the belt applies a force on the plane away from the wall, the cable will apply the same force towards the wall...now just replace the cable with the thrust from the jets and you can see the plane won't move.

Because the question doesn't discuss forces but rather velocity, we can see that if the plane is going 100mph, the belt will be going 100mph in the opposite direction. That means that the plane will eventally get up to takeoff speed and the belt will be going just as fast in the opposite direction.


Here's another brain teaser for you:
You have just died and now you're standing before two identical gates, one leading to heaven and one to hell, each with a guardian in front. Both guardians are identical in every way but one: one of the guardians always lies and one always tells the truth. Neither guardin is nessacerly in front of any specific gate (The one that lies could guard heaven or hell. Ditto for the one that tells the truth but there is only one guadian in front of each gate.) You are allowed to ask one guardian one question only; what question do you ask? (the objective here is to get to heaven)

Highlight below for a hint:
Think like a programmer (eg. flowchart it)
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Post by vicaphit » Mon Apr 16, 2007 1:37 pm

The plane will not take off. air has to be traveling over the wings for liftoff. If the plane is in an air tunnel and on the conveyor it would take off.

As for the second teaser... There is no heaven or hell.

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Re: Ultimate brain teaser......

Post by TristanM » Mon Apr 16, 2007 2:31 pm

Alathald wrote:
Dragon_Cooler wrote:A plane is standing on a runway that can move (like a giant conveyor belt). 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) instantly.
OK the key here is in the wording, it talks about the SPEED of the plane and conveyor belt being equal and opposite, not the forces. If the forces were equal and opposite, the plane would never move. A good way to visualize this is to imagine a cable attached to the front of the plane thats attached to a wall; if the belt applies a force on the plane away from the wall, the cable will apply the same force towards the wall...now just replace the cable with the thrust from the jets and you can see the plane won't move.

Because the question doesn't discuss forces but rather velocity, we can see that if the plane is going 100mph, the belt will be going 100mph in the opposite direction. That means that the plane will eventally get up to takeoff speed and the belt will be going just as fast in the opposite direction.


Here's another brain teaser for you:
You have just died and now you're standing before two identical gates, one leading to heaven and one to hell, each with a guardian in front. Both guardians are identical in every way but one: one of the guardians always lies and one always tells the truth. Neither guardin is nessacerly in front of any specific gate (The one that lies could guard heaven or hell. Ditto for the one that tells the truth but there is only one guadian in front of each gate.) You are allowed to ask one guardian one question only; what question do you ask? (the objective here is to get to heaven)

Highlight below for a hint:
Think like a programmer (eg. flowchart it)
Ask one dude what the other dude would tell you to do, the liar has to lie so he'll tell you to go to hell, the guy who tells the truth will have to tell you to go to hell also, so you go to the door that neither of them points at.

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Post by TristanM » Mon Apr 16, 2007 2:33 pm

vicaphit wrote:The plane will not take off. air has to be traveling over the wings for liftoff. If the plane is in an air tunnel and on the conveyor it would take off.

As for the second teaser... There is no heaven or hell.
You're missing the point - the plane will move, and air will pass over the wings.

Edit - I don't mean that mean either. :)

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Post by Alathald » Mon Apr 16, 2007 2:37 pm

vicaphit wrote:As for the second teaser... There is no heaven or hell.
Don't want to get into a argument about it but the point is suppose there was...you can rephrase the question to 'two doors, one goes to a male strip club, the other a female' and replace the guardians with bouncers...moot point cause Tristan hit the nail on the head.

EDIT: out of curiousty, had you heard that one before Tristan?
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Post by TristanM » Mon Apr 16, 2007 2:51 pm

Alathald wrote:
vicaphit wrote:As for the second teaser... There is no heaven or hell.
Don't want to get into a argument about it but the point is suppose there was...you can rephrase the question to 'two doors, one goes to a male strip club, the other a female' and replace the guardians with bouncers...moot point cause Tristan hit the nail on the head.

EDIT: out of curiousty, had you heard that one before Tristan?
:oops: Ummm...yeah. You wouldn't believe how many times I had to hear it before I could remember the answer though... This was my first chance to get it right in one go - so I had to take it. :mrgreen:

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