Interesting physics/logic riddle

[12AMNSX]

If planes could take off, and potentially land, this way then why do we use or need long runways at airports or aircraft carriers? I'm not an engineer, but would have to assume that most jet engines cannot easily create enough air flow over their wings, basically sitting still, to lift off. Otherwise, Harrier jets and other vertical take-off jets wouldn't need nozzles that can turn vertically to the ground for take off. I would have to assume that a plane taking off this way would be very unstable, dangerous and difficult to control.

Look at it this way - the only thing that matters is the relationship between the plane and the air mass around it. If you were in a plane flying at 100 mph and flying directly into a head wind of 100 mph, the plane would not fall out of the sky. It may be motionless in RELATION to the ground, but in RELATION to the air around it, it is still going 100 mph. The riddle was about the relationship between the plane and the conveyor belt (ground) and did not say anything about the relationship between the plane and its other surroundings (air).

 

[Aero]

Assuming its a spring scale, when you are on it it will show your weight when you jump in the air it will zero again, how does this demonstrate your point

because the fly being on the floor is the same as you being on the scale. Once it takes off its basically the same as someone jumping the air. As i mentioned. the riddle thinks the answer is that the resulting downward force of the lift generated by the fly's wings pushes the same as its mass. Which is not true. my 2 cent.

 

[AdnanVCS]

Yes, Harriers are extremely difficult to fly they are really easy to crash.

 

[NSXrebel]

Yes, Harriers are extremely difficult to fly they are really easy to crash.

And that's why they are called the "Widow-maker".

As for the riddle, the plane can't take off. Its stationary. It doesnt mention anything about tire/wheel speed, only the speed of the aircraft. So whatever speed the aircraft is "moving" at, it gets cancelled out by the conveyor. There is no airflow over the wings to create lift. TAS(True Air Speed) would be zero.

Look at it this way - the only thing that matters is the relationship between the plane and the air mass around it. If you were in a plane flying at 100 mph and flying directly into a head wind of 100 mph, the plane would not fall out of the sky. It may be motionless in RELATION to the ground, but in RELATION to the air around it, it is still going 100 mph. The riddle was about the relationship between the plane and the conveyor belt (ground) and did not say anything about the relationship between the plane and its other surroundings (air).

An aircraft already in flight would not "stop" moving if flying into a head wind of x speed. It would only create more lift, so it's not the same as a "moving" plane on a conveyor.

 

[Vega$ NSX]

As for the riddle, the plane can't take off. Its stationary. It doesnt mention anything about tire/wheel speed, only the speed of the aircraft. So whatever speed the aircraft is "moving" at, it gets cancelled out by the conveyor. There is no airflow over the wings to create lift. TAS(True Air Speed) would be zero.

Read the initial riddle again carefully. Nowhere does it specifically state that the plane is stationary. That is an assumption that you made because you are assuming the movement of the conveyer belt counteracts any forward movement of the plane which is not true. The plane can in fact move foward independent of the conveyer belt. Again, I reiterate, if the wheels of the plane were completely frictionless how can it exert any force to keep the plane from moving forward? What if I replaced the wheels with pads with a frictionless grease or oil on it? The movement of the conveyer belt, no matter how fast they ran, would do nothing to stop the movement of the plane because the thrust is not generated at the interface between plane and belt.

 

[khappucino]

haha assume the plane's wheels maintain perfect contact with the conveyor belt... there is no air streaming over the wings because the plane is stationary (unless its in a wind tunnel), there is no swirling due to vorticity, since the air velocity is zero across the air foil.

basically you don't get lift unless there is airflow over the wings, and if the plane is stationary i don't see how you can get any air with any velocity going across the wings.

(helped my friend learn fluid dynamics, i don't know anything about fluids but i know the math for it from electromagnetics =) )

haha thats my guess, no air flow across the wing, no static airspeed, you can't have lift period =)

 

[khappucino]

i think the real answer is 42 though...

 

[Malibu Rapper]

i think the real answer is 42 though...

Close, but you are actually off by 1. It's 43.

 

[02#154]

The riddle, as worded for this thread, is:

"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?"

While it requires making assumptions about the wheels having friction against the movable runway (and I understand how absolutely frictionless contact patches could make the movable runway a moot point), I think it's more reasonable to make the assumption (that the wheels have friction). The riddle doesn't say "and since we've already made up one imaginary thing (the movable runway), feel free to make up other imaginary things in your answer".

So, using only what was actually stated in the orginal riddle as worded in this thread, and then applying commonly accepted principles of physics, I think the plane would not take off.

To change that fact, one must insert impractical assumptions (a tire that has absolutely no friction against the road) or information that is not already provided by the wording of the riddle.

It's like answering the question: "which is worth more, an NSX or a laptop computer?" Using only the words provided, one has to assume the NSX is worth more. And yes, it's an assumption but a reasonable one. To make it untrue you have to insert some sort of weird circumstance like "well, if the NSX is burned to a crisp and you can't even tell that it's an NSX anymore, then the laptop is worth more".

So... the plane won't take off.:tongue:

J

 

[SaberX]

Read the initial riddle again carefully. Nowhere does it specifically state that the plane is stationary. That is an assumption that you made because you are assuming the movement of the conveyer belt counteracts any forward movement of the plane which is not true. The plane can in fact move foward independent of the conveyer belt. Again, I reiterate, if the wheels of the plane were completely frictionless how can it exert any force to keep the plane from moving forward? What if I replaced the wheels with pads with a frictionless grease or oil on it? The movement of the conveyer belt, no matter how fast they ran, would do nothing to stop the movement of the plane because the thrust is not generated at the interface between plane and belt.

Ahh... I think I see what you're saying Vegas.
Am I correct in assuming the wheels on the conveyor belt would create the same conditions as a plane on a frictionless surface.

So if we reworded the puzzle as "A plane with sleds starts off stationary on a sheet of ice (assume there is 0 friction between the sleds and the ice) would this plane take off?"

I belive it would.

 

[AdnanVCS]

Let's get a poll for NO and YES

 

[robr]

Lets look at this another way, if the plane is moving forward at 20mph, then the conveyor belt is moving backwards at 20mph. Are you guys saying that this will not hold the plane stationary because rotating the wheels in a backward direction does not stop the plane from moving since they are unrelated to the plane movement to start with?

I think I'm starting to get this

 

[NSX-Tuner]

After thinking about this puzzle again, I now understand why the airplane would take off. Unlike a car that drives it's wheels with gears and shafts which ties its forward movement to the conveyor movement's, the airplane's forward movement isn't impeded by the conveyor's movement. The airplane's wheels are simply free-spinning and have no effect on the airplane's forward movement.

 

[AdnanVCS]

Hmm if you where to retract the land gear it would just drop down it has no lift!

 

[NSXrebel]

Hmm if you where to retract the land gear it would just drop down it has no lift!

Exactly, there is no lift to begin with, so it will not take off.

 

[SaberX]

Exactly, there is no lift to begin with, so it will not take off.

Please people... think about this as a plane on sleds on an ice surface. (That is the equivalent of the wheels rolling backwards from the conveyor.) Remember the wheels have nothing to do with the plane taking off - the plane is not a car. It will fly no problem.

 

[02#154]

The plane will only take off if the wheels have zero friction. In the real world (if the real world had a conveyor belt runway, as specified in the riddle) the wheels *do* have friction, the wheels are an indicator of speed even though they are not driving the plane and the plane does not have effective forward movement which is needed for aerodynamic lift.

One has to interject additional conditions to what is provided in the riddle implicitly or can be derived by real-world conditions.

Vegas is right that it will take off *only if the wheels have zero friction*, which isn't ever going to happen. Vegas is also right that if you make the (logical) assumption that the wheels have friction between the runway and themselves that it will not take off.

J

 

[TomCat]

After thinking about this puzzle again, I now understand why the airplane would take off. Unlike a car that drives it's wheels with gears and shafts which ties its forward movement to the conveyor movement's, the airplane's forward movement isn't impeded by the conveyor's movement. The airplane's wheels are simply free-spinning and have no effect on the airplane's forward movement.

 

[nicholas421]

for those of you who still don't get it...

picture yourself on roller skates on a downhill conveyor belt. no matter how fast the conveyor belt spins uphill... you will still accelerate downill... (well at least until the speed overcomes the wheel bearings, but that would be pretty damn fast)

replace you with a jet.
replace gravity will thrust.

voila!

 

[nicholas421]

Vegas is right that it will take off *only if the wheels have zero friction*, which isn't ever going to happen. Vegas is also right that if you make the (logical) assumption that the wheels have friction between the runway and themselves that it will not take off.

J

so are you saying you wouldn't accelerate forward (downhill) in the above rollerskate scenario?

 

[EN93NSX]

"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?"



If you just read the problem and make no other assumptions (i.e. frictionless wheels,etc.), the plane is only moving in relation to the conveyor belt. The wings have no air moving over them, hence no lift is being created. With no air moving over the wings the plane has zero air speed and will not fly.

Like posted above even at full thrust if the wheels are lifted the plane would fall because no air is moving over the wings.

This is based only on the problem as stated w/o making other assumptions

 

[02#154]

so are you saying you wouldn't accelerate forward (downhill) in the above rollerskate scenario?

Yes, I am. I say this because the imaginary conveyor belt is said to be able to match the speed of the wheels of the plane or, in the rollerskate scenario, of the rollerskates.

Remember that in the rollerskate scenario, the acceleration won't be infinite. At some point, the rollerskater will be in "free-fall" and will have reached maximum speed, just like when a parachuter jumps out of a plane: he/she won't continue to accelerate towards the ground forever; eventually, max speed is reached.

J

 

[Viper Driver]

for those of you who still don't get it...

picture yourself on roller skates on a downhill conveyor belt. no matter how fast the conveyor belt spins uphill... you will still accelerate downill... (well at least until the speed overcomes the wheel bearings, but that would be pretty damn fast)

replace you with a jet.
replace gravity will thrust.

voila!

I think that is the best analogy I've seen yet.

For those of you who still think the plane will not take off, I've got something else for you to ponder.

Let's just say you're a windsurfer out in the ocean and the wind is constant. The only thing that changes as you sail down along the beach is that there are many different eddies and currents in the water. Does the direction of the water's current you're travelling through have a 1:1 effect on your speed?

 

[nicholas421]

Yes, I am. I say this because the imaginary conveyor belt is said to be able to match the speed of the wheels of the plane or, in the rollerskate scenario, of the rollerskates.

J

reread the original post... the belt matches the speed of the plane, not the wheels.

if the plane travels 1 meter forward (relative to a fixed position on the ground), the belt needs to travel more than 1 meter back to keep the plane stationary.

 

[02#154]

reread the original post... the belt matches the speed of the plane, not the wheels.

if the plane travels 1 meter forward (relative to a fixed position on the ground), the belt needs to travel more than 1 meter back to keep the plane stationary.

Well, show me a plane that moves forward on the ground without turning the wheels and I'll concede. My opinion on this is based only on the exact words used in the riddle and real-world assumptions. I concede that I'm making assumptions, but they're based on reality, not made up impossibilities. I don't have to "imagine" anything more than the imaginary conveyor belt runway, which is stipulated in the wording of the riddle. In the real-world, when a plane on wheels on the ground moves forward, the wheels turn.

To make the plane move forward without turning the wheels, you have to have non-existent wheels that don't have friction between them and the ground. If one wants to interject with their own special circumstances to make his/her take on this work, then so be it.

J