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Title: Brainteaser: There's an airplane on the runway..
Source: Elsewhere
URL Source: http://www.someplaceelse.com
Published: Dec 1, 2005
Author: I have no idea
Post Date: 2005-12-01 01:10:02 by Jhoffa_
Keywords: Brainteaser:, airplane, runway..
Views: 2339
Comments: 202

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. There is no wind.

Can the plane take off?

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Begin Trace Mode for Comment # 192.

#33. To: Jhoffa_ (#0)

Can the plane take off?

Without reading too many of the responses so far, if I understand right, at whatever speed the plane begins to move forward, the conveyer belt moves backwards.

Assuming the plane is propelled forward by jet or prop engines and not through a drive transmission to the wheels (which would not be a particularly well designed aircraft), then it won't stop the plane from going airborne. The only difference it would make is that the wheels would be spinning twice as fast at the airspeed when it takes off (which I'm assuming won't cause them to blow out, causing the plane to crash).

Neil McIver  posted on  2005-12-01   16:14:23 ET  Reply   Untrace   Trace   Private Reply  

Assuming the plane is propelled forward by jet or prop engines and not through a drive transmission to the wheels (which would not be a particularly well designed aircraft), then it won't stop the plane from going airborne. The only difference it would make is that the wheels would be spinning twice as fast at the airspeed when it takes off (which I'm assuming won't cause them to blow out, causing the plane to crash).

The only way the plane could take off is if it actually did move forward or up. Since the force of the engines is basically parallel with the ground then the only option is moving forward to become airborne. How does a multi-ton object become air born when it has no upward force being applied? The only way it could move forward is if it skidded down the runway and took off that way. I don't think they make planes capable of doing that, so it wouldn't take off. But if they had jet engines powerful enough and planes able to withstand the skidding, then it would take off.

RickyJ  posted on  2005-12-01   22:55:42 ET  Reply   Untrace   Trace   Private Reply  

How does a multi-ton object become air born when it has no upward force being applied? The only way it could move forward is if it skidded down the runway and took off that way.

Moving forward generates the lift required to climb, of course.

Since the plane is not thrusting against the belt, but against the gases being expelled by the engine, it would accelerate.

On further thought, though, the only way the plane could move forward is if the wheels turned fasted than the conveyor belt, and the operation of the belt would make that impossible.

Ergo, as soon as thrust was applied, both the belt and the wheels would accelerate to an infinite speed, at which time the wheels, and probably the belt would both instantly explode, killing everyone and making the question moot. At that point the question would need to be redefined.

Neil McIver  posted on  2005-12-02   12:09:27 ET  Reply   Untrace   Trace   Private Reply  

the only way the plane could move forward is if the wheels turned fasted than the conveyor belt

nope, the wheels and conveyer belt are none issues

wakeup  posted on  2005-12-02   20:01:15 ET  Reply   Untrace   Trace   Private Reply  

nope, the wheels and conveyer belt are none issues

For the sake if this question, yes they are. It is mathematically impossible that the plane could move forward while in contact with cvbelt, the landing gear wheels are in fact solidly attached to the airframe, and are only moving forward to compensate for the rearward movement of conveyor belt. It says so right at the top! :)

Dakmar  posted on  2005-12-02   20:11:00 ET  Reply   Untrace   Trace   Private Reply  

For the sake if this question, yes they are. It is mathematically impossible that the plane could move forward while in contact with cvbelt, the landing gear wheels are in fact solidly attached to the airframe, and are only moving forward to compensate for the rearward movement of conveyor belt. It says so right at the top! :)

in a car, true

in a plane, nope

wakeup  posted on  2005-12-02   21:53:23 ET  Reply   Untrace   Trace   Private Reply  

in a plane, nope

then where are the extra spins of the landing gear wheels going, mister smartguy? Assuming solid construction and a magical fuel supply that plane is stuck on that conveyor belt forever.

Dakmar  posted on  2005-12-02   21:59:33 ET  Reply   Untrace   Trace   Private Reply  

then where are the extra spins of the landing gear wheels going, mister smartguy? Assuming solid construction and a magical fuel supply that plane is stuck on that conveyor belt forever.

The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

As the plane reaches 1 mph relative to the ground (not the belt), let's say its tires are spinning at 2 mph (the outside edge). The conveyor belt must therefore be spinning at 2 mph (the upper edge) to match. All conditions are met.

As the plane reaches 100 mph relative to the ground (not the belt), let's say its tires are spinning at 200 mph (the outside edge). The conveyor belt must therefore be spinning at 200 mph (the upper edge) to match. All conditions are met.

Why the need for infinite rotational speed of the tires? There is no need whatsoever for the speed the tires spin at to be related in any way to the speed the plane is moving. You've got a skateboard sitting on a conveyor belt. The speed that the wheels spin has nothing to do with the speed of the skateboard. Further, the skateboard has a fan sitting on top of it. Nothing is going to counter that force. Picture the inside of the skateboard's ball bearing axles. Picture me accidentally stepping on a skateboard without knowing it is there. You may before it is over, lol. Perhaps you see me at the top of a tall flight of stairs. Now picture how frictionless I picture those wheels once I've reached the bottom. Ouch! Can't say I blame you but I'm not changing my opinion on this.

markm0722  posted on  2005-12-02   22:51:01 ET  Reply   Untrace   Trace   Private Reply  

As the plane reaches 1 mph relative to the ground

It wouldn't. It couldn't:

Thread Rule #1: The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

Dakmar  posted on  2005-12-02   22:57:25 ET  Reply   Untrace   Trace   Private Reply  

As the plane reaches 1 mph relative to the ground

It wouldn't. It couldn't:

Thread Rule #1: The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

There is nothing stopping the plane from moving forward. As the wheels begin to turn, the conveyor belt "exactly" matches the speed of the wheels, whatever that speed ends up being. There is no inconsistency here.

markm0722  posted on  2005-12-02   23:03:24 ET  Reply   Untrace   Trace   Private Reply  

The wheels could never move from their their original stop position, any movement they make is counteracted by The Belt. That's the rules, man.

Dakmar  posted on  2005-12-02   23:12:32 ET  Reply   Untrace   Trace   Private Reply  

The wheels could never move from their their original stop position, any movement they make is counteracted by The Belt. That's the rules, man.

That's not what it says.

The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

If the wheels are rotating clockwise, then the conveyor belt is rotating counterclockwise. If the outside edge of a wheel is turning at 10 mph, then the outside edge of the conveyor belt is merely turning at 10 mph in the opposite direction. Therefore, there is no slippage between the wheels and the belt at any given time.

markm0722  posted on  2005-12-02   23:38:15 ET  Reply   Untrace   Trace   Private Reply  

but from a dead stop, the plane would never be allowed to start moving.

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. There is no wind.

Can the plane take off?

Does "sat on the beginning of" mean, for our purposes, in a state of rest before we started experimenting on it?

Dakmar  posted on  2005-12-02   23:45:20 ET  Reply   Untrace   Trace   Private Reply  

Imagine a plane is sat...

In fact, we should know we are in trouble just reading that far. At one point or another some major assumptions are going to be needed, lol.

(No offense intended Jhoffa_, I know you got it from Elsewhere.)

markm0722  posted on  2005-12-03   1:20:42 ET  Reply   Untrace   Trace   Private Reply  

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