This little puzzle befuddled me for quite some time and whilst it didn’t quite keep me awake at night I did go to bed for a few days thinking about it. This is the 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?
Stop reading here. Go and think about it (unless you are a physics master) as after this I am going to go through my lines of thinking and give the answer.
My conclusions:
1. The plane remains in the same position as any forward propulsion achieved by its (say) jet engines is immediately negated by the opposing motion of the runway. The plane therefore remains stationary. Whilst the forward motion of the plane is achieved by the force of the jet on the air, not on the ground, until the plane has gained sufficient velocity to gain lift from moving through the air it is inextricably linked to the ground by gravity and thus the inverse speed of the runway drags the plane backwards meaning it can’t make any forward progress and thus gain any lift. This was my initial conclusion that I stuck with for quite some time but it doesn’t stand up to close scrutiny as, unlike a car, the forward motion of the plane is created by the force of the jet on the air not by the wheels on the ground.
2. The plane remains stationary but rises. Ok so I never thought this one would apply but am going through it anyway. This can’t work because the plane requires the lift of the air passing its wings in order to become airborne. No amount of thrusting from the jets will make it rise if the plane can’t make any forward progress. It would be like having a plane on a stick stuck in the ground (I like the imagery of that). Besides if this did work surely it would be employed on aircraft carriers and other stuff with short runway constraints.
3. The plane proceeds down the runway regardless of what the runway itself is doing. As the jets are acting against the air not the ground the motion of the surface it is on is largely irrelevant. This answer defies my land and car based thinking but really is the only feasible answer. As the engines are thrusting against the air what the plane is doing relative to the air is the important bit. What the ground is doing is largely irrelevant, think for example of a jet plane taking backwards off an aircraft carrier which is moving forward really quickly.
I have to confess this is a bit of a rob from this post (which itself was a rob from this post, but then the internet is like that) and the subsequently confirmed answer here (at least I got the right answer even if it did take me a few days and a bit of coaxing). But it got me thinking and I thought it was sufficiently engaging to warrant blogging.
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5 comments:
Yes, you seem to be following my initial line of thinking. I was sure the plane wouldn't take off for ages but have since come around. Sure, the plane wouldn't take off if it was making no forward progress as it could generate no lift (see my numbered paragrph 2) but that implies that the runway's motion was particualrly relevant to the planes motion.
I don't think it would take off either. The plane is not moveing. the wheels might be but the actual plane isn't going anywhere. All the energy the plane is putting into moving forwards is being negated by the runway moving in the opposite direction. This means there is no energy left to move the plane upwards.
(please excuse my terrible use of terminology and just read the general meaning of the explaination rather than the words.)
Nick I have been having issues with your RSS recently. whatsup?
Terrible terminology is exscused, I doubt mine is exactly flawless!
The thing is that the force of the jet egines is acting against the air, not the ground. The energy from the jets is going into the air hence propulsion whilst the energy of the moving runway is going into the ground and probably a huge amount of friction in the wheels.
And I don't know what is up with the RSS feed. I have trouble too on mine, Bngs and on girl with a one-track mind which are all on blogger so suspect its their fault.
Yes, the plane clearly has to be moving through the air to generate lift. Also yes, if the jet was switched off the plane would go nowhere whether the conveyor runway was off or moving backwards.
But its not like a car which would also remain stationary on this conveyor which achieves forward propolusion from the ground through the wheels.
The jets are pushing at the air (for all intents and purposes independent of the ground) whilst the conveyor is pulling backwards on the ground. I think i can only relate this again to my (majorly fast and responsive) aircraft carrier with the jet plane taking off backwards (or even sideways).
Yes but surely the moving ground will compensate for any forward propulsion gained from pushing against the air.
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