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[5150]

I'm actually regretting reading this thread, someone pass me some paracetamols, my head is killing me!
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Mars Red '78 Euro 924 n/a
http://www.cardomain.com/id/5150_uk

Graphite? Grey Metallic '85 (late model) 944 2.5

There are two kinds of pedestrians: the quick and the dead.

[Mike924]

[chris24]

sorry mike924, but on page 2 i said the question was a paradox and it really is! it states that the belt will 'counteract' the motion of the plane. definition of counteract by dictionary.com:

To oppose and mitigate the effects of by contrary action...

So the question doesn't make sense. if the belt is moving at only 100mph south and the plane forcing ahead at 100mph north (another weird statement) and yet we are expected to accept that this equally and oppositely causes no plane motion as it is 'counteracted'.

The whole point of wording the question like that must be to spilt peoples opinions down the middle.

pretty successful i'd say!
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1983 - 924 (185K miles) - not mint
1985 - 924 (148K miles) - mint
1990 - 944S2 cab (52K miles)

[Mike924]

Attacking the question is a cop-out.

The trick, IMHO, is, regardless of the actuall wording of the puzzle, to construct the 'thought experiment' in such a way that none of the laws of physics are broken.

The fallacy comes when the runway 'counteracts' the motion of the plane. The laws of physics say it can't. ...And that's the solution to the question.

Maybe we're agreeing, Chris?
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1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd

'There is no substitute for a little grease under your fingernails.' - Chrenan, 924board.org

[timstar92404]

woops
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78 924 sold.
85.5 944

[timstar92404]

the wheelse would not be stopped because the threadmill would be moving the wheels in the same direction as it does when the plane moves forward.

so the plane would fly but if the threadmill is going at 100 mph and the plane is going at 100 mph the wheels would just be spinning at 200 mph and the plane would be going at 100 mph.

so yeah the wheels would just be overheating.
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78 924 sold.
85.5 944

[Carris]

Timstar I think you are wrong. The wheels of the plane would be spinning but the threadmill would be manufacturing thread while this is happening.

The real question is, if the threadmill can produce thread at a speed great enough to match the thrust of the engine would the plane take off and would the pilots trouser seams cope!!?
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Chris

[chris24]

This thread is fantastic!
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1983 - 924 (185K miles) - not mint
1985 - 924 (148K miles) - mint
1990 - 944S2 cab (52K miles)

[kaffine]

I think the real question is at what point do the tires on the airplane blow up? If the tires blow up from overspeed before the airplane gets going fast enough to liftoff it is going to cause one hell of a wreck. However if the tires can withstand double the takeoff speed then it will take off without a problem.
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80 924
80 931

The best desciption of an atom boils down to something unknown is doing we don't know what.
Sir Arthur Eddington

[alxch1n15]

ok, plane moves -----> from 0 mph, trying to gain speed.
tread mill moves <----- from 0 mph, gaining to mph

its been said by mike924 that the movement is generated at the wings, not at the wheels, which is correct. if the wheels truly are independent moving, then they should techincally spin both ways, so the treadmill will, because of friction, will cause the tires to spin backwards. however, theroretically, this shouldnt cause the plain to stay put, because the plane is generating its force at the turbines, on the wings. but if the plane is moving forward, wheels spinning backwards, that means that the plane is stuck where it is because the wheels are providing enough friction, in the opposite direction to stop the plane from moving (given invinsible rubber, of course). another thing to account for is that turbines work by gaining speed, and as they gain speed, they gain horsepower. you have a high potential for horsepower/speed in turbines. if you're not moving, you cannot generate enough horsepower, which will not let you generate enough thrust to gain lift. the 0 factor for movement, coupled with the 0 factor for lift, = 0, no flight. however, if you removed the wheels, which is what would happen, then you can gain flight. the aircraft carrier example is a good one, they do slingshot the planes because they need that extra amount of force to gain enough lift to fly. im almost certain that they launch the planes from the front of the plane, in the direction that the carrier is moving, so all momentum and force is in the same direction.

correct me if im wrong
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'81 924 n/a

[Paul]

[Darin81T]

Turbines don't need airspeed to generate thrust. They generate plenty of static thrust. Infact, the inlets on most jet engines slow the air down before entering the compressor.

Here's the equation for thrust for a turbine engine(or for a rocket engine)

F=m_dot*ue + (Pe-Pa)*Ae

m_dot is the mass flow rate [mass/second]
ue is the exhaust velocity
Pe is the exit pressure
Pa is the ambient pressure
Ae is the exit area

This equation comes from the fluid momentum equation, which can easily be derived by anyone that knows classical physics, vector algebra, and multivariable calculus; kids stuff. However, the thrust equation is much, much simpler and anyone who knows algebra can understand it.

The first term, m_dot*ue, is a momentum term
the second term, (Pe-Pe)*Ae, is a pressure force (if the exhaust stream is subsonic then Pe=Pa and this term goes away)

Now if you want to get more technical we could include the bypass air, since this jet engine is almost certainly a turbofan. The equation remains basically the same, but includes terms for main stream and bypass air; the physics still remain the same.

Just so you know, a jet engine doesn't operate by pushing AGAINST anything, as many people have been saying. It works by exchanging momentum with the fuel that it accelerates out the exhaust nozzle. If a jet engine had its own oxygen source, it would be called a rocket and it would operate just fine w/o anything to push against in the vacuum of space. This is what the first term in the thrust equation tells us.

Aircraft carriers have absolutely nothing to do with this topic. An aircraft carrier is fundamentally no different that any other runway, it just happens to be a lot shorter and could be anywhere in the world within a few months notice.

Seriously, though... somebody already posted an article that is about this riddle. It was written by someone who knows what he's talking about. I think we should consider it the final word on this topic.

Over and out,
Darin Flynn

[jpab924]

NO MOTION, NO LIFT

Suppose we had an airplane which could take off on a windless day at 100 mph (lift off airspeed is 100 mph). Now suppose we had a day in which the wind was blowing 20 mph towards the West. If the airplane takes off going East, it experiences a 20 mph headwind (wind in your face). Since a positive velocity is defined to be toward the tail, a headwind will be a positive wind speed. While the plane is sitting still on the runway, it has a ground speed of 0 and an airspeed of 20 mph.

Wind speed (20) = Airspeed (20) - Ground Speed (0)

At lift off, the airspeed is 100 mph, the wind speed is 20 mph and the ground speed will be 80 mph

Wind speed (20) = Airspeed (100) - Ground Speed (80)

If the plane took off to the West it would have a 20 mph tail wind (wind at your back). This gives a negative wind speed. At lift off, the airspeed is still 100 mph, the wind speed is -20 mph and the ground speed will now be 120 mph.

Wind speed (-20) = Airspeed (100) - Ground Speed (120)

So the aircraft will have to travel faster (and farther) along the ground to achieve lift off conditions with the wind at it's back.

Guys, Ozzie hit the answer on the head, in the first post.

Planes would be dropping out of the sky without any forward motion, which is the #1 ingredient in creating lift!

[jamez]

seriously, read this article. It was posted by someone earlier on this thread, but I think some of you missed it.

http://www.avweb.com/news/columns/191034-1.html

[jpab924]

Guess I`ll have to submit this riddle to these guys--------->

http://dsc.discovery.com/fansites/mythbusters/mythbusters.html