Long rod question

Le_Petit_Lapin said:
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).

Will the plane be able to take off?
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That's an odd version :|

In your case the plane would actually take off. In the more common scenario (where there is no movement of the aircraft relative to its surroundings) it would not.

moon man said:
im sure its something you all wonder about but if i have a metal rod 10 miles long and pull it at say 1 metre/second how long before the other end moves assuming its frictionless and wont stretch, ok its not instant or speed of light so how fast is that motion transfered :)
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If it doesn't stretch then the ends will move simultaneously, simple as. If this doesn't seem realistic to you, it's probably because it's actually impossible for a material 'not to stretch' and so the above model is inaccurate. Any calculation assuming stretching is involved would take into account the 'Young's Modulus' of the material involved - a measure of stiffness [insert joke here ;)].

Nevertheless, since the OP has stated that in his MODEL of the situation, there is no stretching, then yes...both ends will move simultaneously.
 
Grrrrr said:
Ok i might have to eat my pride and say i might be coming round to what you are saying...

If a car had wings and you stuck it on a treadmill it wouldn't take off, there would be no movement relative to the air. The comparable experiment with plane would be sticking it in a wind tunnel with the wind blowing from behind, in this case also there would be no movement relative to the air.

I think
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bingo :)
 
argh...but it isn't a simple question, it does take thinking through clearly and I finally found a good answer here:

http://www.boingboing.net/2006/12/11/airplanetreadmill-pr.html

"The trick to solving these problems is take them in steps. Step number one is turning on the belt with the airplane on it. There may be a bit of initial slippage due do the wheel's spinning before they begin to grab fully (relative to the friction between belt and wheel) and the plane moves backwards. Step two is to bring the plane to a speed consistent with that of the belt. That is to say; make it stop. How much force is require to do this? Very little, only enough to overcome the friction between belt and wheel. Picture it this way; You are on a treadmill wearing roller skates. To stop you from moving backwards all I have to do is put an tiny amount of pressure on your back (a pinky finger's worth would suffice), I don't need to apply 150 founds of pressure because you weigh 150lbs. I need only defeat friction. At this point any additional energy applied would propel the plane forward, and the plane's engine would have plenty of power to spare. It would need a longer runway, but it would not be an exceptional amount"

What I'm wondering now is will the treadmill only slow down the plane a tiny amount if it is turned on before the plane starts its engines or after (i.e does it depend on which starts moving first?). And will it need a tiny bit more power to take off in either case, or does that get negated too once the plane starts moving? I'm too tired to work this out for myself right now.

But the plane will take off, because the treadmill will have very minimal effect on the plane, the exact extent of which will be determined by the bearings in the wheels (and the weight of the plane I guess).

I prefer the rod question though, I have wondered about it myself, people are quite correct a perfectly rigid rod would....violate the laws of physics...and we can be sure about this because it would transmit information instantaneously which can't be done except apparently at the quantum level.
 
Grrrrr said:
Do you all agree that air needs to pass over the wings to cause the plane to take off?

Let's do this step by step. Yes or no to the above
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Yes, air needs to move over the wings to generate lift.

Step 1: Plane is stationary.

Step 2: Somebody flicks the switch on the 500mph treadmill and the plane rolls back slowly as the friction in the wheels take over but the wheels are spinning at 500mph.

Step 3: Pilot hits the burners, the jets light up blue and it shoots off forwards as normal and takes off at ~200mph as it normally would.

Speed at the wings = 200mph. Speed at the wheels = 700mph and very hot wheel bearings :p

I can't believe I'm actually having this conversation :D
 
(Talking about the original question now)

Electrons moving doesn't cause electricity, the charge they carry does. Electrons actually move quite slowly in an electrical circuit
 
Grrrrr said:
Ok i might have to eat my pride and say i might be coming round to what you are saying...

If a car had wings and you stuck it on a treadmill it wouldn't take off, there would be no movement relative to the air. The comparable experiment with plane would be sticking it in a wind tunnel with the wind blowing from behind, in this case also there would be no movement relative to the air.

I think
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Actually the air tunnel thing is quite enlightening here because its almost the opposite of what we are talking here. Make the plane unable to move and blow air at it from the front so you have movement relative to the air mass and thus you have lift from the wings and up she goes. (Except it doesn't because it's tied down in a wind tunnel)
 
EffBee said:
It's all relative. The blades need to be 2000 rpm relative to the air mass its sitting in. If the body is spinning one way at 2000 rpm, and the blades are spinning the other way at 2000 rpm, relative to the air mass the blades are not moving, though they are moving at 2000 rpm relative to the helicopter body. If the blades were 4000 rpm in the opposite direction to the body then you would get lift off.
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Sorry, that's what I meant. If it was just 2000, the blades would be stationary.
 
Jonny69 said:
Yes, air needs to move over the wings to generate lift.

Step 1: Plane is stationary.

Step 2: Somebody flicks the switch on the 500mph treadmill and the plane rolls back slowly as the friction in the wheels take over but the wheels are spinning at 500mph.

Step 3: Pilot hits the burners, the jets light up blue and it shoots off forwards as normal and takes off at ~200mph as it normally would.

Speed at the wings = 200mph. Speed at the wheels = 700mph and very hot wheel bearings :p

I can't believe I'm actually having this conversation :D
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The way i got my head around was by thinking about a plane dropped from a stationary point high in the altitude. In this situation it's also still (horizontally) relative to the air, but i could imagine the plane moving forward in this situation.

OK everyone, i think i'm probably definitely on your side now. Consider my humble pie eaten
 
Im pretty sure that if you had a rod from earth to the moon and a torch, the rod would move instantly or get there quicker than the light.
 
tim80bwi said:
That's an odd version :|

In your case the plane would actually take off. In the more common scenario (where there is no movement of the aircraft relative to its surroundings) it would not.
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AAARGH! It would have to be in a vacuum to not move under its own power!
 
Le_Petit_Lapin said:
Its the original version. :confused:

And in any scenario involving a treadmill and a plane it takes off.
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Except the one where the treadmill moves in such a way as to keep the relative speed of the plane and surrounding air at zero.

Imagine you strap on a pair of wings nd run across your local park at 70 miles per hour - you would generat lift and take off. Now run on a treadmill at your local gym instead and you will be running just as fast but not relative to the gym and everything in it (including the air) so no lift and no takeoff.
 
EffBee said:
Actually the air tunnel thing is quite enlightening here because its almost the opposite of what we are talking here. Make the plane unable to move and blow air at it from the front so you have movement relative to the air mass and thus you have lift from the wings and up she goes. (Except it doesn't because it's tied down in a wind tunnel)
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Blow air from the BACK and the plane stays on the ground.

Blow air from the front and you're only giving it a hand in this case, it'll go up, as its getting air over its wings, but wont move its position on the ground.

EffBee said:
Except the one where the treadmill moves in such a way as to keep the relative speed of the plane and surrounding air at zero. Imagine you strap on a pair of wings nd run across your local park at 70 miles per hour - you would generat lift and take off. Now run on a treadmill at your local gym instead and you will be running just as fast but not relative to the gym and everything in it (including the air) so no lift and no takeoff.
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:facepalm:

I'm unfortunately not a plane, and don't generate motion like a plane would.
 
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Think of a car on a treadmill driving forwards at the same speed as the treadmill is moving backwards. It will not go anywhere.

Now take that car, stick it in neutral so the wheels are free wheeling and therefore are providing no forward force. Now stick a big propeller and wings on the car. Now the propeller is providing the thrust. As the wheel are in neutral, they will just spin with the treadmill, with the prop pushing the car forwards. It doesn't matter how fast the treadmill moves, as the wheels are free wheeling they are providing no backwards force (ignoring the small friction they create) and so the plane will move relative to the air and thus create lift.

I'm waiting for it to click :p

EDIT: Damn fast moving threads.
 
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