It's cool. I was wrong at first too.

 

This is why I originally said that the question did not supply enough information. It never specifically says if the treadmill is moving fast enough that the plane has no relative motion compared to the earth, or if the treadmill is simply moving at the same rate as the plane.

If the plane is moving L to R at 750mph compared to the earth and the treadmill moves R to L at 750mph compared to the earth, yes the plane will fly. As others have stated, the plane will be moving relative to the earth and air and lift will allow the plane to fly. (The whells will simply have to travel 1500mph)

However, if the treadmill moves fast enough for the friction in the wheels/landing gear to exert an equal force to that of the engines thrust the plane will not be moving in relationship to the earth/air and it will not fly.

 

That is what I was originally thinking, but the thing is, no matter the speed of the belt, the wheels can spin just as fast, keeping the plane still. It is the added thrust from the engines (that is NOT a force on the ground) that pushes the plane forward.

 

I don't understand why people are focused on the wheels and the conveyor. For every action there must be an equal and opposite reaction.The action is the thrust of the jets. The conveyor is effecting the wheels and has zero effect on that thrust. So it will in turn have zero effect on the movement of the plane. The plane will fly.

 

HEAD....HURTS....NOW.....


Where's Sigma?? Threw a grenade in the room and left!!

Will this ever be settled???

 

Unless the conveyor is moving at a speed fast enough that the friction in the wheels resulted in a force equal to that of the thrust.

I realize that this is improbable, but it is possible. Unless you can create a bearing with no friction, which if you can I'll buy as many as you can make.

 

The "speed" comes from the thrust of the motors, the speed of the belt is equal and opposite of the motors (thrust). plane is therefore going 0.00mph with no thrust. plane can't take off. if you take the wheels of the plane and sit it on hte landing gear assymbly, the plane is still going hte same speeed (just with more friction and heat) as the belt. so you are still going 0mph and no lift. so the plane wont fly

 

It's an easy experiment if you have a treadmill handy. Take the toy car someone mentioned above, turn the treadmill on, face the car forwards on the treadmill. Now see if you can move the car in the direction it's facing, against the flow of the treadmill. I'll bet you can. You are basically taking the place of the engine's on an airplane. Now it probably looks funny because the wheels are spinning in the opposite direction of travel, but the car moves just fine. The wheels on the plane are still doing what they were designed to do, keep the plane off of the ground to reduce friction between the runway and the plane, irregardless of which they are spinning. The conveyor isn't acting on the plane, it's acting on the wheels.

 

nope, the thrust from tje motors on the plane are making the plane go the same speed as the conveyor, just in the opposite direction.

 

I'll read everyone answer after I post .

No, it cannot fly. The conveyor is not allowing the aircraft to move; aircraft velocity zero (0) = zero (0) lift. Maybe try a Harrier next time .

 

are made in the stated problem..

Quoted problem---- "A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction)."

Assumption: the conveyor control system tracks the plane's ground speed and matches it in the opposite direction. No matter how much thrust the plane's engines apply, the forward ground speed will be countered by the reverse speed control system on the conveyor belt. Thusly, no forward progress of the plane.

Conclusion: The plane cannot move in relation to the ground. it will not fly.

I haven't changed my mind.

 

AAAAHHH! I love this!

Think about this. I have a roller skate on an electric tread mill. I'm holding on to it but I have to hold on to it because if I let go, it will fall off. Why will it fall off? Friction. So... friction in this case is going to require a certain amount of energy to keep the vehicle at rest.

Friction does not greatly increase with speed. It does increase, we should assume, but not enough to sap the planes engines to the point they can't move the plane forward. Back to the car.

You hold the car at rest while the treadmill is moving at 1 mph. Now you increase the speed of the treadmill to 2 mph. Double. The resistance doesn't double, does it? It doesn't become noticeably more difficult to overcome the friction does it? Now gradually start moving the vehicle forward while you increase the speed of the treadmill... x3 x4 x5... it won't be very hard to move car forward and it won't get much harder as you do.

What must be accomplished here is simply overcoming friction. Friction in the tires, friction in the wheel bearing assemblies, and nothing more than that.

EDIT: More pertinent info came to mind after reading EnviroCon's last post.

Let's say the plane needs to go 100 mph to take off. Let's say the conveyer is going 100mph in the reverse direction. The wheels will be spinning at 200 mph when the plane is traveling at 100 mph in relation to the earth. As long as the plane can overcome the friction curve that is being set up at the wheels, there's no problem.

 

The wheels spin. The belt's movement applies a force on the wheels which rotates the wheels. The engines thrust works with air, not the wheels, not the ground. The thrust pushes the plane forward.

The wheels are there to reduce friction. With the belt moving, all that happens is the wheels spin twice as fast for any given speed. It's the thrust that pushes the plane forward.

Think of it like a plane taking off on a frozen lake. Very little friction, but the wheels don't need to grip the ground for the engines to propel it forward.

If you think the thrust won't make the plane fly in this situation, then how would a plane take off in a normal situation? It's not the wheels that bring the plane up to speed for liftoff.

 

Exactly.

 

Ya got me!
Must rest now. Dream about conveyors large enough to park a 747 on.