Plane on a conveyor belt

Winny_Pooh

The first answer on the thread is correct, nothing more to see here. 

monquixote

The question relies on a load of assumptions that don't make sense and any practical apparatus would result in the plane taking off however there is a way to fulfil the criteria and still have the plane take off.

There is a limit to how much frictional force the tyres are going to generate against the runway so eventually the tyres will start sliding against the conveyor through ablation etc. This means the rotational speeds can stay the same, but the plane can move.

Another possibility is that as the conveyor accelerates it will start pulling air along with it which will eventually create lift

Stratavarious

It would overcome the wheel friction easily and move along the conveyor till it reached take off speed. 

Only issue might be wheels failing. 

stickyfiddle

Basically it's a poorly worded question, as if the plane moves forward at all, a theoretical powered conveyor instantly starts moving in the other direction and instantly you get to a rolling speed of infinity mph and the plane still theoretically not moving forwards, which is nonsense. 

In reality, if the conveyor isn't powered and the plane isn't fixed in position by some other means then the plane will move forward and if the plane can move 

dazzajl

What makes this so entertaining is that it’s not so much a physics question as a social experiment. How do people react being told they’re wrong when it feels factual but there’s no way to prove the answer? 

Whenever this comes up on a forum, there simply isn’t enough popcorn. 

Bridgehouse

This question always really frustrates me every single time it pops up.

At no point has anyone ever debated or answered my biggest burning question - how did the plane get onto the conveyor belt in the first place?

viz

monquixote said:

The question relies on a load of assumptions that don't make sense and any practical apparatus would result in the plane taking off however there is a way to fulfil the criteria and still have the plane take off.

There is a limit to how much frictional force the tyres are going to generate against the runway so eventually the tyres will start sliding against the conveyor through ablation etc. This means the rotational speeds can stay the same, but the plane can move.

Another possibility is that as the conveyor accelerates it will start pulling air along with it which will eventually create lift

Stratavarious said:

It would overcome the wheel friction easily and move along the conveyor till it reached take off speed. 

Only issue might be wheels failing. 

stickyfiddle said:

Basically it's a poorly worded question, as if the plane moves forward at all, a theoretical powered conveyor instantly starts moving in the other direction and instantly you get to a rolling speed of infinity mph and the plane still theoretically not moving forwards, which is nonsense. 

In reality, if the conveyor isn't powered and the plane isn't fixed in position by some other means then the plane will move forward and if the plane can move 

I can live with these answers

Physics meets Engineering and shakes hands. Well, bumps elbows anyway. 

Cirrus

viz said:

So long as the question is rephrased as “Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to try (unsuccessfully of course!) exactly to match the speed of the wheels, moving in the opposite direction. Can the plane take off?”

Even that has multiple interpretations - depending on how you measure the speed of the wheels.

If the centre of the wheels - the axis on which they rotate - is moving at 25 MPH in one direction, and the belt is moving at 25 MPH in the opposite direction, then the outer edge of the wheels is moving at 50mph. And that's fine, that could happen.

And you could say in that case that the wheels are moving in space at 25mph, all the conditions of the question are met, and the plane takes off (assuming the tyres don't blow from moving at twice the ground speed across the conveyor belt).

Or you could define the wheels as moving at 50mph, in which case the conditions of the question aren't met, and you're faced with the interpretation that's impossible because the conveyor belt quickly speeds up to infinity, chasing the impossible goal of stopping the plane moving forwards by acting on freely rotating parts of it.