Why the plane takes off in laymans (my) terms

Originally posted by mwmiller
True, it is different except for one point, which is the only one that is important.

The plane moves forward, which means that it will take off or fly if it moves fast enough. Causing the runway to move in the opposite direction will not change what that plane is doing.

If it's slippery, you're absolutely right.

If the wheels turn in the normal way - their speed must be the same as that of the belt - to fulfill the conditions of the original problem - it won't then move relative to an external oberver.

If you want to change the problem, fine but as it stands......

I changed the problem in an effort to show that the wheels and the surface do not change how the plane moves.

If you remove the slippery surface and go back to a regular one, the plane will move forward exactly as it did before, because it is moving itself through the air.

Originally posted by mwmiller
I changed the problem in an effort to show that the wheels and the surface do not change how the plane moves.

If you remove the slippery surface and go back to a regular one, the plane will move forward exactly as it did before, because it is moving itself through the air.

But it's supported by the belt and must move relative to it.

This business of driven or free wheels is not relevant.

Those that insist it is are not reading the problem correctly.

The plane is NOT flying, it is moving on a surface which is itself moving.

To advance in absolute terms, the wheels must rotate at a higher speed that that of the belt - as this is precluded in the terms of the problem, it doesn't happen.

Originally posted by sugiezd
But it's supported by the belt and must move relative to it.

This business of driven or free wheels is not relevant.

Those that insist it is are not reading the problem correctly.

The plane is NOT flying, it is moving on a surface which is itself moving.

To advance in absolute terms, the wheels must rotate at a higher speed that that of the belt - as this is precluded in the terms of the problem, it doesn't happen.

The only thing moving the plane is its' engine as it pulls or pushes itself through the air. This is how it moves, regardless of whether it is flying or not. Once it is moving itself through the air fast enough to develop the required lift, it flies.

Originally posted by mwmiller
The only thing moving the plane is its' engine as it pulls or pushes itself through the air. This is how it moves, regardless of whether it is flying or not. Once it is moving itself through the air fast enough to develop the required lift, it flies.

Very true, except that in this case it is not moving relative to the air (only the belt) and so doesn't generate lift.

Originally posted by sugiezd
Very true, except that in this case it is not moving relative to the air (only the belt) and so doesn't generate lift.

If it is moving in one direction it is moving relative to the air. It is moving relative to the belt as well in the original problem, but if that plane is moving it has to be moving relative to the air and that is the only movement that is important.

Well, I'm off to work again, and I think I have said about all I can to explain my reasoning in this puzzler.

Originally posted by mwmiller
If it is moving in one direction it is moving relative to the air. It is moving relative to the belt as well in the original problem, but if that plane is moving it has to be moving relative to the air and that is the only movement that is important.

Well, I'm off to work again, and I think I have said about all I can to explain my reasoning in this puzzler.

You're actually starting to see it.

If the plane is moving only relative to the belt (the wheels and belt moving in opposite directions at the same speed), then the velocity of the air moving over the wings is zero.

Originally posted by sugiezd
You're actually starting to see it.

If the plane is moving only relative to the belt (the wheels and belt moving in opposite directions at the same speed), then the velocity of the air moving over the wings is zero.

No, actually I'm not.

My position has not changed at all in this thread. I state that if the plane moves in one direction and the belt moves in the opposite direction, the plane will continue to move and the belt will not stop it from moving or from being able to fly.

That is my opinion and I will now leave it at that.

Originally posted by sugiezd
If it's slippery, you're absolutely right.

If the wheels turn in the normal way - their speed must be the same as that of the belt - to fulfill the conditions of the original problem - it won't then move relative to an external oberver.

If you want to change the problem, fine but as it stands......

"A plane is standing on a runway that can move (some sort of treadmill). The plane moves in one direction, while the conveyer moves at the same speed but in the opposite direction. Can the plane take off?"



You're interpreting this question in a bizarre manner.
Where in the original question does it say the wheels have to turn at the same speed as the conveyor?

Keep in mind it's the PLANE we are interested in i.e. "The PLANE moves in one direction...".

You seem to think measuring the speed the wheels are rotating is the only valid way to measure the plane's speed. That's complete BS. The only thing that measures is the plane's speed relative to the conveyor.

Using a GPS system or say a radar gun measures the absolute speed of the plane i.e. the speed relative to a stationary observer. This is the interpretation the vast majority of people in the discussion have taken and a far more valid one IMO.

Do you agree that if we make the frame of reference a stationary observer the plane will take off?

Originally posted by Marsan
"A plane is standing on a runway that can move (some sort of treadmill). The plane moves in one direction, while the conveyer moves at the same speed but in the opposite direction. Can the plane take off?"



You're interpreting this question in a bizarre manner.
Where in the original question does it say the wheels have to turn at the same speed as th ...[text shortened]... e that if we make the frame of reference a stationary observer the plane will take off?

Yes, if that it the frame of reference.

Unfortunately, it isn't - where does it say so in the problem?

I am at a total loss as to why you cannot see that the speed of the plane is relative to the surface of the belt - they are the only two things mentioned in the problem - again, where does it say anything about speed relative to an external observer???

Just as a side issue, it doesn't say anything about the plane accelerating either !

Do you think that you could ask one of your buddies to take a photo of a car on a rolling road. Now, imagine that it's running with a speedo indication of 50 kph - please show the braking forces that stop it driving off the end.

This will be my last post in this thread.

The original scenario:
"A plane is standing on a runway that can move (some sort of treadmill). The plane moves in one direction, while the conveyer moves at the same speed but in the opposite direction. Can the plane take off?"

My answer:
Yes, the plane can take off.

The only information you need to know from the original scenario is given.
It clearly states that the plane moves in one direction.
Nothing else stated in the scenario can alter that.

You also have to understand how a plane works. I can not state that the plane WILL take off unless more information is given, but the plane CAN take off.

Originally posted by mwmiller
This will be my last post in this thread.

The original scenario:
"A plane is standing on a runway that can move (some sort of treadmill). The plane moves in one direction, while the conveyer moves at the same speed but in the opposite direction. Can the plane take off?"

My answer:
Yes, the plane can take off.

The only information you need to kno ...[text shortened]... ate that the plane WILL take off unless more information is given, but the plane CAN take off.

With the information given in the problem, and the obvious inference made that the plane moves relative to the belt surface, the plane cannot move forward.

If you and to add that it is moving relative to a fixed observer and is accelerating, then it will take off - otherwise, no.

You keep saying that you're going to leave.

So, leave the thread.

Originally posted by sugiezd
the obvious inference made that the plane moves relative to the belt surface, the plane cannot move forward.

How can you say it is obvious when only yourself interprets it that way.
Obvious: easily perceived or understood

You assume the planes speed is relative to the belt and measured by it's wheels, despite none of this being mentioned in the problem, because you say it is in context.

In the context of a plane taking off (which is mentioned in the problem) speed means the speed of the plane relative to the surrounding air.
That is all the context you should need.

Originally posted by aging blitzer
How can you say it is obvious when only yourself interprets it that way.
Obvious: easily perceived or understood

You assume the planes speed is relative to the belt and measured by it's wheels, despite none of this being mentioned in the problem, because you say it is in context.

In the context of a plane taking off (which is mentioned in the pr ...[text shortened]... e speed of the plane relative to the surrounding air.
That is all the context you should need.

Nice try but you go two steps to make the relationship - I take only one.

To also forget that acceleration is not mentioned.

There is no difference in how a plane propells itself whether it is flying or taxiing. I think we are all agreed that a plane has to travel fast enough through the air to create lift in order to fly. So just for a minute lets forget that.

Following this argument on, the only difference betweeen a plane flying 10cm off the ground and taxiing (assume this is being done at same speed) is that some part of the plane is in contact with the ground - this could be skids, wheels or even floats.

Therefore if the friction of the piece of the plane on the ground can be overcome by the "pulling/pushing" of the plane through the air then the plane will move forward irrespective of what the ground(treadmill) is doing.

If this is happening at a speed which allows the plane to take off it will.

I was ending my post here but will try and take it one step further -

If this plane travelling at say 100knots is flying 10cm off the ground, and the ground is moving at 100knots in the opposite direction, when the plane lands would it be stationary to the observer - if it would I imagine that air craft carriers would have worked this out rather than a hook and a bloody great big bit of wire!