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A question of physics.....

a simple little question, or is it?

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Martin Harris14/10/2016 01:32:26
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Posted by Gary Manuel on 14/10/2016 01:17:49:

It's a question of logic, common sense and physics.

Ah, but there's the rub - the question does not take the laws of physics into account. If the runway is moving at 150 mph after accelerating from rest with the aircraft then the aircraft will take off normally at say, 150 mph airspeed (ignoring any surface friction derived wind) if the tyres are still intact - with a wheel speed of 300 mph.

But...

The question states that the runway speed always matches the speed of the aircraft's wheels i.e. 150 mph, so logically, the aircraft has not actually moved from its starting position. This is clearly not possible, so the conditions of the question cannot be complied with.

So the answer is neither yes nor no.

Shall we move on to the downwind turn? devil

WolstonFlyer14/10/2016 01:36:36
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If the wheel speed is 300mph the belt is doing 300mph, "the belt is designed to match the speed of the wheels" , not the speed of the plane.

ted hughes14/10/2016 01:37:59
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It is a trick question, Gary.

We know a 747 can take off from just about any surface.

But it can't take off from a surface that is retreating backwards as fast as it is moving forwards- it just can't- it will be stationary.

We know the 747 is powerful, weighs 78 tonnes, wheels have friction etc. but that is just flannel, to make the puzzle hard to solve.

No vehicle can make forward progress on a surface which is constantly matching its speed in the opposite direction.

The plane is stationary and cannot fly.

ted hughes14/10/2016 01:38:52
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Posted by Martin Harris on 14/10/2016 01:32:26:
Posted by Gary Manuel on 14/10/2016 01:17:49:

It's a question of logic, common sense and physics.

Ah, but there's the rub - the question does not take the laws of physics into account. If the runway is moving at 150 mph after accelerating from rest with the aircraft then the aircraft will take off normally at say, 150 mph airspeed (ignoring any surface friction derived wind) if the tyres are still intact - with a wheel speed of 300 mph.

But...

The question states that the runway speed always matches the speed of the aircraft's wheels i.e. 150 mph, so logically, the aircraft has not actually moved from its starting position. This is clearly not possible, so the conditions of the question cannot be complied with.

So the answer is neither yes nor no.

Shall we move on to the downwind turn? devil

It is a logic problem.

Gary Manuel14/10/2016 01:39:01
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I'm finding it quite amusing. I got it straight away. The question is designed to get people with different points of view arguing.

ted hughes14/10/2016 01:40:28
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Okay, now I think I am being wound up.

WolstonFlyer14/10/2016 01:41:05
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I think so ted

Gary Manuel14/10/2016 01:42:20
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Maybe - but the 747 will still take off.

Martin Harris14/10/2016 01:43:09
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Posted by Gary Manuel on 14/10/2016 01:39:01:

I'm finding it quite amusing. I got it straight away. The question is designed to get people with different points of view arguing.

 

But did you, Gary? Your physical explanation of taking off from a moving belt is absolutely correct but this conveyer does not obey the laws of physics!

Edited By Martin Harris on 14/10/2016 01:43:53

WolstonFlyer14/10/2016 01:44:33
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Did you like the video I posted? It is on the previous page, the guy went to the trouble of putting his little plane on a running machine but forgot one important part of the question.

ted hughes14/10/2016 01:47:50
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Posted by WolstonFlyer on 14/10/2016 01:41:05:

I think so ted

Whatever, I've enjoyed this thread, I'm sure I'll still be thinking about it tomorrow!

Gary Manuel14/10/2016 01:49:23
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Posted by Martin Harris on 14/10/2016 01:43:09:
Posted by Gary Manuel on 14/10/2016 01:39:01:

I'm finding it quite amusing. I got it straight away. The question is designed to get people with different points of view arguing.

 

But did you, Gary? Your physical explanation of taking off from a moving belt is absolutely correct but this conveyer does not obey the laws of physics!

Edited By Martin Harris on 14/10/2016 01:43:53

My point exactly Martin wink

Stuart C14/10/2016 01:51:18
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It matters not which way the conveyor turns the wheels, as there is no way that un-braked wheel rotation can impart a decelerating force to the airframe. The plane takes off.

Martin Harris14/10/2016 01:55:45
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Posted by Gary Manuel on 14/10/2016 01:49:23:
Posted by Martin Harris on 14/10/2016 01:43:09:
Posted by Gary Manuel on 14/10/2016 01:39:01:

I'm finding it quite amusing. I got it straight away. The question is designed to get people with different points of view arguing.

 

But did you, Gary? Your physical explanation of taking off from a moving belt is absolutely correct but this conveyer does not obey the laws of physics!

Edited By Martin Harris on 14/10/2016 01:43:53

My point exactly Martin wink

Sorry - I thought from your first post in the thread that you had decided the question was valid and had concluded that the answer was yes.

Edited By Martin Harris on 14/10/2016 01:59:50

Gary Manuel14/10/2016 02:15:45
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Posted by Martin Harris on 14/10/2016 01:55:45:
Posted by Gary Manuel on 14/10/2016 01:49:23:
Posted by Martin Harris on 14/10/2016 01:43:09:
Posted by Gary Manuel on 14/10/2016 01:39:01:

I'm finding it quite amusing. I got it straight away. The question is designed to get people with different points of view arguing.

 

But did you, Gary? Your physical explanation of taking off from a moving belt is absolutely correct but this conveyer does not obey the laws of physics!

Edited By Martin Harris on 14/10/2016 01:43:53

My point exactly Martin wink

Sorry - I thought from your first post in the thread that you had decided the question was valid and had concluded that the answer was yes.

Edited By Martin Harris on 14/10/2016 01:59:50

I did - and I stick by it because it's correct from a physics point of view.

I also recognised that the question was worded in such a way that different people would interpret it differently and would argue all day (and night) about it.

john stones 114/10/2016 08:26:25
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Beginning to look like folk are mocking one another, but that just my opinion, no physics involved.

The Wright Stuff14/10/2016 08:40:55
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I've only just seen this thread, and I'm utterly amazed it's gone on for 9 pages...

As a professional physicist, I'm going to adopt the 'translate to a simpler problem with the same answer' approach.

Imagine instead the conveyor suddenly starts to move backwards relative to the initially stationary plane. What happens? Does the plane immediately accelerate backwards with the belt?

No. Its inertia would want to keep it stationary, but the wheels would spin up as if it was travelling forwards, to counteract the belt's motion. Eventually, as the wheels got faster, friction would transfer a small amount of this energy to the aircraft, causing a very slow acceleration backwards. As others have said above, the mass of the plane really doesn't care about what the belt is doing. If the engines were on, the friction in the wheels would be easily overcome. Provided the wheels and tyres could withstand travelling at twice the take off speed, the 747 COULD take off.

Colin Leighfield14/10/2016 08:43:22
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Ted Hughes. If the engines are running at take-off power it will move forward and fly. It makes no difference whether the conveyor is moving backwards or forward or at any speed. Apart from mechanical friction from tyre to conveyor and wheel bearings there is no mechanical relationship between wheel rotational speed and aircraft motion unless the brakes are applied. The wheel speed is a consequence, not an instigator. If you park the plane on ice and lock the brakes on, the plane will probably take off under power because the braking effect of the wheels is cancelled because of the lack of slip-resistance of the ice. If the ice was an ice-flow moving backwards, it would be also irrelevant, the plane will take off under power. If the conveyor goes backwards at 150 mph and the plane has a take-off speed of 150 mph, it will still take off, but the relative difference in speed between plane and conveyor will be 300 mph.

Bob Burton14/10/2016 08:50:00
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Posted by The Wright Stuff on 14/10/2016 08:40:55:

Provided the wheels and tyres could withstand travelling at twice the take off speed, the 747 COULD take off.

But as the plane will never move forward on the conveyor belt due to it matching the speed of the wheels how could it ?

Colin Leighfield14/10/2016 08:56:59
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There is no mechanical relationship between the wheels and the airframe so they don't influence what the plane does. It is wholly the other way round. The speed of the wheels is a consequence of the relative difference between the speed of the airframe and the speed and direction of the surface that the wheels are in contact with. The speed of the airframe is only a function of the relationship between engine thrust and drag, modified by the frictional consequences of the wheels, which is negligible unless brakes are applied. This only changes if there is a mechanical drive through the wheels or brakes are applied.

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