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A question of physics.....

a simple little question, or is it?

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ted hughes13/10/2016 20:04:00
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466 forum posts

It would not fly.

Imagine the conveyor belt started with the plane not using its engines for thrust.

The plane would stay stationary, just the wheels would spin.

As soon as the plane used thrust, it would move backwards if the conveyor belt was faster than the thrust could provide, but if they were balanced (as in the problem), the plane would be stationary.

Edited By ted hughes on 13/10/2016 20:04:37

Martin McIntosh13/10/2016 20:06:54
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2871 forum posts
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I agree with that. Just as a car on a rolling road cannot move forwards.

Dave Hopkin13/10/2016 20:20:13
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Posted by ted hughes on 13/10/2016 20:04:00:

It would not fly.

Imagine the conveyor belt started with the plane not using its engines for thrust.

The plane would stay stationary, just the wheels would spin.

As soon as the plane used thrust, it would move backwards if the conveyor belt was faster than the thrust could provide, but if they were balanced (as in the problem), the plane would be stationary.

Edited By ted hughes on 13/10/2016 20:04:37

Apart from the facts

a) the wheels of the plane are not driven so what happens at the wheel/ground interface is irrelevant

b) Mythbusters put the conundrum into real life... guess what the plane flew....

**LINK**

Now can we let this thread die ?

Josip Vrandecic -Mes13/10/2016 20:30:55
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Thanks alot Dave .....nothing to say....... except embarrassed ashamed embarrassed

Jos

Martin Harris13/10/2016 20:35:01
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It always intrigues me when someone calls for a thread to be closed - presumably because it bores them? Is it (except in the case of a mod.) compulsory for them to look at it?

john stones 113/10/2016 20:39:28
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Posted by Martin Harris on 13/10/2016 20:35:01:

It always intrigues me when someone calls for a thread to be closed - presumably because it bores them? Is it (except in the case of a mod.) compulsory for them to look at it?

Don't start asking tricky questions you'll set some one off teeth 2

Don Fry13/10/2016 20:40:01
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But Dave, A. there is friction, and not irrelevant. Friction melts things. B. they did not. They did not go to school. They are journalists. Check the date first. If the belt is built by a good engineer, it will not fly. ( wind sheer caused by the belt, giving lift, excepted).

But let it die. My karma is intact. I see the system in its beauty. Except for shear induced winds.

ted hughes13/10/2016 20:55:45
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466 forum posts
Posted by Dave Hopkin on 13/10/2016 20:20:13:
Posted by ted hughes on 13/10/2016 20:04:00:

It would not fly.

Imagine the conveyor belt started with the plane not using its engines for thrust.

The plane would stay stationary, just the wheels would spin.

As soon as the plane used thrust, it would move backwards if the conveyor belt was faster than the thrust could provide, but if they were balanced (as in the problem), the plane would be stationary.

Edited By ted hughes on 13/10/2016 20:04:37

Apart from the facts

a) the wheels of the plane are not driven so what happens at the wheel/ground interface is irrelevant

b) Mythbusters put the conundrum into real life... guess what the plane flew....

**LINK**

 

Now can we let this thread die ?

Because you are wrong. I know it is hard to understand.

The myth busters were working on a different problem.

They were matching conveyor belt speed with speed of the plane.

The OP problem did not mention speed of the plane.

It is concerned with the speed of the wheels, which is quite a  different problem.

 

Edited By ted hughes on 13/10/2016 20:56:55

Bob Burton13/10/2016 21:02:05
178 forum posts

Why so much discussion ?

The conveyor matches the speed of the wheels moving in the opposite direction therefore the plane will never move forward so will never take off from the conveyor belt.

ted hughes13/10/2016 21:09:28
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466 forum posts

You are right, Bob, but others are getting confused thinking the conveyor belt is matching the speed of the aircraft, which was not in the original question.

Stuart C13/10/2016 22:29:48
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4 photos

The conveyor matches the wheel speed - not the axel speed. The plane takes off with very fast spinning wheels, since no amount of wheel speed will cause retardation of the a/c. ( Hence the conveyor has the same dimensions as a runway).

Colin Leighfield13/10/2016 23:01:47
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The wheels don't drive the movement of the plane, their rotational speed is only relevant to the difference in speed between the conveyor and the plane. Whether it flies or not is purely a function of its' speed relative to the air. That is purely dependent on the balance between engine thrust and airframe drag. What the conveyor is doing has no influence on that, all it influences is the speed that the wheels rotate at.

ted hughes13/10/2016 23:36:21
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466 forum posts
Posted by Stuart Coyle on 13/10/2016 22:29:48:

The conveyor matches the wheel speed - not the axel speed. The plane takes off with very fast spinning wheels, since no amount of wheel speed will cause retardation of the a/c. ( Hence the conveyor has the same dimensions as a runway).

The plane may well have fast spinning wheels, but the conveyor always matches them. It is a logic puzzle, not a matter of physics, despite the thread title.

ted hughes13/10/2016 23:37:32
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466 forum posts
Posted by Colin Leighfield on 13/10/2016 23:01:47:

The wheels don't drive the movement of the plane, their rotational speed is only relevant to the difference in speed between the conveyor and the plane. Whether it flies or not is purely a function of its' speed relative to the air. That is purely dependent on the balance between engine thrust and airframe drag. What the conveyor is doing has no influence on that, all it influences is the speed that the wheels rotate at.

The plane won't have any air speed because it is not moving. It is a logic puzzle.

ted hughes13/10/2016 23:46:54
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466 forum posts

A very interesting thread, reminds me of the Monty Hall problem, which had top academics laying their mathematical accreditation on the line.

Monty Hall was an American game show host.

At the end of the game, the winning contestant was shown three doors.

Behind one was a car, behind two, goats.

The contestant was asked to choose a door (lets say he chose B, out of A, B and C.

Monty Hall then opened one of the remaining doors to reveal a goat.

The contestant was then asked if he wanted to change his mind and choose the remaining door to try to win the car.

The question posed to the world was: would changing your mind improve your chances of winning the car, or was the original choice as good as any.

The logical and correct answer was that changing your mind gave a 33% better chance of obtaining the car- but many academics could not see this.

Stuart C13/10/2016 23:54:07
121 forum posts
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I was wrong about fast spinning wheels. I now postulate that the wheels never turn, and the conveyor exactly matches the a/c speed till lift off.

ted hughes13/10/2016 23:54:48
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466 forum posts

When the contestant was asked to choose the first time, he was guessing one in three.

The second time, he was guessing one in two.

So it was always wiser to guess again!

ted hughes13/10/2016 23:57:46
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466 forum posts
Posted by Bob Burton on 13/10/2016 21:02:05:

Why so much discussion ?

The conveyor matches the speed of the wheels moving in the opposite direction therefore the plane will never move forward so will never take off from the conveyor belt.

This is the correct answer to the problem.

onetenor14/10/2016 00:07:43
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1900 forum posts

A) which way is belt moving ? B ) Brakes on to start with ?? At which point is plane throttle opened.. Those apart

; if enough air at sufficient speed passes over the wings in the right direction the plane will fly irrespective of the source of the speed. Wether belt or engine thrust

Stuart C14/10/2016 00:09:31
121 forum posts
4 photos

Ah! "The opposite direction", this is where I fell down. The opposite direction means the conveyor moves forward to prevent wheel rotation. (I think!)

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