Quick Physics Test..

[AndrewOW]

OK.... imagine this:

 

In the original scenario we have a plane completely stationary on this conveyor runway.

 

As soon as the plane moves forward, the conveyor moves backwards to compensate.

 

Therefore, wheels do not move.

 

If wheels do not move from their position on the earth, plane does not move and so will be unable to create lift as there is no air flowing over or under the wings.

 

 

The wheels on the plane aren't being powered, so won't have a bearing on the planes forward thrust ooer. They'll just rotate faster and faster, but won't slow the process of forward motion. Maybe

[Gaz6002]

The wheels on the plane aren't being powered, so won't have a bearing on the planes forward thrust ooer. They'll just rotate faster and faster, but won't slow the process of forward motion. Maybe

 

Rubbish. They are being powered by the 2 huge engines connected to the wings.

[tbourner]

'The plane cannot take off!'

 

...I need to go and put my skill/knowledge to use and make an Aircraft safe to fly!

 

OMG. I'm taking a cruise next time I go anywhere!!

[Gaz6002]

OMG. I'm taking a cruise next time I go anywhere!!

 

 

Why? He's right isn't he?

[AndrewOW]

Rubbish. They are being powered by the 2 huge engines connected to the wings.

 

 

What part of my statement didn't you understand? Obviously I meant that the plane is powered by engines.

 

I was merely pointing out that the wheels AREN'T powered, and will spin independently as fast as it would take for the plane to move forward.

[Gaz6002]

The wheels on the plane aren't being powered, so won't have a bearing on the planes forward thrust ooer. They'll just rotate faster and faster, but won't slow the process of forward motion. Maybe

 

 

The wheels can never move in this situation.

 

 

Push your fists together and see what happens.

[caseys]

OK.... imagine this:

 

In the original scenario we have a plane completely stationary on this conveyor runway.

 

As soon as the plane moves forward, the conveyor moves backwards to compensate.

 

Therefore, wheels do not move.

 

If wheels do not move from their position on the earth, plane does not move and so will be unable to create lift as there is no air flowing around the wings.

 

Gaz,

 

Think of the air the jet engines are displacing. The plane is not putting power out via the wheels, but rather out via the back of the turbines - force against the air which is totally different and not affected by the movement of this 'treadmill/runway from hell'. The wheels themselves are freewheeling, hence not delivering power to the treadmill.

 

To try and use another analogy imagine you have an escalator like in the airports - nice and flat and long - BUT it's partially broken. The stairs are going backwards but the bannister isn't moving.

 

The wheels of the plane are your legs. Maintain a constant speed, you stay stationary. Run faster, let's say like the treadmill the stairs compensate and speed up too. So if you were only using you legs (akin to delivering power via the wheels of the plane) yes you would stay stationary relatively speaking.

 

Now imagine your legs somehow automatically freewheel like a plane's wheels (let's say you're on a skateboard or rollerskates with no friction) and you decide to move yourself with your arms (the jet engines in this case) and pull yourself along yanking on the bannisters which are not moving backwards (the jets working against the air which in the experiment is not blowing backwards) - which remember are stationary relative to the stair part.

 

Think you'd move along? If the answer's no, please point out the bit where you think is wrong and you think you would in fact be still stationary.

[Gaz6002]

What part of my statement didn't you understand? Obviously I meant that the plane is powered by engines.

 

I was merely pointing out that the wheels AREN'T powered, and will spin independently as fast as it would take for the plane to move forward.

 

See above post.

[johnnyknox]

OK.... imagine this:

 

In the original scenario we have a plane completely stationary on this conveyor runway.

 

As soon as the plane moves forward, the conveyor moves backwards to compensate.

 

Therefore, wheels do not move.

 

If wheels do not move from their position on the earth, plane does not move and so will be unable to create lift as there is no air flowing around the wings.

 

You're right..the plane needs to be moving to create lift but the conveyor is only working against the wheels...so if the plane turns on it's afterburners it will push itself forward giving itself enough forward motion to create lift and take off.

 

The best example I have read is the toy car on a conveyor belt. If you hold the car with your hand on the conveyor belt the wheels will spin but the car will remain still. If you push the car forward the wheels will spin faster as you do so, however the car will still move forward as you push it.

 

In effect the plane engines are creating the same effect as the hand on the toy car in this example.

[Gaz6002]

Gaz,

 

Think of the air the jet engines are displacing. The plane is not putting power out via the wheels, but rather out via the back of the turbines - force against the air which is totally different and not affected by the movement of this 'treadmill/runway from hell'. The wheels themselves are freewheeling, hence not delivering power to the treadmill.

 

To try and use another analogy imagine you have an escalator like in the airports - nice and flat and long - BUT it's partially broken. The stairs are going backwards but the bannister isn't moving.

 

The wheels of the plane are your legs. Maintain a constant speed, you stay stationary. Run faster, let's say like the treadmill the stairs compensate and speed up too. So if you were only using you legs (akin to delivering power via the wheels of the plane) yes you would stay stationary relatively speaking.

 

Now imagine your legs somehow automatically freewheel like a plane's wheels (let's say you're on a skateboard or rollerskates with no friction) and you decide to move yourself with your arms (the jet engines in this case) and pull yourself along yanking on the bannisters which are not moving backwards (the jets working against the air which in the experiment is not blowing backwards) - which remember are stationary relative to the stair part.

 

Think you'd move along?

 

Again, if the thrust of the engine makes the plane begin to move forward then my point stands. The forward movement is transfered via the wheels, which can't by definition of the problem move so hence the plane can't move.

[Gaz6002]

You're right..the plane needs to be moving to create lift but the conveyor is only working against the wheels...so if the plane turns on it's afterburners it will push itself forward giving itself enough forward motion to create lift and take off.

 

The best example I have read is the toy car on a conveyor belt. If you hold the car with your hand on the conveyor belt the wheels will spin but the car will remain still. If you push the car forward the wheels will spin faster as you do so, however the car will still move forward as you push it.

 

In effect the plane engines are creating the same effect as the hand on the toy car in this example.

 

No, thats incorrect. There will be no lift as the plane isn't going anywhere. Every time it moves forward the force is counter-acted by the runway, hence no air pressure to give lift in the first place.

[caseys]

Again, if the thrust of the engine makes the plane begin to move forward then my point stands. The forward movement is transfered via the wheels, which can't by definition of the problem move so hence the plane can't move.

 

Ok, let's go on the skateboard/escalator analogy. Forget anything about taking off, we're trying to get the relative speed concept going.

 

Imagine you are on the escalator, yes it speeds up whilst you're on the skateboard if the skateboard's *wheels* speed up.

 

Now imagine there is a rope you are holding onto on this 500 mile long escalator. The rope is not moving, it is stationary. Nor is the air moving

 

Pull yourself along the rope. You will get to the end eventually yes? Which means you have moved relative to the bottom part of the escalator, even if it is moving.

[Kranz]

weeehhahahkljj, i did not log off lkaj;lfjeiorj;ojl k;jnldkj jakljf;jaofjpaj;fkljal;kjf; klaj; kljfaof;klajf

 

I think someone left the computer logged on in the crew room LOL

 

Naughty stewardesses

[Gaz6002]

Ok, let's go on the skateboard/escalator analogy. Forget anything about taking off, we're trying to get the relative speed concept going.

 

Imagine you are on the escalator, yes it speeds up whilst you're on the skateboard if the skateboard's *wheels* speed up.

 

Now imagine there is a rope you are holding onto on this 500 mile long escalator. The rope is not moving, it is stationary.

 

Pull yourself along the rope. You will get to the end eventually yes? Which means you have moved relative to the bottom part of the escalator, even if it is moving.

 

Absolutely. But read the question. Speed of the plane/wheels is directly proportional to the reverse speed of the belt.

[Gaz6002]

The idea is that you can't move off the runway without the runway reacting by equaling the force.

[caseys]

Absolutely. But read the question. Speed of the plane/wheels is directly proportional to the reverse speed of the belt.

 

I have read the question, and to quote

 

This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

 

At no point does it state which part of the plane relatively speaking the conveyor belt's speed is inversed against.

 

I, like Ian C have read it as inversely to the relative rotational speed of the wheels. Not of the relative speed of the body of the plane to the earth/body it is based on. I think unfortunately that part of the theory has been badly worded and can possibly be interpreted very openly.

[Rob]

Again, if the thrust of the engine makes the plane begin to move forward then my point stands. The forward movement is transfered via the wheels, which can't by definition of the problem move so hence the plane can't move.

 

For gods sake, no it isn't. The wheels on a plane are simply there for it to stand on when not flying, and to allow it to roll easily down the runway, or conveyor belt.

 

Imagine this, the conveyor belt runs the other way, down the runway with the plane at 200mph, ignoring friction in the wheel bearings, the plane still takes off at the same speed and not twice as fast...

or...

the conveyor belt exactly matches the ground speed of the plane so the plane moves down the runway under its own propulsion but the wheels never turn at all...

or...

The plane doesn't have wheels but a multi-directional ball system, now the convetor belt is running at 90deg to the plane and is 2 km wide, the belt shoots across but the plane still tracks down it and the plane takes off as if it is on a stationary surface.

 

Have you got it yet?

[Rob]

The idea is that you can't move off the runway without the runway reacting by equaling the force.

 

 

No, the force from the plane is not reacting agianst the runway, it is reacting against the air in which the plane sits. The weight of the plane via gravity reacts against the runway (until lift cancels it) but the driving force (which is horizontal) pushes against the air, not the ground.

[AndrewOW]

So much for a quick question about physics!

[johnnyknox]

The idea is that you can't move off the runway without the runway reacting by equaling the force.

 

But the only contact the runway has with the plane is via the plane's wheels. The conveyor cannot act against the forward thrust of the plane's engines.

 

I know what you mean...the plane cannot take off unless it is moving forward...but in this case it can move forward.

[Ewen]

I assume yet another answer would just get lost in the scramble, but I'm bored.

Wether its a prop or jet plane, the plane will tend to move forward when engines are powered up (FERTIAEAOR) The only thing stopping it moving forward is its contact with the ground. This contact is via wheels....assuming the brakes are off, the wheels are free to rotate, thus the planes contact with the ground can be ignored, and forward motion is a given.

The addition of a conveyor belt underneath the wheels is irrelevant to this forward motion, as the only part of the plane in contact with the belt is the wheels, which as before, are free to rotate.

The speed of the fwd motion of the plane depends purely on the thrust from the engines....the fact that the belt moves at the same speed of the plane but in the opposite direction has no effect on the fwd motion of the plane, again because it is only in contact with the wheels...thus the wheels begin to spin at the combined speed of the fwd motion of the plane and the reverse motion of the belt, neither affecting plane nor belt.

Assuming said belt is long enough, the plane will continue its fwd motion and accelerate until it reaches take off speed and, er, take off.

[Gaz6002]

I have read the question, and to quote

 

 

 

At no point does it state which part of the plane relatively speaking the conveyor belt's speed is inversed against.

 

I, like Ian C have read it as inversely to the relative rotational speed of the wheels. Not of the relative speed of the body of the plane to the earth/body it is based on. I think unfortunately that part of the theory has been badly worded and can possibly be interpreted very openly.

 

How can you think that? At the end of the day, the idea is that if the wheels of the plane (which move at the same speed as the plane) are moving at a speed then the runway will react by moving at the opposite speed.

 

For gods sake, no it isn't. The wheels on a plane are simply there for it to stand on when not flying, and to allow it to roll easily down the runway, or conveyor belt.

 

Imagine this, the conveyor belt runs the other way, down the runway with the plane at 200mph, ignoring friction in the wheel bearings, the plane still takes off at the same speed and not twice as fast...

or...

the conveyor belt exactly matches the ground speed of the plane so the plane moves down the runway under its own propulsion but the wheels never turn at all...

or...

The plane doesn't have wheels but a multi-directional ball system, now the convetor belt is running at 90deg to the plane and is 2 km wide, the belt shoots across but the plane still tracks down it and the plane takes off as if it is on a stationary surface.

 

Have you got it yet?

 

No.

 

I'm not talking about flying here. I'm talking about the plane moving in the first place. What happens when you push someone on rollerskates down the road? They move. What happens when you push that same person, but on non-friction rollers instead of a road? Nothing. Every action has to be cancelled out by a reaction.

 

Do you get what I'm saying?

[johnnyknox]

Ok...another example:

 

You can put a normal car on the dyno because the engine powers the wheels. The rollers act against the wheels, so the car will remain stationary...(well, in most cases )

 

but if you put a jet car on a dyno it will push itself off because the power is not going to the wheels, it is being pushed by the jet in the back

[Gaz6002]

Ok...another example:

 

You can put a normal car on the dyno because the engine powers the wheels. The rollers act against the wheels, so the car will remain stationary...(well, in most cases )

 

but if you put a jet car on a dyno it will push itself off because the power is not going to the wheels, it is being pushed by the jet in the back

 

How can you tell the difference between the jet engine and the power being put through the rear wheels? In a fully controlled environment you can't, as the body that is excerting the force is still the same body.

[Rob]

How can you think that? At the end of the day, the idea is that if the wheels of the plane (which move at the same speed as the plane) are moving at a speed then the runway will react by moving at the opposite speed.

 

 

 

No.

 

I'm not talking about flying here. I'm talking about the plane moving in the first place. What happens when you push someone on rollerskates down the road? They move. What happens when you push that same person, but on non-friction rollers instead of a road? Nothing. Every action has to be cancelled out by a reaction.

 

Do you get what I'm saying?

 

 

If you push someone on a set of rollers as you describe, they will move forward off the rollers, because the person pushing is not part of that system. The force being delivered is has no reaction upon the rollers. this is the same as the thrust from the engine, it pushes the air and the plane WILL move forward and the wheels will rotate at twice the rotational speed because of the roller moving backwards.