Stupid physics question

[Jupitus]

Jup - The belt COULD NOT negate the speed of the plane. The way it is worded is self-contradictory. If the belt could actually stop the plane then the belt must ALSO have stopped, which means it isn't providing any counter-force in the first place. Please read my earlier posts and Danya's. It's quite clear. Even if you did reword it to make more sense you would fall into the argument above which would require the plane to have its brakes on the wheels which would be pretty stupid on a takeoff.

The belt doesn't stop the plane, just prevents it from ever reaching take-off speed required.

 

[Will]

The belt doesn't stop the plane, just prevents it from ever reaching take-off speed required.

How, by magic?

 

[Jupitus]

How, by magic?

How the belt keeps up with the plane was never explained, so maybe

 

[Will]

Wij explained it best, the belt is a nonsense question, since it doesn't affect the plane.

 

[Jupitus]

Wij explained it best, the belt is a nonsense question, since it doesn't affect the plane.

Plane = attached to wheels, wheels = attached to belt, belt rolling backwards.


ok... try this:

I'm on a plane at Heathrow, on the runway.

The pilot turns up the thrust then lifts the undercarriage. We fall to the ground... yes or no?

Let's try that again... this time he doesn't lift the gear.... we roll along, slowly to begin with, and all of a sudden, the ground starts rolling backwards so we stand still effectively, then he lifts the undercarriage... do we fall to the ground... yes or no?

 

[old.user4556]

Hnnng,

It does affect the plane, the plane must reach an airspeed of 160 mph in order to take off, or a forward motion relative to the ground of 160 mph in order to take off. Since the ground (rolling conveyor) will ALWAYS negate the forward rolling speed the plane can never reach take off velocity since it's at 0 mph relative to the airspeed.

 

[Tilda]

christ alive <bangs head into wall>

Close the thread please

 

[old.user4556]

G - The plane does NOT have its brakes on the wheels. Therefore all the belt would do is cause a bit more friction in the faster turning wheels.

That's correct it doesn't, but the belt is negating the forward motion of the plane (as i've said) so won't allow take off speed.

Wheels turn faster, conveyor moves faster, plane doesn't move regardless of thrust. All the thrust will do is make the wheels spin faster and faster and faster. The conveyor belt does so in sync, negating the acceleration.

No take off.

 

[old.user4556]

christ alive <bangs head into wall>

Close the thread please

Why Tilda? Because you can't understand it?

It's a very interesting discussion, not helped by some people who can't be arsed participating and calling for it to be ended/closed.

 

[Lamont]

The problem is that everyone is describing the 'speed' of the plane and the conveyor belt, and not the 'velocity'

Speed means nothing in a problem such as this, you can go at a speed of a million miles per hour in a circle and effectively get nowhere.

Velocity however has direction, and must be measure relative to a point or in this instance an object.

The problem states that the plane moves forward, but does not state what it moves forward relative to. If it moves forward relative to the conveyor, then theoretically if the conveyor itself is moving backwards at the same velocity but relative to the air, then the plane has zero velocity relative to the air, and as has been pointed out by many people this would result in no airflow over the wings, and thus no lift.

However, if the plane moves forward relative to the air, then it doesn't matter what velocity the conveyor is moving at (assuming there is no force transferred from conveyor to plane) there is still a lift generated, and thus depending on the lift compared to the weight of the plane, take-off is possible.

So, to summarize (and it may have been said before, I couldn't be bothered looking through 13 pages ) the question paints a mental image, but not a sufficient mathematical one to enable an absolute answer.

 

[Tilda]

Why Tilda? Because you can't understand it?

It's a very interesting discussion, not helped by some people who can't be arsed participating and calling for it to be ended/closed.

Now I KNOW why you don't understand it, you haven't read the thread!

I get it, and have even explained it several times earlier in the thread, if you'd read it, you'd know that.
I've explained in a multitude of ways why it works, if you cant get it from the 15 different explanations theres really nothing more I can say that will make you understand.

 

[Tom]

Ok G, think about this.

You're sat in a car. Moving at 0mph.

Out of the window, you stick a little stabiliser wheel on a bit of metal, weld it to the doorframe, and it rests a vertically mounted conveyor belt - the belt behaves in the same way as the original question. The belt is of course at 90' to the road surface in this example.

You push the accelerator, and accelerate to an indicated 10mph.

The belt then starts moving at 10mph in the other direction.

Remember, the force pushing the car along is not accelerating the little stabiliser wheel at all.


How fast is the car moving, relative to the road surface?

 

[old.user4556]

Tilda, I've read the thread and given it careful consideration.

Here is the (or a) proper physics explanation:

http://forum.physorg.com/index.php?showtopic=2417&st=1155&#entry42611

Again, the plane will never fly.

 

[Tom]

Oh and I demand that you rate my thread. Or give me some rep. Or something. Call me a twat if you like.

 

[Jupitus]

Twat

 

[Tom]

Fuck you noob!*





















































*

 

[old.user4556]

I'm not quite following your example Tom?

 

[Tilda]

Tilda, I've read the thread and given it careful consideration.

Here is the (or a) proper physics explanation:

http://forum.physorg.com/index.php?showtopic=2417&st=1155&#entry42611

Again, the plane will never fly.

He is incorrect, i dont have the time nor the inclination to go through his post, however i'll pick on this which he has in his sig:

"Simplest Example of why plane will stay over same spot in earth...EVERYONE MUST READ THIS!!!!
Man pulls heavy box on a carpet....another man pulls carpet in opposite direction...."

This is wrong, the man the first man, is not pulling the box while standing on the carpet, this is acctually a good example for why a car dosn't move on a treadmill.

A correct example would be, a man standing on ground pulls a heavy box along some carpet with a rope. Another man pulls the carpet in the opposite direction. Does the box move towards the 1st man? Yes.

Therefore, when the planes engines are on and it accelerates (the 1st man pulling) it moves relative to the ground (the carpet/treadmill being pulled away is irrelevant) and thus gets a positive airpseed and can take off.

 

[Insane]

Id say it would, purely by stupid reasoning.

if the control sensor is reading the plane on the conveyor belt, then it would read it as 0 at all times.

as the plane accelerates the belt moves (this everyone agrees on)
but the sensor would then read the plane as travelling at 0 instead, and would then stop the belt from moving.
plane would then take off because the belt stops moving

now im gonna plug my fingers in my ears and go "na na na im not listening" if anyone goes to dispute this.

another theory for people to argue over now.

 

[Tom]

I'm not quite following your example Tom?

I'm trying to demonstrate that the force which propels the vehicle (the engine) is entirely separate from the speed of the stabiliser wheel and the belt.


The force that propels an aircraft is thrust from the engines.

The wheels on the undercarriage are free wheeling - their speed is entirely unrelated to the force being applied via the engines. If this were false, then when the aircraft took off and the wheels slowed down, the aircraft would fall from the sky.

The belt cannot have any affect whatsoever on the speed of the aircraft, because the aircraft is not driven from the wheels - it is driven from the engines on the wings. The engines on the wings push against the mass of the aircraft, and not the friction from the tarmac, unlike a car which is propelled by the wheels.

 

[Tilda]

This picture represents the correct way.

The plane pulls itself (or the man pulls the box) not through contact with the carpet/treadmilll, but through the air.
The fact the treadmill/carpet is moving against it has no effect other than to speed the wheels up.

This is different to a car on a treadmill, which depends on its contact with the ground to move. Analogous to a man in a box trying to pull himself forwards by grabbing onto the carpet being pulled out from under the box, he wouldnt move(assuming the carpet matches his forward motion).

 

[Tilda]

And one final post before I go to bed to convince the non-believers, copied from the physorg forum thread:

"Imagine, if you will, a car and an airplane sitting side by side on an icy runway, with the ice just starting to melt--basically, the slipperiest conditions you could realistically get.

At the same moment, the driver and the pilot release their brakes and go to full throttle. We all agree that the car will go nowhere, just spinning its wheels. What will happen to the plane? It might be a little squirrely, but it will accelerate normally."

Because the plane pushes against the air, not the ground to go forwards.

 

[old.user4556]

That picture is wrong, in my opinion.

The man pulling the box is not relative to the carpet in that example, if the man were to be pulling the box whilst on the carpet (the plane is on the tarmac which is moving backwards, like the carpet) then that would be the same as the plane example. The thrust (man) is not relative to the conveyor/carpet, where as in the example of the plane (the thrust being engines)the engines are relative to the moving tarmac.

For it to be similar to the plane example, the man would have to be pulling the box whilst he was standing on the carpet (which is being pulled the other way) - and thus the box relative to our "off-carpet" point of view would stay stationary (thus no airspeed and no lift).

 

[old.user4556]

And one final post before I go to bed to convince the non-believers, copied from the physorg forum thread:

"Imagine, if you will, a car and an airplane sitting side by side on an icy runway, with the ice just starting to melt--basically, the slipperiest conditions you could realistically get.

At the same moment, the driver and the pilot release their brakes and go to full throttle. We all agree that the car will go nowhere, just spinning its wheels. What will happen to the plane? It might be a little squirrely, but it will accelerate normally."

Because the plane pushes against the air, not the ground to go forwards.

That's not the same situation. In the slightest.

 

[Louster]

Oh man I'm impressed there are people that still don't get this.

I like the bit in the article linked to earlier: "... and was fascinated to see that the argument seemed to split between those who had some engineering or math background, all of whom said the airplane would takeoff and fly without any problem; and those with some other background, who visualized the airplane as having to push against the conveyor in order to gain speed.".

Big G - you've demonstrated that you're thinking about it in exactly the same way you'd think about a car. Read. Through. The. Thread. PLEASE.

 

[Tom]

I give up, sorry Big G but you'll have to think about why you're wrong

 

[old.user4556]

Louster, HOW MANY FUCKING TIMES - i've read the thread, i'm putting forward my own theories.

 

[Wij]

Tilda, I've read the thread and given it careful consideration.

Here is the (or a) proper physics explanation:

http://forum.physorg.com/index.php?showtopic=2417&st=1155&#entry42611

Again, the plane will never fly.

And then the theory gets trashed by other posters.

How does the treadmill exert any Newtons of force to stop the plane moving through the air unless the brakes are on ? Only a little is transferred due to friction (hence why the plane would not stay still if the belt was on and the plane was still.) However unless the brakes were ON so that 100% (or near enough) of the motion of the belt was transferred to force opposing the plane's movement through the air then the plane would still accelerate. Simple as that. There are so many reasons why you are wrong here. Give it up.

 

[Wij]

For it to be similar to the plane example, the man would have to be pulling the box whilst he was standing on the carpet (which is being pulled the other way) - and thus the box relative to our "off-carpet" point of view would stay stationary (thus no airspeed and no lift).

NO - THAT IS HOW A CAR WORKS !!!

The example where the man doing the pulling is not on the carpet was perfect. The engine is not on the ground. The wheels are. Similarly the bloke is not on the carpet. The box is.

*will thread to be closed*

 

[Louster]

Louster, HOW MANY FUCKING TIMES - i've read the thread, i'm putting forward my own theories.

That's too bad, because I was rather giving you the benefit of the doubt by assuming you hadn't really read the thread, rather than having read the thread and not understanding any of the huge amount of really obvious and solid reasoning contained therein.