Stupid physics question

[Sockstuff]

But the belt moves at the speed of the plane, not at some made up 100 mph number.

It doesn't matter what the number is 1mph through to 8947592mph. if the belt moves backwards at the same speed at the same speed the plane doesn't move forward.

 

[WPKenny]

This is throwing a staionary object that the plane can be tied to into the mix.

Nope, it's equating the pull of the elastic to the same force that the jet engines are creating.

Fuck it, change the elastic for a bit of cable and a winch motor. Change the paper for the ground opening up in an earthquake. Change the skateboard for a 4x4 jeep. You think just cos the ground's opening up between the car and the winch that the winch pulls the car at a different speed? It's the same as the thrust from a jet engine only it's air being "wound" through the engine instead of cable round a drum.

 

[Danya]

If the plane doesn't move, the belt doesn't move though, at any speed.

Re-read the question: The belt moves at the speed of the plane.
Thus, if the plane is motionless, so is the belt. For the belt to be travelling at 1 mph, 100 mph, or 8947592 mph, the plane must also be moving at that speed.

 

[WPKenny]

If the plane doesn't move, the belt doesn't move though, at any speed.

Re-read the question: The belt moves at the speed of the plane.
Thus, if the plane is motionless, so is the belt. For the belt to be travelling at 1 mph, 100 mph, or 8947592 mph, the plane must also be moving at that speed.

That answers the question, but socky still isn't understanding the physics behind it.

Come on Sockstuff, you can do it!

 

[TdC]

there is no belt</matrix>

I know I'm being amazingly deep here, but think of it as if there was no belt. What does the plane do? Do the plane's wheels care if there is a belt or not? A plane is not a car. A car uses traction to transfer engine power into a forward motion via it's wheels. A plane uses thrust to apply the engine's power into a forward motion using what? not it's wheels matey. the whole trick is that the wheels are moot.

I know it doesn't apply, but how does a skiplane move forwards?

 

[Danya]

Physics wasn't working, so I tried logic.

 

[Sockstuff]

I'm fed up with this. I know how the mechanics of the question work in my head. I know the correct physics of the situation. I am right - I win. End of. I AM RIGHT AND THATS IT. I CBA NOW. (_(_) (_|_) (_)_)

 

[Wij]

If the plane doesn't move, the belt doesn't move though, at any speed.

Re-read the question: The belt moves at the speed of the plane.
Thus, if the plane is motionless, so is the belt. For the belt to be travelling at 1 mph, 100 mph, or 8947592 mph, the plane must also be moving at that speed.

Hooray - someone else actually gets it

The question does not make sense. The plane could not be still.

C'mon socky - think...

P.S. CLOSE THE THREAD !!!

 

[juliet]

ok.

lets assume this plane is big and thus has some big engines. big engines can produce anything up 500,000 newtons of thrust.

at 0 mph the plane has maximum thrust and 0 newtons of friction as it is not moving. as the plant starts to move, then so does the belt at exactly the reverse velocity of the plane. so when hte plane does +1 mph the belt does -1 mph and so the wheels must accommodate 2 mph.

at +2 mph the belt moves at -2 mph so the wheels accommodate 4 mph

so at plane speed of 150mph, the belt is doing -150 mph and the wheels are accommodating for 300 mph

lets assume that this big plane takes off at 200 mph, thus the wheels will be spinning to accommodate 400 mph.

untill takeoff the wings have absolutly nothing to do with the movement of the plane, it moves horizontally along the surface of the moving runway.

at take off speed, the plane lifts off the runway and so the runway then has nothing to do with the plane.

so in order for the plane not to take off, the frictional force in wheels must be higher than then thrust.

so if the wheels were spinning to accommodate 400 mph would they have a frictional force of more then 500,000 newtons? the answer is no, as they are tested way beyond this threshhold.

so in conclusion the plane does take take off, the only change in behaviour from normal is that it accellerate less quickly, and thus the moving runway would need to be much much longer than any normal runway.

i say that wheels spin to accommodate x mph, because wheels dont spin in mph, they spin in degrees/sec.

also to end this, i'd like to say that this question is bullshit, as it has too few items of information.

it doesnt detail the thrust of hte engines, it doesnt detail the size of the wheels, it doesnt detail the frictional coefficient of the wheels, it doesnt say how long the runway is, it doesnt say how heavy the plane is.

all i can say is that if the plane flys and take off under normal circumstances, then it will take off just fine with a moving runway.

 

[Sockstuff]

I'm fed up with this. I know how the mechanics of the question work in my head. I know the correct physics of the situation. I am right - I win. End of. I AM RIGHT AND THATS IT. I CBA NOW. (_(_) (_|_) (_)_)

I TAKE IT BACK I WAS WRONG... i get it now! The wheels just spin faster.... right yes....
YES

BTW it was TDC that made me see the light... Dray, juliete, kenny - you suck, but wij sucks more - tdc wins at life.

 

[Wij]

Thank you.























CLOSE THE THREAD !!!

 

[Sockstuff]

I started to write this pm to tdc... but er yeah...

I know I'm being amazingly deep here, but think of it as if there was no belt. What does the plane do? Do the plane's wheels care if there is a belt or not? A plane is not a car. A car uses traction to transfer engine power into a forward motion via it's wheels. A plane uses thrust to apply the engine's power into a forward motion using what? not it's wheels matey. the whole trick is that the wheels are moot.

I know it doesn't apply, but how does a skiplane move forwards?

Correct. BUT. Say the plane was stationary on the belt with its brakes on and the belt moved back... the plane would go back with it. If it then takes the brakes off it continues to go back with it until the engines come on it will then have en ough thrust to be stationary. It applies more thrust to move forward and the belt moves faster.....oh fuck i get it..

 

[Danya]

Hooray - someone else actually gets it

The question does not make sense. The plane could not be still.

C'mon socky - think...

P.S. CLOSE THE THREAD !!!

The question makes sense, but only if the plane moves.

 

[ECA]

Surely the correct answer is, If I shove my cock in something, how many objects _wont_ take off?

 

[Danya]

Surely the correct answer is, If I shove my cock in something, how many objects _wont_ take off?

Try a switched on blender, let us know the answer.

 

[Tom]

Hooray - someone else actually gets it

The question does not make sense. The plane could not be still.

C'mon socky - think...

P.S. CLOSE THE THREAD !!!

SHUT UP

*cackles*


The plane will take off, every time. Live with it.

 

[old.user4556]

I find it's incredibly simple; it won't take off.

Presuming the plane needs ~160 mph to take off?

Plane accelerates down a runway at full throttle, it gets to 150 mph (just short of takeoff speed) - turn on the conveyor belt (which matches the speed of the plane, remember) - forward motion of the plane through the air stops completely, no air over the wings, no take off?

Since we're at maximum thrust and further forward acceleration can't be applied - any increase of rolling speed will be negated by the ever increasing conveyor belt.

Yes, it's not powered by the wheels (as discussed) but the plane is still connected to the ground until such times that it has generated 160+ mph forward speed (which is constantly being negated by the rolling treadmill) and is independent of the treadmill.

Imagine you're sitting on a freewheeling car on the same tarmac and pull a jet engine out your pocket and turn it on - you would apply additional forward thrust that would turn push the car forward and turn the wheels faster, but the rolling road would only move faster to negate the additional thrust provided by the engine.

Edit: I must stress that it doesn't matter the wheels aren't powered; the plane still needs to create a 160+ mph rolling forward motion to generate lift. As I say, the ever-increasing conveyor negates any forward rolling speed.

 

[Tom]

G. The plane is connected to the belt by wheels. Wheels which are free-spinning.

The plane is thrusting against the air - not the belt. The speed of the wheels is irrelevant.

The only ways that the belt can make a difference to the speed of the aircraft are by drag on the wheels (friction), and turbulence.

The plane will take off.


https://forums.freddyshouse.com/showpost.php?p=2708834&postcount=46

 

[Tom]

Imagine you're sitting on a freewheeling car on the same tarmac and pull a jet engine out your pocket and turn it on - you would apply additional forward thrust that would turn push the car forward and turn the wheels faster, but the rolling road would only move faster to negate the additional thrust provided by the engine.

The road would move faster, but it can do nothing to counteract the thrust from the rocket!

 

[old.user4556]

In my opinion that is incorrect.

Yes, it's thrusting against the air but that thrust against the air is transformed into a rolling motion and the plane must achieve a 160 mph forward motion to be able to take off?

So the conveyor belt starts to move at 160 mph - the plane is standing still, so it can't take off.

According to your theory Tom, the thrust will be increased to allow the plane to move faster to overcome the -160 mph of the belt? but the thrust is not infinite on a plane and as the thrust is increased to increase the forward rolling motion of the plane, so will the conveyor negating the forward motion.

Remember the engines are fixed to the plane, which is being pushed at -160 mph.

Think of it conversely, a plane sits stationary on a conveyor belt doing 160 mph backwards. As soon as thrust is applied it will reach a relative 0 mph air speed eventually.

Lets assume the conveyor belt is moving in reverse at rolling speed and air speed 2000 mph (impossible for a passenger jet), the plane will reach it's maximum forward thrust and do (lets say 400 mph forward) - that's a rolling speed AND airspeed of -1600 mph.

The plane can't take off.

 

[old.user4556]

The road would move faster, but it can do nothing to counteract the thrust from the rocket!

Yeeeesssss, but you're forgetting that although the rocket isn't connected to the road the object it's pushing is connected - so any additional forward motion is negated by the opposing conveyor belt.

 

[Xgkkp]

I think this will go absolutely nowhere because for every person who realizes that it will, in fact take off, there are four more who come straight into the thread from the first post and go "NO IT WON'T HURR" and need to be explained to again what they could have learnt just by reading the damned thread.

They also seem to have an uncanny ability to ignore or dismiss any facts which counteract their 'argument'

 

[old.user4556]

I've read the thread extensively and i'm putting forward my theory.

I'm not a physicist, i'm not a mathematician; i'm suggesting my opinion.

That's allowed, if you don't like it you can always fuck off?

 

[Jupitus]

I'm still here, and I'm still in the no camp.

I can see where the yes camp get their argument. It is flawed.

The rolling road will negate the speed of the plane. Whatever thrust the engines have to make the plane 'move' extremely fast will be counteracted by the fact that until it takes off, it's wheels are in contact with the strip, and the strip is moving backwards at an equivalent speed.

Yes... if the pilot whacks the engines to full throttle quickly the conveyor belt will need to speed up equally quickly, but assuming it does (and the question states this is the case) the plane will not gain any air speed.

The yes camp are basically saying 'the plane is independant of it's wheels and the thrust from the engines will give it forward speed'. This is true, but only if the plane is allowed to get it's wheels off the runway. To do so, it has to reach sufficient airspeed, which it can't.

Edit: Unless there happens to be a very big gust of wind

 

[old.user4556]

Imagine those airport conveyor belts for passengers, that guide you through the terminal. Imagine you put your luggage on one of them, and walk alongside dragging your luggage the wrong way.

If the belt was moving at 4mph, and you're walking about 4mph, the luggage comes with you at 4mph - 8mph relative to the belt. If you started walking at 8mph, then your luggage will be moving at 8mph, 12mph relative to the belt.

But Tom, you're not relative to the moving platform that the luggage is on and indeed the luggage is not moving relative to you (but it is to the platform).

A flawed comparitive would be that the plane which is travelling at near take off speed of 150 mph (negated by a -150mph conveyor) is travelling at 0mph relative to you standing on the non moving ground (for a given level of thrust). You get yourself a big truck at the side of the moving conveyor which is tied round the front wheel of the plane and you start to pull the plane forward at 20 mph. The plane is now doing 170 mph but since it's only doing airspeed 20 mph it doesn't take off.

As soon as the big truck stops pulling (in this case you at the side of the luggage platform) the plane will return to it's given speed of 150 mph for the level of thrust applied.

It's all about relativity.

 

[old.user4556]

In addition,

You are standing still at the side of the platform, meanwhile the engines providing thrust are moving at -150 mph in the other direction (both mph and most importantly air speed) or however fast the conveyor is going (which can be infinite) - so your comparison is flawed.

 

[old.user4556]

The bottom line is that the plane can never achieve a relative airspeed of 160+ mph and thus will never take off since the original example clearly states that the conveyor belt will always match the forward speed of the plane regardless of thrust, so therefore an airspeed of 0 mph is all that can be obtained.

In my opinion.

K.

Thx.

 

[Jupitus]

The bottom line is that the plane can never achieve a relative airspeed of 160+ mph and thus will never take off since the original example clearly states that the conveyor belt will always match the forward speed of the plane regardless of thrust, so therefore an airspeed of 0 mph is all that can be obtained.

In my opinion.

K.

Thx.

Yup... unless a huge gust of wind picks the plane up so the conveyor can't negate the thrust... or the thrust is so massive that it can shear the grip of the tyres on the conveyor sufficiently to gain the lift off speed...


As long as the plane is in contact with the conveyor it will NOT gain the air speed needed for take off.

 

[Wij]

FFS - this is why the thread needs to be closed. I, and others, have explained this before.

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.

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 YES crowd has won the argument.

CLOSE THE THREAD !!!!

 

[Will]