Physics Question

[mugs]

Originally posted by: giantpinkbunnyhead
Holy thread necromancy Batman!

How is this still being debated?!? Whatever, it's still funny to read all this.

OP needs to update the OP to include the correct answer. But I'm not sure the OP believes the correct answer is correct, so that might not happen.

 

[sxr7171]

No, not unless that conveyor belt stops. It would be an awesome take off then. The turbines might be running at full thrust, but what use is that thrust when you cannot generate lift?

Yes, I know an answer is probably in this thread, but I just scrolled down and posted before reading any of it.

If the conveyor belt also moved along with it (hypothetically) a mass of air above the surface of belt at the speed that the belt is travelling at then there would be lift; in other words a hypothetical air conveyor belt.

 

[Confused]

I've got to say, I'm VERY impressed that this thread has received so many replies without ANY flaming or personal insults, and we've had two very interesting discussions going (plane and 0.999... = 1)



As far as I'm concerned in this scenario, the plane will take off.

The wheels are purely to keep the fuselage of the plane off the ground. The wheels can spin freely (granted, with a small amount of friction, but in this instance neglible). The force moving the plane forward is from the turbines - which don't require friction against the ground (unlike a car or a human). Therefore, this forward force from the engines will move the body of the plane forward, and the conveyor belt will just spin the wheels at twice the air speed of the plane.

 

[notfred]

This thread's already about 3 million posts long, but I have to give my input. The plane WILL take off. The engines of an airplane move it relative to the air around it, not relative to the ground, like many people have said before.

Let's make an analogy. Pretend the plane is propelled forward by a giant winch. There's a rope attached to the front of the plane, and that rope is attached to a winch at the far end of the conveyor. Now, when the winch starts winding in and pulling the plane forward, do you think it matters how fast the conveyer under the plane moves? It doesn't the winch is going to keep pulling in rope at the same rate, and move the plane forward. The only thing that the conveyer will change is the rotation speed of the wheels, but the plane will keep moving forward at a constant rate because it's being pulled by a rope.

The engine on an airplane is like the rope, except the rope and the winch is air instead the engine pulls the plane through the air, independent of the movement of the ground.

 

[letdown427]

Originally posted by: Confused
I've got to say, I'm VERY impressed that this thread has received so many replies without ANY flaming or personal insults, and we've had two very interesting discussions going (plane and 0.999... = 1)

Seconded.

Mugs, how you've managed to put up with all those 'it won't move' posts is admirable!

Also, to the people discussing the infinite power conveyer, as has been said already, the question states it matches the planes speed. Therefore ANY talk of high speed conveyer actually proves it can take off, as for the conveyer to be going high speed, so must the plane. not the plane's wheels. the plane. As was also discussed earlier, you can't argue that the conveyer matches the plane's wheel speed, as this leads to, well frankly a bit of a cock up.

Hence, if you wish to continue to argue that the plane can't take off, then I'd advise dropping that argument, as that is not relevant to this question. If you wish to start a new question saying "if an infinitely powerful conveyer that provides infinite friction was used, could it stop a plane taking off?" then feel free. But the arguement of conveyer going at huge speed is irrelevant.

Any mention of the conveyer going at any speed other than zero, merely proves that the plane can take off, as it is moving. The question clearly states the conveyer speed is identical to the speed of the plane. This cannot refer to the wheels as a paradox is encountered.

Hence, the only way you can prove the plane couldn't take off, is by somehow proving that the conveyer can oppose the thrust of the engines. The only way it can do this, is by friction. As if it starts moving, then the plane can take off, as it is moving, and thus you've shown that it can move, and so the rolling resistance can be overcome.

Say the friction of the conveyer was sufficiently high to prevent the wheels from turning, effectively glueing them. Tto form an equation for this using physics and any rough estimates at even the largest plane's mass, and the rough corresponding forward thrust from its engines, produces a coefficient of friction that is too high to be considered anything other than fantasy, let alone something that could be created using the rubber from the tyres and whatever material the conveyer is made from.

Please can this end soon? Hehe, it's fun though.


I think i've seen a way of combining the 0.999... = 1 and the OP!!

So, for the conveyer to have any effect different from a normal runway, it must be moving, correct? Correct.

OK, so for it to be moving, so must be the plane. As stated. Correct? Correct.

So, it has speed. Let us say this speed is 0.000000................1 mph. Well, that is equal to 0, so let us instead say it has speed 0.000000000.....2 mph.

So, one must conclude therefore that it has accelerated from 0, rest, to 0.000........2 mph.

To get to 0.000......2mph, it accelerated, from 0, to 0.000..1mph, to 0.00.....2mph

Therefore, it can accelerate. It can overcome the force produced by the conveyer. In fact, for the conveyer to produce any force different to a normal runway (from which a plane can take off ALSO ), the plane must first overcome that force.

In essence, the question answers itself. I'd say it is worded fantastically.

Please, I do not see how this can be argued further? The conveyer speed matches that of the plane. Not the wheels, the plane. If you argue the conveyer has any speed, and thus any input into this question, then you have accepted that the plane is moving.
For the plane to move, it must have overcome the input of the conveyer. The input of the coneyer is governed by the planes speed. The plane will have taken off long before the conveyer reaches any real speed at which rolling resistance could even possibly be considered. The plane cannot reach a speed where rolling resistance from the wheels could be considered. Planes just can't go that fast. Even if it could reach such a speed, aerodynamic drag would have annihilated it by then. Not to mention friction.

 

[Rayden]

The problem really is that the initial question is not precise enough. However, given the poor question, I have decided that the plane can take off. My initial thought was no, the plane will be stationary and could not possibly take off due to the lack of air flow over the wings.

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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). Can the plane take off?"

The problem arises when we assume that the belt moves at a speed sufficient to keep the plane stationary. That is not stated anywhere in the problem. It says that the conveyor belt will move backwards at X mph for every X mph that the plane moves forward.

So if the plane is moving forward at 10mph the belt moves backwards at 10 mph. How much will that slow the plane down? It will not slow it down enough to make its speed zero, relative to the surrounding atmosphere. Because wheels can be considered to have minimal resistance, one can assume that the plane is barely slowed down by the conveyor belt.

NOWHERE IN THE PROBLEM DOES IT STATE THAT THE CONVEYOR BELT MOVES AT A SPEED SUFFICIENT TO REDUCE THE PLANE'S VELOCITY TO ZERO.

If it were, then I would agree that the plane could not take off. However I believe that the speed of the conveyor belt will have little to no affect on the overall forces on the plane.

Think about it from a purely physics point of view.

The plane's engine provies X pounds of thrust in a forward direction giving it a velocity V. The conveyor belt begins moving backwards with velocity V. That puts X pounds of force on the wheels. However those wheels do not transmit X to the plane. This means the plane will move forward despite the conveyor belt.


The problem is in finding a common ground over what the question states, not our physics.

 

[sao123]

I didnt read this whole thread...
but i immediately searched for the post froma certain DrPizza, and immediately knew he had it right.

I give him 5 beers.

:thumbsup::beer::beer::beer::beer::beer:

 

[tealk]

I did this last week..the plane takes off.

 

[BudAshes]

basically all this question comes down too is whether jets need wind moving past their engines to suck in enough air to take off. All the rest of it is inconsequential.

 

[HomeBrewerDude]

Depends on how wide and long the conveyor belt is. If it is wide enough and long enough then it would eventually would some of the air above it which would provide some air movement over the wings of the plane. Of course, to get enough air movement, the belt would have to be moving very fast and would probably catch on fire before the plane could take off.

 

[Armitage]

Originally posted by: Xafgoat
basically all this question comes down too is whether jets need wind moving past their engines to suck in enough air to take off. All the rest of it is inconsequential.

1. Nowhere does it say that it's a jet engine aircraft. The type of engine is completely irrelevant.
2. A jet engine doesn't rely on "wind moving past the engines" to operate.

 

[Argo]

Yes it can, the jet engines create push against the air, not the ground.

 

[LeadMagnet]

When I was on a aircraft carrier, we had to ferry a squadron of wooden Marine recon planes to the Persian Gulf, they were not equiped with the correct catapult equipment. So the ship couldn't just throw the plane to 150knots so it would fly. We didn't know if we could get enough wind over the wings for them to fly. The first plane was piloted by the squadron CO - and if he didn't make it then the others wouldn't try, and we would crane them off at a port.

So to get the planes off the ship we turned into a 25knot wind and cranked up the ship to 30knots+++ , so we had a 55knot++ wind coming over the bow. The first plane was on the back of the carrier, locked his breaks, and put his engine to full throttle. He released the breaks and the plane rolled about 50 feet of the 1000 feet available and he slowly lifted off the deck while the ship pulled out from underneath him. He then just floated off the port side and very slowly flew away.

 

[aigomorla]

this is easy to explain. just hold a piece of paper at one side of the tips. Then blow over the top of the paper. The paper lifts. That is the same principle for flight. Without air being forced over the wings, the plane can not lift.

 

[SophalotJack]

I jsut looked at this thread... didn't get to read most of it.

But my two cents... the plane would not be able to take off. There is no lift being given to the wings.

However, another plane behind it might be able to get some lift-off. But then it would surely fall back down to the ground very hard, seeing as though it merely got pushed up into a slightly higher elevation (which would be air that is not moving).

Think of it like a helicopter that has air traveling directly downward into the rotors. The rotors are simply cancelling out the downward force of the air. It's how a lot of helicopters crash when they hover too much.

 

[So]

How can there even be an argument here. Assuming a frictionless bearing on the plane wheels, the plane's forward motion is generated by the thrust from the engines against the AIR and the plane will accellerate and takeoff.

 

[SophalotJack]

So, you have to be kidding right?

There is a massive difference between moving a vehicle forward on a reverse-moving plane compared to that same vehicle taking off.


A jet powered car would do the same thing.... but we know for sure that it wouldn't take off because it's not an airplane.

Don't let the "wings" fool you all... you need "lift." Which can only be attained if wind is hitting the wings. Seeing as though the only wind generated is behind the airplane (from the jet engines), there is no lift.


The thing that the people (who think this airplane might take off) are severely not understanding is that just because a plane moves forward (countering the reverse covneyer belt) doesn't mean that the airplane's wings are getting any lift. The air in front of the plane is still perfectly still (except the air immediately around the jet intakes, and that air doesn't get to the wings at the intake speeds).


Maybe if there was a hurricane force wind blowing toward the airplane.... then it might take off.


Man, how to people not get this?

I suppose parachutes can open up on the ground based on the way most people think.

 

[So]

Originally posted by: SophalotJack
So, you have to be kidding right?

Nope.

There is a massive difference between moving a vehicle forward on a reverse-moving plane compared to that same vehicle taking off.

What is the difference. Once again, assuming frictionless wheels? The engines exert a force on the air to accelerate the plane, not on the road. The speed the ground is moving at has NO effect on the speed at which the plane moves (once again -- in theory, frictionless wheels)

A jet powered car would do the same thing.... but we know for sure that it wouldn't take off because it's not an airplane.

No, but it would run along the gound at the same speed, treadmill or not.

Don't let the "wings" fool you all... you need "lift." Which can only be attained if wind is hitting the wings. Seeing as though the only wind generated is behind the airplane (from the jet engines), there is no lift.

the 'wind' isn't being generated by the engines...and I'm not being fooled by them. The engines exert a force on the air and the plane will accelerate normally.

Maybe if there was a hurricane force wind blowing toward the airplane.... then it might take off.


Man, how to people not get this?

I suppose parachutes can open up on the ground based on the way most people think.

So, assuming frictionless bearings on the wheels...explain to me how the treadmill impedes the plane in any way.

I think you need to bust out your freshman physics book again

 

[juiio]

The original question needs to be clearer. Instead of saying

"This conveyer has a control system that tracks the plane speed ",

it needs to say:

"This conveyer has a control system that tracks the plane's wheels angular velocity"

The plane's velocity and the plane's wheel's angular velocity are not the same. The plane will still be moving forward relative to an observer (the atmosphere) even if the conveyer belt is moving backwards at a velocity equal to the wheel's angular velocity. There is no force exuded on the plane by the plane's wheels.

 

[So]

Originally posted by: juiio
The original question needs to be clearer. Instead of saying

"This conveyer has a control system that tracks the plane speed ",

it needs to say:

"This conveyer has a control system that tracks the plane's wheels angular velocity"

The plane's velocity and the plane's wheel's angular velocity are not the same. The plane will still be moving forward relative to an observer (the atmosphere) even if the conveyer belt is moving backwards at a velocity equal to the wheel's angular velocity. There is no force exuded on the plane by the plane's wheels.

Perfect! Said it better than I could have.

 

[letdown427]

All this crap about, oh it needs air over the wings to provide lift. Don't you think we know that? I mean, we all understand how a plane manages to fly. The air moving over the wings comes about from the plane moving. Tha plane does move. It does take off. There have been at least 5 analogies from other people that clearly explain why this is. But as no-one cna be bothered to read the thread anymore, cliffs are:

IT ISN'T A CAR, THE WHEELS ARE IRRELEVANT, THEY DO NOT DRIVE IT FORWARDS.

THE ENGINE'S THRUST MOVES THE PLANE FORWARDS. THIS IS INDEPENDENT OF THE CONVEYER.

THE PLANE CAN TAKE OFF

 

[So]

Originally posted by: letdown427
<snip>

IT ISN'T A CAR, THE WHEELS ARE IRRELEVANT, THEY DO NOT DRIVE IT FORWARDS.

THE ENGINE'S THRUST MOVES THE PLANE FORWARDS. THIS IS INDEPENDENT OF THE CONVEYER.

THE PLANE CAN TAKE OFF

Did you mean to edit?

 

[letdown427]

I thought I'd post that again as no-one is going to read all of the posts in the thread, and I don't presume for a second anyone will even bother reading all of that, so i thought I'd try to get the point across at the bottom, in the hope people will maybe see the fact that they're thinking of the plane as a car with driven wheels and see the error of their ways.

 

[So]

Originally posted by: letdown427
I thought I'd post that again as no-one is going to read all of the posts in the thread, and I don't presume for a second anyone will even bother reading all of that, so i thought I'd try to get the point across at the bottom, in the hope people will maybe see the fact that they're thinking of the plane as a car with driven wheels and see the error of their ways.

Sory, but you probably should trim of the repeated stuff. I for one, saw it moments before you reposted. The last part, which was new was good though.

 

[g8wayrebel]

I read all the posts.
You are still wrong.
The plane would never have forward motion. If you are on a treadmill running 11 m/s (just under the world record) does the wind blow your hair back and your hat off? NO , it does not. There is no wind.
A plane , no matter the propulsion system, would not have any either.
It would initially move slightly forward to cause the conveyor to start according to the OP who says the conveyor "matches the speed " of the plane. From that point forward , the forward motion of the plane would be negated by the rearward motion of the conveyor.
The fact that the plane has wheels merely allows it to roll and accelerate on the conveyor faster.. Even if it were on it's belly , if it had enough thrust , it would begin to move and start the conveyor.
From that point forward , the result would be the same.It would be sationary relevent to the atmosphere and never develop lift.
THIS BIRD WON"T FLY!

This has been an extremely interesting question.. I am going to take this to another forum and see if I get any answers worth repeating.
BTW , I have a treadmill and am trying to convince a friend of mine to bring an RC plane over to prove the point. I will not stand in front of it , but don't expect any damage to plane or basement in the process. On second thought , guess I'll move the treadmill outside. There is more air out there. Yeah that's it , there's more air outside. LOL
If this takes place (doubtful) I will put it on video for all.