You have a plane and a conveyor belt.

[blahblah99]

Originally posted by: TitanDiddly
Do I need to get a video interview from a flight instructor with a doctorate in aeronautical science so I can put this thread to rest?

No, all you need explain to them is F=MA. but apprently they are having trouble comprehending even that simple formula.

 

[illusion88]

I'm always the last one to the party.
:brokenheart:

 

[loic2003]

Originally posted by: Tom

Originally posted by: loic2003
Idiots. The wheels would turn twice the speed as they would upon a normal takeoff, but the aircraft would take off anyway. This is ignoring the fact that the wheel bearings/tyres might not take all that speed.

How can anyone even be confused by this?

Stand still and throw a paper plane. It flies the speed your arm throws it.
Stand on a conveyorbelt and the plane will fly at (speed of your arm) - (conveyor speed)
Walk against the conveyor so your overall speed is actually 0 and throw. Plane flies at the same speed as #1.

Simple.

Your description of walking on a treadmill and throwing a plane is not the same, which is why you get a different result. In your scenario, the person takes the place of the plane, not the paper airplane.

In the original question, suppose the pilot throws a paper airplane out the window, that would be what you are describing, the fact that his paper airplane might fly, doesn't make the full size plane take off.

I won't call you an idiot for your mistake though.

As far as the wheel bearings issue, this is another example, like many in this thread, of confusing a hypothetical scenario with given parameters, with what would happen in the real world. It isn't relevant.

No, you've understood incorrectly. In my example, the person represents the aircraft's undercarriage (wheels), the paper plane represents the aircraft. My point is, that the wheels compensate for any movement of a conveyor.

Lets use another example. Imagine a small boy with a little model aircraft that has rolling wheels. The boy sits on the floor, rolls the aircraft along the floor, then lifts it into the air (complete with a 'neeeyyyooow!' sound for effect). In this example, the boy's hand represents the thrust of the aircraft, and him picking it up represents the lift.

So the kid turns around and sees a running machine that's running. First off he holds the plane steady on it. The only force pushing his hand backwards is the friction of the wheels and bearings against the surface of the treadmill. We'll call this negligable force.
Then the kid pushes the plane forward on the treadmill, it still requires the same force as when he did it on the floor (apart from above friction from wheels) and he can still lift the plane up off the treadmill as the force required is not reliant on the wheels at all. The force pushing the aircraft forward comes from the engines of the plane (kid's hand), so it makes no difference.

I really can't make this any simpler or clearer. If you still struggle to understand this increadibly simple scenario, I think you need to go back to school or something. Seriously.

 

[loic2003]

I think someone is taking the p!ss...

 

[Jack Ryan]

Originally posted by: PurdueRy

Originally posted by: KMDupont64

Originally posted by: GuitarDaddy

Originally posted by: KMDupont64
Soooo, you are in a seaplane aiming up river (against the current), you need 100 mph of airspeed to take off, the speed of the water is exactly whatever speed (in the opposite direction) you would be going if the water wasnt moving. (If you had enough thrust to go 50MPH, the water would move 50MPH the other direction).

How does the plane take off?

In this example the plane wouldn't take off. The difference in this example and the one in the OP is this plane has stationary pontoons that tranfer the force of the moving water to the plane. In the OP the plane has free spinning wheels, therefore the force of the conveyor belt is absorbed by the spinning wheels and not transfered to the plane.

Thank you.

You do realize you just supported the plane taking off in the OP's case right? Maybe that's what you were supporting...but I always thought you disagreed

Yes, and I am glad that you got what I was trying to say. In theory, the plane will take off. I was simply changing the "visual" of the question to prove that in real life, this is a ridiculous question and would not work.

 

[quakefiend420]

Originally posted by: D1gger
The wheels are not driving the plane forward, the engines are, so the plane moves forward until the air passing over the wings gives it enough lift to take off. The wheels spinning on the conveyor belt have absolutely nothing to do with the physics of a plane flying.

bingo...why is this still being discussed?

and no way i'm wading through 550 replies to figure it out

 

[Jack Ryan]

Originally posted by: Chiropteran

Originally posted by: GuitarDaddy

Originally posted by: KMDupont64
Soooo, you are in a seaplane aiming up river (against the current), you need 100 mph of airspeed to take off, the speed of the water is exactly whatever speed (in the opposite direction) you would be going if the water wasnt moving. (If you had enough thrust to go 50MPH, the water would move 50MPH the other direction).

How does the plane take off?

In this example the plane wouldn't take off. The difference in this example and the one in the OP is this plane has stationary pontoons that tranfer the force of the moving water to the plane. In the OP the plane has free spinning wheels, therefore the force of the conveyor belt is absorbed by the spinning wheels and not transfered to the plane.

I don't agree. The example is impossible and flawed.

Water moves at a constant velocity.

The force provided by a plane's engines moves the plane with constant acceleration.

There is no such thing as "enough thrust to move forward at 50mph". There could be enough thrust to accelerate at 500mph per minute, but a force will not give a constant velocity until it is canceled out by friction.

In theory the water could prevent the plane from moving forward through friction if it was moving much much faster than the plane was trying move, but the water moving 50mph isn't going to slow the plane down by 50mph.

This is where you need to take the Einstein wannabe hat off and look at real life. How much juice do I need to give the engine to go 50 mph??? That is a valid question. There is an upper limit to how fast a plane can go.

That being said, real life has friction/gravity. If in the original post the plane is some magical thing that is not pulled down by gravity and the wheels simply "touch" the conveyor belt, then yes, the plane will take off. Not that hard to understand.

But in real life, where a plane weighs quite a bit and requires that friction to be placed on the wheels to roll forward to a speed fast enough to take off, a conveyor belt will prevent that plane from taking off.

In my opinion, everyone is right. If the plane can take off on the conveyor belt in real life, why do planes have wheels today?

 

[Jack Ryan]

As I read through some of the posts, I believe there are a few people get the fact that the wording of the question is a little different than what most of you googled the answer to. Bravo to you who dared to use reading comprehension!

 

[Gdepp519]

lol.. these peoples comments are hilarious

 

[Chiropteran]

Originally posted by: KMDupont64
This is where you need to take the Einstein wannabe hat off and look at real life. How much juice do I need to give the engine to go 50 mph??? That is a valid question.

No, it isn't. In reality, a plane that wants to go, for example, 50mps applies force to accelerate to 50mph and then reduces that force to just enough to overcome friction. Constant force = constant acceleration, not any singular velocity. This isn't general relativity, this is basic physics stuff you were taught in 4th grade.

Originally posted by: KMDupont64
There is an upper limit to how fast a plane can go.

Due to friction. If you add friction to the equation, the whole problem breaks down because it is impossible for a treadmill to move arbitrarily fast if you include friction.

 

[aigomorla]

actually, i reread the question and i take back my original answer. This question is different from the original one presented.

Anyhow the answer is NO. IF the thurst emited from the rear of the plane, the trust will not provide air to flow under the wings. THE QUESTION EVEN STATES THERE IS NO AIR BEING FORCED UNDER THE WINGS without air being forced under the wings, lift can not happen.

If you still think the plane can take off, think of it this way, a Jet fighter has thurst located in the rear, yet when on a air craft carrier, the carrier must be facing the directing of the wind and then sling shoted off the carrier. If you try it the other way arround, with the wind, the jet would probably be shot right off the carrier and into the sea.

 

[jagec]

Originally posted by: aigomorla
actually, i reread the question and i take back my original answer. This question is different from the original one presented.

Anyhow the answer is NO. IF the thurst emited from the rear of the plane, the trust will not provide air to flow under the wings. THE QUESTION EVEN STATES THERE IS NO AIR BEING FORCED UNDER THE WINGS without air being forced under the wings, lift can not happen.

If you still think the plane can take off, think of it this way, a Jet fighter has thurst located in the rear, yet when on a air craft carrier, the carrier must be facing the directing of the wind and then sling shoted off the carrier. If you try it the other way arround, with the wind, the jet would probably be shot right off the carrier and into the sea.

No, they would still fly unless the wind was REALLY strong, but that's a terrible example because figher jets have an insanely high stall speed. Besides, you're implying that fighters can't take off on a calm day :roll:

The plane moves forward...therefore air passes under the wings...therefore it takes off.

 

[SlitheryDee]

Originally posted by: aigomorla
actually, i reread the question and i take back my original answer. This question is different from the original one presented.

Anyhow the answer is NO. IF the thurst emited from the rear of the plane, the trust will not provide air to flow under the wings. THE QUESTION EVEN STATES THERE IS NO AIR BEING FORCED UNDER THE WINGS without air being forced under the wings, lift can not happen.

If you still think the plane can take off, think of it this way, a Jet fighter has thurst located in the rear, yet when on a air craft carrier, the carrier must be facing the directing of the wind and then sling shoted off the carrier. If you try it the other way arround, with the wind, the jet would probably be shot right off the carrier and into the sea.

Specifically the OP says that the SOURCE OF THRUST doesn't provide the airflow. It's the consequent opposite movement of the plane that does that.

Another dumb analogy:

Imagine you're firing a high powered rifle. There's a very common term associated with firearms that deals with the force the shooter is subjected to every time they fire. That term is recoil.

Thinking that the source of thrust on the airplane must be blowing air over the wings for the plane to take off is akin to thinking that you'd have to be shooting yourself to feel the force of the gun. The thrust doesn't directly create the lift.

It just pushes the plane through the air fast enough for lift to occur. Think of it as like therecoil of a gun only constant.

 

[MBrown]

No. There would be no lift.

 

[spidey07]

To use another example...

You can all the thrust in the world, but the conveyor is acting like a big rope holding the plane in place.

plane can't move, it's stated in the scenario that it can't. by definition, it can't.

 

[SlitheryDee]

Originally posted by: spidey07
To use another example...

You can all the thrust in the world, but the conveyor is acting like a big rope holding the plane in place.

plane can't move, it's stated in the scenario that it can't. by definition, it can't.

Where does it say that the plane can't move? It says that there's a conveyer belt moving in the opposite direction at the same speed that the wheels are spinning. That DOES NOT restrict the plane's movement.

 

[Banzai042]

Originally posted by: spidey07
To use another example...

You can all the thrust in the world, but the conveyor is acting like a big rope holding the plane in place.

plane can't move, it's stated in the scenario that it can't. by definition, it can't.

Um, how? All i see is that the conveyor belt is moving backwards at the exact same speed as the plane moves forward. Once again, i present the following analogy for your consideration: You take a piece of rolling luggage and put it on a moving walkway moving at 5 mph, you then walk beside the walkway (on non moving ground) in the opposite direction at 5 mph while holding onto the luggage. Now, as you are moving and pulling the luggage along, is the luggage moving? Answer: It very obviously does, why? Because the wheels are free spinning, and provide very very little friction relative to the external force being exerted to move it forward. The airplane is in an IDENTICAL situation. The force that the engines exert on the wings can be considered an external force on the wings, thus the plane will accelerate forward and will eventually reach takeoff speed

 

[Tom]

As one of the people who argued vociferously the plane doesn't move, until I read a post with sufficient clarity to bring me out of my fog..


Forget about the conveyor belt. The engines will push/pull the airplane through the air, unless it is fixed to an immoveable object.

The conveyor belt would have no effect on this.

 

[rizzaz]

THE PLAIN DOES NOT TAKE OFF IN THE SCENARIO DESCRIBED!!!!!!

ok slightly different way of looking at it...

If the plane was stationairy, engines off - no thrust, and the conveyor belt starts to move backwards, the plane would move with it i'd say, all agree? hence the plane starts going backward, think of it like a wheelchair on a conveyor belt instead

then the planes engine starts up providing thrust.. which pushes the plane forward.. like someone pushing the wheel chair, the harder you push the faster it goes..

at a certain point the forward push equals the conveyor belt speed... like pushing a wheel chair whilst standing on a running machine.... hence you are effectively stationary

at this point we reach the senario described

"The belt compensates for the rotation of the wheels in reverse, as in the belt moves in reverse exactly as fast as the wheels move forward"



However the planes engines can push really hard ( harder that you could push a wheelchair!) so it can keep getting faster until it runs down the runway and takes off like normal, its no different to a normal take off except that you would need a longer runway!!

Except now we have exceeded the described senario.. the planes wheels are rotating faster than the runway.

so yes the plane could take off.. except it doesn't because all the thrust it provides is compensated for by the conveyor belt.. it pushes harder, the conveyor speeds up etc..

it's like pushing a wheel chair on a running machine which automaticaly speeds up as you run faster....


Yes the friction in the wheels is low but its still there!! and the mass of the plane is a big factor.. the matbox car example is decieving... it's easy to push a matchbox car, but can you push a plan along??


its a very silly idea anyway....

 

[cKGunslinger]

Originally posted by: spidey07
To use another example...

You can all the thrust in the world, but the conveyor is acting like a big rope holding the plane in place.

plane can't move, it's stated in the scenario that it can't. by definition, it can't.

Incorrect. Did you guys even read the Matchbox car + treadmill + finger analogy? It very accurately describes this scenario in a real-world, try-it-yourself application.

Treadmill is moving with a free-wheeled matchbox car sitting on it. The car would move backwards (due to friction) unless you very gently touch your finger to the top of the car. Now you are holding the car in place with very little effort.

Now, if the treadmill is moving at 20 MPH, are you in turn having to move the car 20 MPH to match the speed? No, you are simply holding the car still and the wheels are turning at 20 MPH.

Now, the the treadmill is moving at 200 MPH, are you in turn having to move the car forward at 200 MPH to match the speed? No, you are simply still holding the car still and the wheels are turning at 200 MPH.

Now, you want to push the car forward. Do you have to push the car faster than 200MPH to overcome the treadmill? No, you can simply push gently on the car (moving you finger/hand/arm at a very slow speed) and it (the car) will move forward, relative to both the world and it's location on the treadmill (middle to front, etc). The reason for this is that you are acting on the body of the car, and not the wheels.

No, you want to push that car faster still, do you need to manually push the car at 210MPH to overcome the treadmill and reach 10MPH? NO!

Now imagine if you had a mini-jet engine attached to the car. It acts the same as your finger. It does not need infinite amounts of thrust to overcome the treadmill speed, because the treadmill speed is irrelevant. Even if the treadmill continually accerates, it will only make the wheels spin faster - you will not have to magically counteract the increasing speed with you hand or the jet engine.

3 Pages again? Jeez, people.

 

[Kev]

Originally posted by: spidey07
To use another example...

You can all the thrust in the world, but the conveyor is acting like a big rope holding the plane in place.

plane can't move, it's stated in the scenario that it can't. by definition, it can't.

The conveyor acts upon the wheels. The wheels have nothing to do with affecting the motion of the plane whatsoever. Think of them as being there for support only.

Now if you designed some kind of retard plane that took off powered by its wheel movement and then immediately crashed upon liftoff, then yeah, the conveyor would hold that retarded plane in place. But since this is a non-retard plane, the thrust will cause the plane to move forward REGARDLESS OF ANY FORCE ACTING UPON THE WHEELS.

This is mind bogglingly simple to understand and it really makes me said that it takes 500+ posts for some of you dimwits.

 

[Kev]

Originally posted by: cKGunslinger

Originally posted by: spidey07
To use another example...

You can all the thrust in the world, but the conveyor is acting like a big rope holding the plane in place.

plane can't move, it's stated in the scenario that it can't. by definition, it can't.

Incorrect. Did you guys even read the Matchbox car + treadmill + finger analogy? It very accurately describes this scenario in a real-world, try-it-yourself application.

Treadmill is moving with a free-wheeled matchbox car sitting on it. The car would move backwards (due to friction) unless you very gently touch your finger to the top of the car. Now you are holding the car in place with very little effort.

Now, if the treadmill is moving at 20 MPH, are you in turn having to move the car 20 MPH to match the speed? No, you are simply holding the car still and the wheels are turning at 20 MPH.

Now, the the treadmill is moving at 200 MPH, are you in turn having to move the car forward at 200 MPH to match the speed? No, you are simply still holding the car still and the wheels are turning at 200 MPH.

Now, you want to push the car forward. Do you have to push the car faster than 200MPH to overcome the treadmill? No, you can simply push gently on the car (moving you finger/hand/arm at a very slow speed) and it (the car) will move forward, relative to both the world and it's location on the treadmill (middle to front, etc). The reason for this is that you are acting on the body of the car, and not the wheels.

No, you want to push that car faster still, do you need to manually push the car at 210MPH to overcome the treadmill and reach 10MPH? NO!

Now imagine if you had a mini-jet engine attached to the car. It acts the same as your finger. It does not need infinite amounts of thrust to overcome the treadmill speed, because the treadmill speed is irrelevant. Even if the treadmill continually accerates, it will only make the wheels spin faster - you will not have to magically counteract the increasing speed with you hand or the jet engine.

3 Pages again? Jeez, people.

This is a really good example and anyone who has trouble understanding after reading this should honestly just give up.

 

[jagec]

Originally posted by: rizzaz
Yes the friction in the wheels is low but its still there!! and the mass of the plane is a big factor.. the matbox car example is decieving... it's easy to push a matchbox car, but can you push a plan along??

Copy/paste from another site. Friction is VERY small. I suggest a rule where anyone arguing the plane CAN'T take off has to show their calculations, instead of just reasoning it out in their heads.

I found the formula's for all the people saying the wheel resistance wouldn't allow the plane to take flight.

The rolling resistance is found roughly by a formula like this: the weight in newtons (1lb = 4.45 newtons) divided by the number of wheels, times actually wheel resistance (usually below 4% or 0.04), times the speed in meters per second (1mph = .45 meters/hour), equals the resisting force.
rr=(weight/wheels)x(.04)x(speed)

The force of thrust to move a plane is found by the formula like this: mass in newtons times speed in meters/hour equals the force.
f=(weight)x(speed)

A plane weighing 1,335,000 newtons (300,000 lbs) divided by 3 wheels multiplied by the wheel resistance .04 and then multiplied by the speed of 4.5 meter/hour (10 miles/hour) has a rolling resistance of 80,100 newtons (18,000 lbs) acting against it. The conveyor moving in the opposite direction at the same speed of 4.5 meter/hour this would double the wheel speed and make it an opposing force of 160,200 newtons (36,000 lbs) acting against it.
rr=(1335000/3)x.04x(4.5+4.5)=160200

A plane weighing 1,335,000 newtons moving at 4.5 meter per seconds has a forward force of 6,007,500 newtons.
f=1335000x4.5=6007500

Therefore the forward force of 6,007,500 newtons minus the rolling resistance of the conveyor going in the oppose direction is 160,200 newtons leaving a forward force of 5,847,300 newtons.
(f=6007500)-(rr=160200)=5847300

The opposing force, on the plane will never equal the opposing force applied to the wheels. Therefore the opposing force on the plane by the conveyor will subtract from the forward force but not equal it.


And just for everyone, don't nit pick on the spelling or technicalities like "the propellers actually don't push on air... " whatever! I am just trying to get the point across.

I have come to the conclusion that there is a select group of people(atl5p, fargo boyle) arguing the scientific data and laws of physics just for the fun of seeing all the people argue. If you are doing that, stop it. I can't help of think of the character in dogma who knows that god exits talking a nun out of her faith.

 

[JujuFish]

Originally posted by: rizzaz
"The belt compensates for the rotation of the wheels in reverse, as in the belt moves in reverse exactly as fast as the wheels move forward"



However the planes engines can push really hard ( harder that you could push a wheelchair!) so it can keep getting faster until it runs down the runway and takes off like normal, its no different to a normal take off except that you would need a longer runway!!

Except now we have exceeded the described senario.. the planes wheels are rotating faster than the runway.
so yes the plane could take off.. except it doesn't because all the thrust it provides is compensated for by the conveyor belt.. it pushes harder, the conveyor speeds up etc..

I bolded the part where you've gone astray. Look closer at the sentence you quoted. It does not say that the the conveyor matches the angular velocity. "...as the wheels move forward" can only be logically read as the linear velocity with respect to the surroundings/air.

 

[rizzaz]

Originally posted by: jagec

Originally posted by: rizzaz
Yes the friction in the wheels is low but its still there!! and the mass of the plane is a big factor.. the matbox car example is decieving... it's easy to push a matchbox car, but can you push a plan along??

Copy/paste from another site. Friction is VERY small. I suggest a rule where anyone arguing the plane CAN'T take off has to show their calculations, instead of just reasoning it out in their heads.

I found the formula's for all the people saying the wheel resistance wouldn't allow the plane to take flight.

The rolling resistance is found roughly by a formula like this: the weight in newtons (1lb = 4.45 newtons) divided by the number of wheels, times actually wheel resistance (usually below 4% or 0.04), times the speed in meters per second (1mph = .45 meters/hour), equals the resisting force.
rr=(weight/wheels)x(.04)x(speed)

The force of thrust to move a plane is found by the formula like this: mass in newtons times speed in meters/hour equals the force.
f=(weight)x(speed)

A plane weighing 1,335,000 newtons (300,000 lbs) divided by 3 wheels multiplied by the wheel resistance .04 and then multiplied by the speed of 4.5 meter/hour (10 miles/hour) has a rolling resistance of 80,100 newtons (18,000 lbs) acting against it. The conveyor moving in the opposite direction at the same speed of 4.5 meter/hour this would double the wheel speed and make it an opposing force of 160,200 newtons (36,000 lbs) acting against it.
rr=(1335000/3)x.04x(4.5+4.5)=160200

A plane weighing 1,335,000 newtons moving at 4.5 meter per seconds has a forward force of 6,007,500 newtons.
f=1335000x4.5=6007500

Therefore the forward force of 6,007,500 newtons minus the rolling resistance of the conveyor going in the oppose direction is 160,200 newtons leaving a forward force of 5,847,300 newtons.
(f=6007500)-(rr=160200)=5847300

The opposing force, on the plane will never equal the opposing force applied to the wheels. Therefore the opposing force on the plane by the conveyor will subtract from the forward force but not equal it.


And just for everyone, don't nit pick on the spelling or technicalities like "the propellers actually don't push on air... " whatever! I am just trying to get the point across.

I have come to the conclusion that there is a select group of people(atl5p, fargo boyle) arguing the scientific data and laws of physics just for the fun of seeing all the people argue. If you are doing that, stop it. I can't help of think of the character in dogma who knows that god exits talking a nun out of her faith.

He calculates the force the conveyor exerts at 36000 lbs! thats quite a bit!! I think this calculation is ~correct even though he states 4.5 meters per hour instead of meters per second.

then he messes up he says force = mass * speed.... which it doesn't... force= mass *acceleration!!!!!

http://www.grc.nasa.gov/WWW/K-12/airplane/newton2.html


so he's all confused, don't just copy and paste indescriminantly!!