A plane on a runway, how smart is r3vlimited?

[dirty30]

who is on the plane?

[Fidhle007]

Wait, I'm changing my mind. I don't think it will take off at all. As a matter of fact I think it might go backwards...

:p

[DarkWing6]

400th Post
This should hopefully end it for you guys.

http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

Airplane on a Conveyor Belt

A riddle was proposed on the Neal Boortz show today:
If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots, will the plane be able to take off, or will it just sit stationary relative to the ground, with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?
Astoundingly, Neal and the rest of his crew took the position that the plane would sit there stationary! Good God… this man is a pilot and has a law degree! I could understand a random high school dropout being fooled by this, but a pilot?
Then I googled the riddle, and found a thread on Airliners.net that has been raging on, with the vast majority of people taking Neal’s position… that the plane would not be able to take off.


Their argument is this, to quote one poster:

Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, its providing an exactly CANCELLING force pushing the aircraft back.

The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.
Gah… people are freakin’ stupid.
Update: There is a variation on this riddle that says that the conveyor belt matches the speed of the plane. It doesn’t matter… the plane still takes off. The conveyor belt could be going 5 times as fast as the plane, and the plane would still take off. You’d get into issues about tires blowing out, but assuming that the wheels can take the strain, the airplane would still take off.

[DarkWing6]

This is somewhat true considering that the what I've been arguing is that the weight of the plane on the wheels will determine friction.

And yes.. Wheel size DOES matter because they are acting as a gear in a sense.

But it is not like a car beacasue it is not using the ground to propel itelf forward. The reason a car will not move forward is beacsue it uses the ground to propel itself forward and the treadmill would zero that GROUND SPEED out. A plane uses the air to propel itself forward so the ground under it will not be working directly against what is creating the forward motion of the airplane.

Friction and gravity so come into play, but only very minimally. They will slightly slow the plane down, but not enough to cause it not to move forward.

[DarkWing6]

How many of you people making arguments have taken (and passed) a basic class on physics?

I have! Pretty much the only science class I was ever good at. :D

[RobertK]

My only issue with equate's explanation is this:

All of the force from the jets are on the wheels, as if.. gasp.. it was like a car.

This is somewhat true considering that the what I've been arguing is that the weight of the plane on the wheels will determine friction.

And yes.. Wheel size DOES matter because they are acting as a gear in a sense.

How many of you people making arguments have taken (and passed) a basic class on physics?

[DarkWing6]

Nobody listen to Ben. He understands but wants to keep this thread going, so he keeps arguing.

[BENdashdash]

My only issue with equate's explanation is this:

All of the force from the jets are on the wheels, as if.. gasp.. it was like a car.

[gstuning]

The final post.

Rules of the world.
Bearings create friction.

Part 1.
airplane is tied to a rope wich is tied to a weight or strain gauge wich is then tied to the solid ground, it´s also sitting on a conveyor belt,.
By setting the belt in action two things happen.
The plane WILL sit still as it´s tied to the ground,
The strain gauge will show added force from the plane as the friction of the wheels is trying to pull the plane backwards, as the tires are rolling backwards because the belt is rolling backwards.
This we all know and agree on,

What we know now is that the conveyor belt exerts backwards motion to the plane by the bearing friction,

Part 2.
Everything is the same, now if you where to put the conveyor belt to the airplanes double take off speed only backwards as before,
Now the wheels should be putting more backwards motion to the plane but it´s still tied to the ground, so it´s not going anywhere.

Part 3

Now we need to know if the planes engines have enough thrust to overcome the backwards motion and relieve the strain from the rope and gauge,
i.e to overcome the backwards motion of the belt, wich is turning at the planes double take off speed. (double take off speed is the same wheel speed if the belt is always moving at the same speed backwards as the plane is moving forward when it´s trying to take off)

Part 4 and Conclusion
If the planes thrust can overcome the belts speed backwards.
The plane as has overcome the friction and is thus capable of flying .

There is no dissputing this as all the variables have been covered.

The only reason the airplane will not fly is if it can not overcome the friction of the wheel bearings going backwards at twice the take off speed as is required of the original question.

Now I´m going to assume that ALL planes have this cabability as they all easily enough over come the bearings at the take off speed anyway.
And thus the final is that indeed the plane will fly.

Now there are no othere variables in this but the bearings friction as the weight of the airplane is the same as on normal ground. and the take off speed does not change, nothing is diffrent exept the speed of the wheels.

Now you can take that to the bank and cash it

Don´t post anymore just re read it until you DO understand

[Fidhle007]

...Wheel bearing fricition will keep it from moving...:drink:

I even proved in that the plane could move forward with my car example in my earlier thread. I guess it's all a matter of how you look at the question, really.

[Justin B]

Ive been saying all along it isn't and why, but "they" choose not to listen. =s I think this is why the other "plane" threads get so damn big. This one is certainly on its way (+1)

[Fidhle007]

I'm still not sure why this is so complicated...

Nice example though, equate.

[Justin B]

The plane would take off, this is why.

First off, what is the point of landing gear with wheels? Think about this, lets say a plane (747 for argument sake) is trying to take off, but its just laying on its belly with no landing gear or wheels. Whats going to happen when it tries to take off? The plane won't, why? Friction, Imagine those engines that normally propel the plane down the runway on nice rolling wheels trying to push a metal hog on its belly scraping the ground, the plane would never get up to speed to take off. Imagine pushing your car in neutral vs. pushing your car while its on the ground with no wheels on, you probably wont move it anywhere by hand.

Second, planes don't just "lift off" and fly, again for arguments sake lets say this plane has to get up to 150mph to take off. This is how it would work: This plane on a normal runway, would be at the end. Assuming 0 wind (and I am sure lacking a few variables here but again just to demonstrate a point bear with me) the plane would have to get up to 150mph, to get 150mph of wind across its wings to get enough lift to leave the ground.

So this is why we have wheels, just to take away friction to get the plane up to 150mph as quickly and as efficiently as possible.

And we can all agree on that wheels DON'T propel the plane at all (like the rear wheels of a honda ;) ) They are there just to hold the plane up.

Now, exchange that runway for a treadmill. It's supposed to be setup like this: If the plane were going 150mph the belt would go 150mph in the opposite direction. We all can agree on that.

So what causes a plane to fly? Lift, we all again can agree for this plane we need 150mph of wind across the wings to cause sufficient lift to allow the plane to take off.

Put the plane at one end of the runway, the belt is still since the plane is not moving.

Now as the plane starts to thrust its engines and try to get up to speed, the belt spins faster and faster. However, its really irrelevant how fast the belt spins.

Like I said before, the point of the wheels is not have friction on the plane so it can go down the runway as fast as possible, and just hold the plane up.

So, the airplane on the belt, its still being held up by the wheels (although they are spinning really fast now) and it still does not have any abnormal friction of drag, it would be the same as if it were on a runway.

The airplane would be able to engage its engines and take off since the wheels have no effect on the speed of the plane, again they are only there to hold the plane up and reduce friction so the plane can actually take off.

Think of it like this. What about those planes that land and take off in water? Lets put one of them on the belt. But under the plane lest say it has an infinite amount of Petroleum Jelly or something, assuming this would not cause enough friction to prevent the plane from taking off what would happen? The plane again would engage its engines and just slide on the jelly until it got up to enough speed to take off.

This is super hard to explain, but you have to think of the wheels and the plane as 2 separate things.

Like I said, the speed of the belt is irrelevant. It could be going at 1mph or 10,000 mph, the wheels would still spin, they would still hold the plane up, and it would still not have any friction to allow the plane to take off.

Very nice explanation. Just what we have been saying all along though in different examples and situations. Hopefully they'll understand that one of these days. If not, we'll end up with the longest r3v thread evar bickering back and forth, not that I would have a problem with that at all or anything. Post counts! LOL

Nice to see some are finally posting up as much info as I have been, all I can do for this though is just say ....

see?

[E30 Reaktionär]

here is another picture for this thread

[equate975]

The plane would take off, this is why.

First off, what is the point of landing gear with wheels? Think about this, lets say a plane (747 for argument sake) is trying to take off, but its just laying on its belly with no landing gear or wheels. Whats going to happen when it tries to take off? The plane won't, why? Friction, Imagine those engines that normally propel the plane down the runway on nice rolling wheels trying to push a metal hog on its belly scraping the ground, the plane would never get up to speed to take off. Imagine pushing your car in neutral vs. pushing your car while its on the ground with no wheels on, you probably wont move it anywhere by hand.

Second, planes don't just "lift off" and fly, again for arguments sake lets say this plane has to get up to 150mph to take off. This is how it would work: This plane on a normal runway, would be at the end. Assuming 0 wind (and I am sure lacking a few variables here but again just to demonstrate a point bear with me) the plane would have to get up to 150mph, to get 150mph of wind across its wings to get enough lift to leave the ground.

So this is why we have wheels, just to take away friction to get the plane up to 150mph as quickly and as efficiently as possible.

And we can all agree on that wheels DON'T propel the plane at all (like the rear wheels of a honda ;) ) They are there just to hold the plane up.

Now, exchange that runway for a treadmill. It's supposed to be setup like this: If the plane were going 150mph the belt would go 150mph in the opposite direction. We all can agree on that.

So what causes a plane to fly? Lift, we all again can agree for this plane we need 150mph of wind across the wings to cause sufficient lift to allow the plane to take off.

Put the plane at one end of the runway, the belt is still since the plane is not moving.

Now as the plane starts to thrust its engines and try to get up to speed, the belt spins faster and faster. However, its really irrelevant how fast the belt spins.

Like I said before, the point of the wheels is not have friction on the plane so it can go down the runway as fast as possible, and just hold the plane up.

So, the airplane on the belt, its still being held up by the wheels (although they are spinning really fast now) and it still does not have any abnormal friction of drag, it would be the same as if it were on a runway.

The airplane would be able to engage its engines and take off since the wheels have no effect on the speed of the plane, again they are only there to hold the plane up and reduce friction so the plane can actually take off.

Think of it like this. What about those planes that land and take off in water? Lets put one of them on the belt. But under the plane lest say it has an infinite amount of Petroleum Jelly or something, assuming this would not cause enough friction to prevent the plane from taking off what would happen? The plane again would engage its engines and just slide on the jelly until it got up to enough speed to take off.

This is super hard to explain, but you have to think of the wheels and the plane as 2 separate things.

Like I said, the speed of the belt is irrelevant. It could be going at 1mph or 10,000 mph, the wheels would still spin, they would still hold the plane up, and it would still not have any friction to allow the plane to take off.