Airplane and Conveyor Belt Debate

[simon009988]

i not sure what u mean but i think the plane wouldn't take off because a plane only files because there is a difference in air pressure on the wings and if only the runway is moving and not the air molecules the plane would stay still (if only the tires were moving exactly in opposite to the runway)

 

[mathman]

The ability of a plane to take off depends on its speed relative to the air, so what is happening on the ground is relevant only as it affects air speed. An example of this principle is taking off from an aircraft carrier. The carrier is pointed against the wind and the plane takes off against the wind. Similarly at airports the preferred direction for takoff is against the wind. An analogous logic is used for landing, also against the wind, to get minimum ground speed for given airspeed.

 

[mewmew]

I understand that, I know how a plane flys(well, the basics atleast) and know that this problem comes down to if the plane can move or not. I, as well as others I have talked to, say that the conveyor belt will only make the wheels spin faster but have little effect on the plane. The wheels aren't locked like on a car, so the plane can generate forward movement relative to the Earth independent of how fast the wheels move. Say the plane is going 100 m/s relative to the earth, the coveyerbelt is going -100 m/s relative to the earth, so the wheels, or a point on the wheel, should spin at 200 m/s, if this was a car it would be easier, but because its a plane the situation is harder to imagine.

 

[berkeman]

Who cares what the wheels are doing. If the plane is not moving relative to the Earth and the associated air, the wings cannot generate lift. This seems like a pretty straightforward puzzle. Why are you guys debating it somewhere? Care to make a little wager... :-)

 

[mewmew]

It is not as easy as you make it sound, just because the belt moves backwards and the plane is moving forward(with equal and opposite speed) doesn't mean the plane is going to feel the backwards motion, because it has wheels independent of its engine that can spin however fast they want. It isn't really a straight forward question.

 

[berkeman]

Wanna bet? You got a PayPal account? :-)

 

[mewmew]

No where in the question does it say the plane is stationary in respect to the earth.

Here is what I am thinking:

The moving belt should only cause a small change in the planes speed, but it will cause the wheel of the plane to spin. If you have a treadmill and something on wheels you can try it, as it is easier to imagine or even actually do. If you turn on the treadmill you can put something on it that has wheels like a tonka truck and then with little force move it in the opposite direction the treadmill is moving, can you not? If this is the case than certainly the plane could do it because its engine is just like your hand and then it could of course take off.

I am more interested in what the forces all are that cause this to happen and what is going on with those forces, as I don't really understand all of that part of the question.

 

[Danger]

How bloody many threads do we have on this thing, anyhow?
The plane will take off unless the belt is moving so fast that the wheel bearings seize up.

 

[berkeman]

No where in the question does it say the plane is stationary in respect to the earth.

Well, I guess it was where you said this:

Suppose you have an airplane on a runway that is a huge conveyor belt. The conveyor belt moves with the opposite velocity of the plane at all times, what happens when it tries to take off?

"Opposite velocity at all time" is the part where I guess you threw me. You need to define your reference frame better (the earth?), and define what the velocity of the treadmill is with respect to it, and what the velocity of the plane is with respect to the reference frame.

 

[RandallB]

So on another forum we are in a huge argument

I hope you’re talking about some other forum web site, not double threading on this one.

Now, I think we have ...
one in which the runway moves with the opposite speed tangental to the outside of the tire.

No this would be the same a stationary runway, the tangential speed of the tire in contact with the runway is always ZERO. The top of the tire would be moving forward at double the speed of the axle. Tires don’t leave skid marks on takeoff.

This is a logic problem that requires assumptions: assuming zero wind and the airplane speed unlike a car is measured against the wind and (because no wind) the stationary markers on the sides of the treadmill runway.
And since the runway only moves as a complement to the movement of the airplane it’s only important the runway move backwards at takeoff speed. As defined in the problem it only gets there if the plane also does so moving forward & therefor takes-off.
Note also that the bottom of the tire is still moving at Zero with relation to the treadmill which is moving backwards. The axle moves with the plane so the top of the tire is moving at 4 times takeoff speed.

Pretty simple, what’s to argue about?
I assume since you can afford one expensive runway you can afford wheels that don’t seize up when over spun.

 

[mewmew]

How bloody many threads do we have on this thing, anyhow?
The plane will take off unless the belt is moving so fast that the wheel bearings seize up.

I searched and found nothing, searching for conveyor, airplane, and plane. Also, if you read through I made it clear that I pretty much know what the answer is, as I posted an example and what not. I am interested in the forces involved that make it so.

I hope you’re talking about some other forum web site, not double threading on this one.

Yes, I am.

the tangential speed of the tire in contact with the runway is always ZERO.

Why does the tangental speed have to be 0 for a tire rolling on a regular runway?

My main question, as stated above in an earlier post, was what where the forces acting on the system. I have pretty much figured that out so it doesn't much matter, its nice though to see some people who know what they're talking about explain things sometimes though. If this question is a sore topic or anyone has a link to the other thread please tell me and Ill close this one.

 

[Integral]

See https://www.physicsforums.com/showthread.php?t=101259" thread.

 

[Chris Rox]

A Plane is sitting on a treadmill that is set to

Move at exactly the opposite speed and direction to that of the plane


will the plane be able to take off?


People have been arguing with me for days no saying that it wont.

this is my theory and i would like a few of you on here to let me know if i am right or wrong.


The treadmill is set to move in the opposite direction to the plane but at the same speed

so if the body of the plane is traveling at 20 mph to the left then the belt of the treadmill is traveling at 20 mph to the right

people have been constantly arguing with me that this means the plane will be standing still then.

This is my theory.

the plane is powered by jets which use the exhaust gases to provide the thrust.

the wheels on the plane arent powered and do not provide any drive or forward motion

as the plane apllies the thrust will it be able to move forward and eventually take off?

i am the only one out of about 40 people that says yes, they are all saying no it will sit still because the treadmill is going in the opposite direction to the plane at an equal speed.


the wheels of the plane and the treadmill are in no way related to the jets of the plane


so if the plane is traveling at 200 mph to the left then in theory the treadmill will be traveling at 200 mph to the right

so

will the plane be sitting still?? Or will the wheels be turning at 400 mph in the direction of the treadmill and the plane will be traveling at 200 mph and be able to take off??



PLEASE HELP, IT FEELS LIKE ME AGAINST THE WORLD IN THIS ARGUMENT!

 

[ZapperZ]

Er... what is with this question that seems to be popping up every couple of weeks? Is some website running a contest or something?

Please read these two threads that have appeared already. Continue your discussion in the last thread.

https://www.physicsforums.com/showthread.php?t=103024
https://www.physicsforums.com/showthread.php?t=101259

Zz.

 

[Chris Rox]

thanks to everyone who agreed the plane would take off


i knew the plane would take off,

people were too busy worryong about lift and air pressure, the plane has to move first to achieve both of these

the point being argued here is wil the plane move in the first place to be able to achieve these factors??

the answer

yes

so will the plane be able to take off??


yes

 

[w_benjamin]

actually, it won't fly.

 

[Danger]

How do you figure that? This thing has been put to bed.

 

[w_benjamin]

The amount of energy the conveyor exerts on the plane is directly proportional to the amount of energy the plane exerts on the conveyor effectivly cancelling each other out.

 

[russ_watters]

The amount of energy the conveyor exerts on the plane is directly proportional to the amount of energy the plane exerts on the conveyor effectivly cancelling each other out.

While that's true, that amount of energy (force, really) is relatively small because the wheels of the plane spin freely. Because of that, the conveyor cannot, in reality, exert a strong enough force to stop the plane (though it can spin the wheels so fast they burn up).

Again, you have to be careful with the wording of the problem. The consensus here is that the wording of the problem says the plane moves forward with respect to the ground and the conveyor moves backwards with respect to the ground, making the wheels spin at twice the plane's forward speed.

Let me reiterate that all of the difficulty with this problem is all in the wording and the problem's adherence to reality. Had it been better stated (both here and in whatever forum it was first posted in), it would be very, very simple. Ie, modified to match reality, the question would simply be:

Can a conveyor belt prevent a plane from taking off?

The answer from a theoretical standpoint (assuming negligible friction in the wheels) is a simple no. From a more practical standpoint, it's probably still no, but there is always the chance that the wheels could burn up before the plane reached takeoff speed.

 

[w_benjamin]

This is where we might agree to disagree. To me, wheel speed is defined as how much distance a wheel COULD travel at a given rpm. This is how race teams and testing facilities for cars define it. When they're testing a vehicle on a dyno, the vehicle speed is considered 0 while the wheel speed is whatever the have going at that particular time. It is also how they plot data for tire grip when a tire is spun up. At the start the vehicle speed is less than the wheel speed. The car eventually builds vehicle speed until the car moves forward at the same rate as the wheel.

 

[w_benjamin]

Remember, if you put a car on a set of rollers, and run it for 150,000 miles, the car didn't go anywhere, but does the drivetrain stil have 150,000 miles on it?

 

[w_benjamin]

The original wording of the question is thus:

"Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

Can the plane take off? "

 

[Danger]

In the case of a car on a chassis dyno, look at it this way. If you have that sucker spooled up as fast as it can go, sitting still, and push it from behind with another vehicle, it's going to move forward. Think of the propellor or jet engine as that other vehicle.

 

[w_benjamin]

An imperfect scenario as the second car is not turning the wheels of the first. In the question, it states the conveyor will not turn any faster than the wheels of the plane will. Therefore, the conveyor does not contribute to the turning of the wheels. If the conveyor does not turn the wheels of the plane, what does? (I'm going somewhere with this, trust me.)

 

[Wishbone]

im new to this debate, however, from the question at hand, can we assume that the plane is stationary? Are we dealing with a treadmill situation where the force you push on the ground, there is a separate force of the belt pushing you backwards? If this is true, then the plane obviously won't take off...

 

[Danger]

An imperfect scenario as the second car is not turning the wheels of the first.

And the engine of an aeroplane has no connection to the wheels.

 

[w_benjamin]

Ah, Newton's Cradle! The force acting upon an object does not need to be connected to the object. The thrust of the plane begins to try to move forward. This includes the wheel. Since the wheel is in contact on the ground it can either a)skid or b) roll, depending on which is easier. The thrust is indeed acting to turn the wheel.

 

[Danger]

Okay... let's try it this way then. Suppose you have one foot on solid ground, and the other in a roller skate on an opposing treadmill. If you push forward with your anchor leg, what will happen?

 

[w_benjamin]

If the treadmill works as stated in the question, it won't move.

 

[w_benjamin]

And you're not pushing against a solid object in the question. You're pushing against a fluid(believe it or not), so the scenario again an imperfect one.