The surface it's on, or the surface next to the surface it's on? Not stated.

OK. It's simpler to have one point of view.

Are you saying the question is written from one single point of view?

No, if it's not stated you can't chose what to relate it to, that's adding to the question. It can only be relative to where it was if it moved, which the question states it did. :litebulb:

It was on the treadmill.

Ask the question. The point is: The question can only be what it is, nothing more.

It dosn't matter what the speed is because it is 'matched' by the treadmill. There is an equal and opposite momentum.


That's what the question said.

...> It the treadmill is made to match the forward speed of the plane, only in the opposite direction.

It doesn't have to. Only one of the interpretations is physically possible. Reality isn't an additional assumption.

The question doesn't ask you to explain how the hypothetical treadmill accomplishes what is stated.

Occam's razor.

I choose not to include a magic treadmill.

As Flint is saying, there are two different ways to look at the problem. I think that scenario #1 is difficult to achieve in the real world.

Scenario #1 is not possible. That is something that you have to factor in to the question. Like a math proof, if you follow one assumption, and it leads to a contradiction, you can eliminate that possibility.

Ok, first of all, don't ask Cecil. Beanie may know the answer, but Cecil won't


http://www.cartoondepot.com/pages/im...0&%20Cecil.jpg

Now, concerning Labrat's ass ... I mean bicycle analogy, one thing is missing. The airport moving walkway would speed up to counteract the push that you give the bike.

I'm so out of this thread once it passes the 200th post. mark my words.

I'm so tempted to make six consecutive posts, but I'll be nice and leave it at one.

No it can't. The only thing the runway can do is speed up the wheels. A push will still move the bicycle forward.

I was expecting someone to do that. You surprised me, Mr. Dallas, with your restraint. Unlike the treadmill, which provides no restraint for the plane.

imagine this, but six times...
 

Yeah, and in doing so, make the wheels turn faster. But the bike would keep on keepin' on with you pushing it.

Here is something you can test--if you're near a large airport.

If you are pulling one of your airport bags with the wheels on it down the airport terminal, then walk onto (or next to with the bag still on the thingie) a moving walkway thingie, and continue to walk at the same speed what happens? The bag does not stop moving because the tread underneath it is going in the opposite direction. It continues forward because YOU are pulling it, while the wheels spin faster than they were when they were on solid ground. You=the plane's thrusters. the bag = the plane on wheels.

TRY IT!!!!!!!

And with that, this thread is finished :)

Can the treadmill possibly exert enough force on the plane to counteract the force of the thrust?

1) Pie is correct that any force exerted on the wheels is subsequently exerted on the plane (although LabRat's drawing is better).

2) By SteveDallas' example, we know that
Fthrust => Ftreadmill
otherwise the plane would be pushed backwards. If Fthrust > Ftreadmill, the plane must accelerate forward and then eventually take off.

3) The force exerted by the treadmill on the wheels is a friction force, and therefore limited to:

Ftreadmill <= μR * Weight of plane

Where μR is the coefficient of rolling friction.

There are three different coefficients of friction that we could use: static, rolling, and kinetic. Static means that the plane is not moving at all w/r to the treadmill. With kinetic friction, the plane and the wheels are sliding forward, as in Maggie's story of brakes on ice.

μK < μR < μS

I'm not sure what the coefficient of rolling friction is for a 747, but the largest μR listed on Wikipedia is 0.03, and that's for a bus on asphalt. I assume that μR for a plane would be much smaller, but I'll use 0.03 for effect. According to Boeing's site, for a 747-400, the maximum takeoff weight of the plane is 3886 kN. Each of the 4 engines produces a maximum of 281 kN of thrust, for a total of 1124 kN of thrust.

So

Ftreadmill <= 3886 kN * 0.03 = 116.6 kN
Fthrust = 1124 kN

1124 kN > 116.6 kN

I'm not entirely sure, but it looks like this plane is going to move forward, and the treadmill can't go fast enough to stop it, because the plane will just start sliding. Since the plane is moving forward, air goes over the wings, and the plane takes off.


Can anyone get μR for a plane wheel?

Nuh-uh.:p

/me didn't get the memo in time

Woops! The walkway doesn't speed up. :redface:

*Wheels!
 

The wheels effectively disconnect the plane from the treadmill. the movement of the plane is relative to the planet they are both sitting on. the treadmill can spin any direction and rate you want it to and will have _no_ effect on the airplane or it's ability to move forward at an increasing rate and eventually generate enough lift to fly.

Interpretation #1: The question doesn't allow you to move forward, relative to the treadmill.
Interpretation #2: The question does allow you to move forward, relative to the treadmill.
Interpretation #3: In this variation of Interpretation #1, you slide forward, relative to the treadmill.

It has exactly that effect, if you interpret the question as stating that the plane's forward motion is relative to the treadmill.

Ja, aber,,,
 

But now aren't we back to the flaw in the premise?

aren't we back on the treadmill?

Nope -- landing is an entirely different problem.

We're running a rat race on a treadmill with a LabRat

By the way, the question doesn't say it's a jet.
Just as it doesn't say the planes movement is relative to any thing.:p

Airplane on a catapult has zero relation to his problem. F=ma . Plane must have sufficient "a" to achieve airspeed on a short runway. Planes engines do not provide enough '"F" . So catapult adds more "F" . Carrier planes are catapulted on a runway (not a treadmill) as defined by numbers in post 152 .

We simply do the answer as demonstrated in post 152 and then change the answer to that 'observers' reference.

We do same thing with electricity. Which is ground? Earth? Breaker box? Computer motherboard? Any one can be ground. We can define any point as the reference point. We select any reference point only to make the problem easier to solve. Same applies to this problem.

We have three possible 'grounds' - points of reference. Four if we complicate the problem by considering an observer. The observer is completely unnecessary to the question - airspeed - velocity between airplane and air.

Air is a perfect reference point because a clearly defined relationship exists between air and the plane. And we have a known relationship between air and the runway or treadmill. Break a problem into parts. We do all numbers relative to air AND therefore have a simple answer.

Obviously - and this is just too simple for all the speculation - obviously -As the engines throttle up, the plane does take off whether it is on a runway or on a treadmill. Obviously because what the treadmill does is completely irrelevant to (independent of) airplane and air. Obviously plane's airspeed only involves a relationship between air and airplane. Obviously observer’s location and what treadmill does are completely irrelevant. Obviously wheels make that treadmill movement completely separate of (independent - not connected to) airplane and air. Obviously location of observer is completely separate from (independent of) airplane and air.

It is quite scary that so many cannot grasp these obvious and simple relationships. Some of these replies are making me feel like a genius. That scares me.

J Just because they did not tell you which to use as the reference point, then you cannot arbitrarily choose one? Nonsense. See the previous post. Choose air as the reference point. As engines throttle up, velocity of air to airplane increases until takeoff speed is achieved. All this occurs regardless of runway, treadmill, or catapult.

Engine defines a relationship between airplane and air. That makes the problem simple and completely solvable (once we include numbers for engine force, plane mass, and minimum speed for takeoff). It's just not that complex. This is a trivial high school physics problem where a runway / treadmill is completely irrelevant.

So many unable to grasp this so simple problem is scary.

Of course it does. The classic F=ma relationship and the classic v=ma . Foreward speed is defined once we have numbers for these simple equations. And again, treadmill and observer will only confuse one with irrelevant parameters.

Clearly this problem has so confused so many and yet no one has yet asked about landing. Are there terrorists among us?

Just another reference point to keep us confused.

the classic v = ma? I'm not familiar with that one. Are you confusing yourself with v=at or p=mv?

The problem is not explicit, therefore you must do whatever tw tells you to do.:right:

hm,,,
 

But the force here is relevant only so far as it contributes to air moving over the airfoil, no? So the movement of the treadmill, not contributing any positive or negative to getting air moving about the airfoil, contributes nothing, either positive or negative to the ability of the plane to fly.
I think.

Depending on what you interpret the plane's forward motion as being relative to, you get a different answer. The question is designed to make you feel like a genius. But, I'm sure you're the one who finally found the "right" answer, huh?

Oh, my dear Ghod...and you know what? That person's not much different today. :-)
At least in that respect. :-)

What TW Meantto say

It's George Jr's fault...mental midget....seven seconds... top management... limbaugh disciples... no WMD...

The original question says the treadmill matches the planes speed. It cancels out. The plane isn't moving forward to create airflow over and under the wings so it isn't going to fly.

That is actually good to hear. I always felt like she lost some of her personality through the transition. Like Scott would push back, but Lisa wouldn't. I liked that push back sometimes.

As long as the "it won't fly" people are continuing to be maroons, let me point out that an airplane sitting on the runway with the engine *off* (or with no engine at all) will take off all by itself if the wind speed exceeds the stall speed. This is why we tie aircraft down: wheel chocks aren't sufficient in the case of a lightly wing-loaded aircraft in the presence of a stiff breeze.

With sufficient wind speed, an airplane can take off (or be landed) with *zero* forward motion relative to the ground. There's a standard airshow demo that's done in STOL-type airplanes (Piper Cub, or maybe a Maule or Zenith CH-701) that involves hovering and even flying backwards when the wind speed is high enough.

Ya forgot MBAs that ordered that stupid treadmill. ;)

Maggie, The original question dosn't say the wind speed exceeds the stall speed.

i read the question....thought about it..decided that the rotation of the wheels nullified the movement of the treadmill..then i decided to wait to see what Happy Monkey would say, and knew i was right when he agreed.


the only way this scenario works out to no forward movement, and thence no lift is if the wheels are locked, and fastened tight to the surface.

as happy monkey said. it's a trick question

Oh, Lisa's still the same person, no question about it. I think it's natural to lose some assertiveness during transition--especially male to female--because that power is being consumed elsewhere. But then begins the process of integrating core motivations with a new set of social expectations and norms. The strategy is largely unchanged, but often the tactics are different. Learning the new tactics isn't instantaneous.

One of the cruelest things that happens to a transitioning transsexual is the important people in your life who find it easier to cope with your transition by behaving as if the person they knew is now dead; taken over zombie fashion by some other entity. In the cases I know, t'aint so.

you said taint

NOW the plane/treadmill debate is OVER!

No, it doesn't. But I'm trying to shed light on the fact that lift is a function only of airspeed (and angle of attack). Groundspeed is irrelevant, and airspeed and groundspeed are two different things.

The other thing that some folks are missing is that airspeed is created by thrust...and that the operation of thrust has nothing to do with the ground or how the airplane is supported before takeoff (wheels, treadmills, skis, skids, floats, etc.) at all.

If thrust had anything to do with the ground, it wouldn't work while flying. Duh.

"taint" != "t'aint", just as "wont" != "won't"

it's a trick question.
 

it doesn't matter that the treadmill matches the planes speed. The wheels aren't driving the plane. The treadmill will only affect the rotation of the wheels, which will only affect the movement of the plane if the brakes are applied and the surface of the treadmills won't allow a skid. In which case the tires will eventually blow and the plane will skid on it's rims.

The treadmill can make the wheels be still by matching the forward movement of the plane. it can make the wheels roll backward while the plane moves forward by moving faster than the speed of the plane. it can make the wheels move forward by moving slower than the speed of the plane. But until the plane becomes a car with a drive shaft mechanicallly linked to the engines the movement of the wheels will have no effect on the movement of the plane.

I guess I am just too stupid to get it then. Thanks hideouse. You tried. :)

It's very counterintuitive if the only wheeled vehicles you've ever been in moved by applying power to the wheels. But imagine an airboat in a swift current...the propellor operates on the air, not on the water.

Someday come along with me for a ride in the Cardinal...feeling it happen makes a difference.

Is his name now MaggieL?:p ;) :D

Nope. Read your Cellar history.

Eppur si muove.

You are a good sport Sky.:)

The question states that the treadmill matches the plane's speed. Wheels be damned. If you believe the question, you believe the treadmill matches the plane's speed, regardless of what the wheels are doing. The question is: what is the plane's speed relative to? And this information is not provided. It's only a trick question in the sense that it tricks you into thinking there is any determinable answer. As it is stated, there isn't.