A harmless little riddle

Pherekydes wrote:

Hahahaha. The engines push air out the back of the engines. The fact that they are attached to the wings is immaterial.

Only immaterial if you have no understanding of aerodynamics. Not immaterial if you want the a plane to fly. The air must travel over the top of the wing and under bottom. The ENTIRE wing, not just a small section of it. That is why larger wings generally give more lift

wow. wrote:

If we take the plane off the runway, and put it on a giant treadmill that MOVES AT THE SAME SPEED as the plane would move were it on stationary ground, then by definition the force in the direction provided by the thrust of the engine is EXACTLY MATCHED and thus cancelled out by the opposite force of the treadmill.

You are confusing force with speed. The force created by the engines to accelerate the plane to 200 mph is greater than the force on the plane created by a treadmill spinning at 200 mph. The thrust is NOT exactly matched. It is not even close to being matched. The plane will move forward, and it will eventually achieve enough speed to generate lift.

o.k the plane will take-off as long as it can overcome any

frictional-force between the tires and the treadmill

there

hope you flyers are happy

DCer wrote:

My question is this, and please answer if you know:

The thrust is provided by the engines using air like a rope, we all know this...

But because the wheels ARE needed for the absence - or near absence - of friction, how can the plane continue to move forward if the treadmill matches the wheels' speed?

I mean, due to gravity, the wheels ARE inded necessary for the plane to move forward, right? I guess my question is, if the wheels are spnning at 300mph, and so is the treadmill, how can the plane move forward, assuming the treadmill always matches the speed of the wheels?

thanks

There are two pairs of forces at work in this problem: gravity and lift, and thrust and drag.

Gravity obviously pulls the plane down toward the ground.

Lift is needed to overcome gravity and carry the plane into the air. As many on both sides have correctly stated, lift is generated by the movement of air over the wings (airspeed).

Thrust is provided by the plane engines. That is the engines and the engines alone push the plane foward. This is true for all planes whether they have skis, floats, or wheels. Thrust, when a sufficient amount is applied, moves the plane foward and produces both airspeed and groundspeed (though they may not be the same).

Drag, because the wheels on a plane's landing gear are free spinning and are designed to be as close to frictionless as realistically possible, comes from only one source in this problem, and that is air resistance.

Since the runway (be it treadmill or standard runway) cannot apply a drag force to the plane via. the near-frictionless wheels (unless the brakes are engaged), the runway can not oppose the applied thrust. Since there is no force to oppose the applied thrust the plane will move foward. Once the plane has attain a sufficient airspeed the magnitude of the lift force, produced by the motion of air over the planes wings, will become larger than the magnitude of the gravitational force and the plane will take off.

Think about how a seaplane takes off against a tide or current.

DCer wrote:

My question is this, and please answer if you know:

The thrust is provided by the engines using air like a rope, we all know this...

But because the wheels ARE needed for the absence - or near absence - of friction, how can the plane continue to move forward if the treadmill matches the wheels' speed?

I mean, due to gravity, the wheels ARE inded necessary for the plane to move forward, right? I guess my question is, if the wheels are spnning at 300mph, and so is the treadmill, how can the plane move forward, assuming the treadmill always matches the speed of the wheels?

thanks

There are two pairs of forces at work in this problem: gravity and lift, and thrust and drag.

Gravity obviously pulls the plane down toward the ground.

Lift is needed to overcome gravity and carry the plane into the air. As many on both sides have correctly stated, lift is generated by the movement of air over the wings (airspeed).

Thrust is provided by the plane engines. That is the engines and the engines alone push the plane foward. This is true for all planes whether they have skis, floats, or wheels. Thrust, when a sufficient amount is applied, moves the plane foward and produces both airspeed and groundspeed (though they may not be the same).

Drag, because the wheels on a plane's landing gear are free spinning and are designed to be as close to frictionless as realistically possible, comes from only one source in this problem, and that is air resistance.

Since the runway (be it treadmill or standard runway) cannot apply a drag force to the plane via. the near-frictionless wheels (unless the brakes are engaged), the runway can not oppose the applied thrust. Since there is no force to oppose the applied thrust the plane will move foward. Once the plane has attain a sufficient airspeed the magnitude of the lift force, produced by the motion of air over the planes wings, will become larger than the magnitude of the gravitational force and the plane will take off.

Think about how a seaplane takes off against a tide or current.

You guys are hilarious. READ the riddle again.

Plane generates (turbine generated) thrust moving it forward at some speed relative to ground. Treadmill moves backwards at equal and opposite speed.

Plane is still moving forward relative to ground and treadmill, but wheels are spinning at twice the rate of normal runway plane.

Plane takes off no problem, but wheel rubber catches fire and landing is going to be a bitch.

DOH!

Ah yes, much in the same way that the runner on the treadmill produces much greater force than the treadmill imparts on the runner, therefore allowing the runner to run off the front of the treadmill. If a runner is on a treadmill with a top speed of 15 mph, and he's running 15 mph, the force he is producing is balanced by the opposite force that the treadmill is producing; that's why he stays in one place. Is this or is this not how a treadmill operates? Now, if he were to increase his force against the treadmill by running 20 mph, and the treadmill can't keep up (read: match his force) then yes, he is going to move forward on the treadmill, obviously. However, if the treadmill can go 20 mph, is he going to start moving? well, in this world i'm pretty sure the answer is no. so..... assuming that the treadmill matches the plane as it does the runner, the same holds true.

YOU are the one who is confused.

DCer wrote:

Thick as a brick wrote:

Might as well give up, track-vol. They don't get it.

Answer my question above please.

You too track-vol...

OK, I think here is our disconnect. . . .

The riddle says plane one way, treadmill another. At some point (when the plane begins to travel relative to ground, the treadmill can no longer keep speed with plane wheels, but it can keep pace with plane.

If the treadmill going 150, the plane CAN and eventually WILL travel, through the air, relative to the ground 150 mph, when this happens, the actual wheels of the plane will be spinning at 300 mph, because the treadmill 150 and the plane MOVING 150. If the plane was just sitting there on the treadmill, NOT moving relative to the ground, wheels would spin 150. But turn the thrusters on and it will travel forward relative to the ground.

Even if you attempt to speed up treadmill to match wheel spinning speed (300 mph) the plane will still be traveling forward relative to the ground at 150, just now it's wheels will spin freely at 450 mph and so on.

The only part wrong about that is that at 150 ground/air speed, the plane would take off and no longer be on the treadmill.

Not my best explanation, but does answer your question.

"HERE WE GO AGAIN" If I put a link on here to CNN, or NBC news or Fox news, because it was on the internet, would that alone make it wrong? because it was on the internet?

Like Snopes.com, one of the links I put up was to a site that answers things like that. If that is your best arguement for 'no flight', you have a weak arguement.

WOW, but the plane operates on the treadmill NOTHING like a runner would. If it were a car, than you would be right, but the wheels of the plane do NOT drive or provide force or propulsion like our legs do or the wheels of a car does. Planes don't drive, they jets pull them through air, even when on the ground.

THe plane will clearly fly, as obvious as the day is long. . .

Go to the links, read the article. . .

The aviation people in their message boards have all agreed the plane will fly (they understand how planes move):

The plane flies.

Now that we've established that the plane will fly (all newcomers who disagree, go search on google), there's one more important point. The treadmill must exert a torque on the wheel to make them spin faster-this is friction between the treadmill and the wheel. You have to consider this force as a force as well as a torque. So, if there is a large torque, there is a large force, which can be made very large (this is a static friction force, mu*weight of airplane=huge). To do this, the treadmill would have to be accelerating at a=F_p(r/I-1/M_p), where F_p is the force of the engines, r is the radius of the wheels, I is the moment of inertia of the wheels, and M_p is the mass of the plane.

I stopped reading this thread after the following post:

"An airplane propels itself by exerting a force on the atmosphere."

Most stupid thing I've ever heard.

wrong again wrote:

Only immaterial if you have no understanding of aerodynamics. Not immaterial if you want the a plane to fly. The air must travel over the top of the wing and under bottom. The ENTIRE wing, not just a small section of it. That is why larger wings generally give more lift

The engines push against the air. The engines move forward. The engines are attached to the A/C. The A/C moves forward. The wings, which are attached to the A/C, move forward. Lift is developed. Simple isn't it, once you break it down?

try to understand, before the airplane has reached takeoff speed THROUGH THE AIR, it will not fly. OK, everyone gets it, a plane's engines push on air, no one is saying they don't. IT DOES NOT MATTER HOW THE FORCE IS DEVELOPED. let me say that one more time: IT DOES NOT MATTER HOW THE FORCE IS DEVELOPED.

When the plane is sitting on the stationary treadmill, gravity is holding the plane to the surface of the treadmill. Now, if we start the treadmill going, the wheels are not going to just start spinning while the plane remains stationary, the plane is going to move backward with the surface of the treadmill. If you don't think that would happen, then you can't possibly understand the rest of this scenario.

So if we now have the plane moving backward on the treadmill (and relative to the ground and air surrounding), what must be done to compensate for this motion? answer: a force must be applied to the aircraft in the opposite direction. so we turn on the engines, and the thrust that is developed begins to slow the airplane's backward speed on the treadmill. eventually, the engines are going to produce enough thrust so that the aircraft stops moving backwards. now, at this point guess what? the airplane has produced sufficient force to match the speed of the treadmill. in order for the airplane to continue to accelerate, so that it begins to move forward on the treadmill, it must now produce MORE FORCE than when it was at the equilibrium point. however, given the nature of the question - which specifies EQUAL speed - it cannot.

i got to 3rd page and people were typing intelligent posts both for and against it flying, i still don't know, i would say know though, it needs wind.

wow, your answer makes an assumption. you state that because the treadmill is moving at the same speed as the airplane, it exerts a force equal to the thrust of the engines on the plane. i'm going to tex out the equations to show that this unjustified assumption is false. stay tuned.

Here is the answer:

Every single plane that takes off IS sitting on an enormous treadmill, the earth. All a plane must do is to reach XX mph (depending on mass) and it can fly no matter if it is moving with or against the rotation of the earth. Therefore, in the riddle, the treadmill is the earth. The plane will fly.

well, this analogy doesn't quite work, because the air spins with the earth and doesn't move with the treadmill.

Pherekydes wrote:

The engines push against the air. The engines move forward. The engines are attached to the A/C. The A/C moves forward. The wings, which are attached to the A/C, move forward. Lift is developed. Simple isn't it, once you break it down?

It is quite simple. A shame you still don't understand it.

Incidentally, the engines don't move forward. Have you forgotten the riddle already?