A harmless little riddle

The plane will move through the air (and relative to the ground).

Since you think that it won't answer me this: What is stopping the plane from moving?

Hint: I know why you think it won't take off. You think there will be no airspeed (movement of the plane relative to the air) to generate lift. I am asking you why you think there will be no airspeed. Is it because of the treadmill? How?

cgaites wrote:

The plane will move through the air (and relative to the ground).

Since you think that it won\'t answer me this: What is stopping the plane from moving?

Hint: I know why you think it won\'t take off. You think there will be no airspeed (movement of the plane relative to the air) to generate lift. I am asking you why you think there will be no airspeed. Is it because of the treadmill? How?

Why do you think that there IS airspeed? The question implies that the airplane would be moving at takeoff speed relative to the ground (treadmill) but at zero airspeed relative to the air around it. In this situation, the plane will not fly.

track-vol wrote:4) The treadmill, even if it speeds up to 300 mph, will not slow the plane down, it will only cause the wheels to spin faster.

5) The treadmill can NOT nullify this, no matter how fast it speeds up to just keep the fuselage from scraping the ground.

8) The treadmill can't stop the plane. How does the treadmill exert force on the plane? It doesn't. It will just make the wheels spin faster, not keep the plane from moving forward. Malmo has said so, the airplane people in their thread have said so, most every *inteligent* poster has said so. The rest of you still thing the earth is flat.

Facts 5 and 8 are untrue. This is the crux of the problem and simple physics shows that they are untrue.

Ask Malmo, airplane people, and every *intelligent* poster if an airplane sitting on a treadmill with its engines off can be made to move backwards. Assuming their answer to that question is yes, how can that fact be made to reconcile with the declaration of facts 5 and 8?

The fact that you Fly-ers believe that you are inarguably right makes you the stupidest people on this thread. The question, as posed at the beginning of this thread, can be interpreted in two different ways. One of these results in the plane flying, and one doesn't.

so you have a couple of links that agree with your analysis..??

WTF??

Talk about reaching for straws

track-vol:

i am on your side! please read my posts again. the answer to the riddle as it was posed is yes, the plane will take off. the wheels are completely irrelevant-the pilot could be stomping on the brakes as hard as he wants, just the wheels would leave long black marks on the tarmac.

i was saying that the treadmill does have the capability to stop the airplane. when the treadmill exerts a force on the wheels, you have to consider it both as a force and as a torque. as a torque, it just makes the wheels spin, but the direct force can be made large enough to stop the plane. the force just has to be sufficiently large, and if the treadmill just equals the plane, then that force is much too weak.

I think everyone on here understands that the wheels have nothing to do with the plane's engine/power/thrust/etc, get see past that perceived argument.

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

argh... wrote:

the answer to the riddle as it was posed is yes, the plane will take off. the wheels are completely irrelevant-the pilot could be stomping on the brakes as hard as he wants, just the wheels would leave long black marks on the tarmac.

WRONG. As posed, the riddle is ambiguous. It can be interpreted two different ways. You are only looking at one of them.

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...

DCer wrote:

I think everyone on here understands that the wheels have nothing to do with the plane's engine/power/thrust/etc, get see past that perceived argument.

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

The riddle doesn't state that the treadmill matches the speed of the wheels. The treadmill matches the speed of the plane. The riddle doesn't specify if the speed of the plane is relative to the ground or the air. Therefore, there are two different interpretations, which lead to two different answers.

and to further clarify... the wheel's speed is caused by the thrust of the engines...

so where is the extra force come in to "push" the plane forward..???

ans: there is no extra force, thus the plane remains stationary relative to the ground, producing no air speed

A. Dent wrote:

What if the treadmill constantly accelerates?

Doesn't matter, it can be going 1 million miles per hour, the wheels will just spin and grin, the plane however will travel forward as the jet engine or propellers push/pull the plane through the air.

The quick and obvious answer is that it will just sit still relative to air, but that is not the correct answer. Upon further inspection, easy 10 and trackVol and the rest are right, we were wrong. Heck, some 'jrm' guy figured it out on the first page of this now 24 page thread.

I believe the plane will fly.

here we go again!!! wrote:

and to further clarify... the wheel's speed is caused by the thrust of the engines...

so where is the extra force come in to "push" the plane forward..???

ans: there is no extra force, thus the plane remains stationary relative to the ground, producing no air speed

The riddle can be interpreted to mean that the plane is moving forward in space at the same speed the treadmill is moving backwards. Because the plane has to be moving forward in space as long as the treadmill is moving, the only effect on the wheels is that they would have to be spinning twice as fast.

I'm a believer now wrote:

A. Dent wrote:

What if the treadmill constantly accelerates?

Doesn't matter, it can be going 1 million miles per hour, the wheels will just spin and grin, the plane however will travel forward as the jet engine or propellers push/pull the plane through the air.

The quick and obvious answer is that it will just sit still relative to air, but that is not the correct answer. Upon further inspection, easy 10 and trackVol and the rest are right, we were wrong. Heck, some 'jrm' guy figured it out on the first page of this now 24 page thread.

I believe the plane will fly.

THERE IS NO "ANSWER," IT DEPENDS ON HOW YOU INTERPRET THE QUESTION. NO ONE IS "RIGHT."

DK wrote:

The riddle doesn't state that the treadmill matches the speed of the wheels. The treadmill matches the speed of the plane. The riddle doesn't specify if the speed of the plane is relative to the ground or the air. Therefore, there are two different interpretations, which lead to two different answers.

where is the air moving at a different speed than the ground in this example? if we assume it is a calm day, then the ground and the air have the same velocity so the speed of the plane in relation to the ground and air is identical

you are assumimg no friction on the wheels and the treadmill...

but there is friction

and "that" is the difference

I'm a believer now wrote:

Doesn't matter, it can be going 1 million miles per hour, the wheels will just spin and grin, the plane however will travel forward as the jet engine or propellers push/pull the plane through the air.

This isn't even true. The entire weight of the plane is resting on the treadmill until it gets to takeoff velocity. It would have to overcome the speed of the treadmill at some point.

another frustrated flyer wrote:

where is the air moving at a different speed than the ground in this example? if we assume it is a calm day, then the ground and the air have the same velocity so the speed of the plane in relation to the ground and air is identical

It's a treadmill. The treadmill can be going backwards at 150 mph, and the air directly above it won't be going anywhere. The plane can be moving 150 mph in relation to the ground, but 0 mph in relation to the air around it.

holy crap i cannot believe this is still being argued. "blah blah blah the plane pushes against the AIR, man, not the ground..." are you kidding me? does anyone here realize that there is a force called GRAVITY that exerts force on matter (assuming that this example takes place on earth)?

when the airplane is at rest on the ground (or stationary treadmill) the force of gravity pulling the plane down is balanced by the force of the runway (treadmill) pushing up on the wheels - newton's laws people, the same amount of force pulling the plane down (the plane's weight) is also being applied by upward by the runway; if the forces were not equal, a plane sitting on the runway would either be crushed into the ground by gravity or would float off the ground. at rest, no force is being exerted horizontally, hence no movement.

in a normal runway takeoff, when the engines of an airplane are engaged, they provide thrust that begins to accelerate the aircraft down the runway. we now have a positive force in the direction opposite the engines' thrust. the reason an airplane requires a runway, however, is that the plane needs room to accelerate to a speed where the surrounding air is moving over the surface of the wings fast enough to to produce a lifting force (due to the differential curvatures of the top and bottom of the airfoil) that exceeds the gravitational pull on the plane's mass, resulting in a net force in the upwards direction. UNTIL SUCH LIFT IS GENERATED, THE AIRCRAFT WILL NOT LEAVE THE GROUND.

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. recall that when the airplane was resting, the force of the runway (treadmill) on the wheels was the same as the force of the plane on the ground. the fact that the engines thrust against the air has NO IMPACT WHATSOEVER on the outcome as long as GRAVITY has not been counteracted. The only way to produce lift is by increasing the speed of the air flowing over the wings. If the force of the aircraft's thrust is met by an EQUAL force in the opposite direction, the wheels will spin, yes, but the airplane will not move forward in space, hence no airflow, no lift, NO FLYING. an aircraft at rest on the treadmill trying to accelerate is no different than a person running on the treadmill, until gravity is overcome, the force of the engines is INDEED BEING APPLIED THROUGH THE WHEELS INTO THE GROUND. think not? what do you think would happen if the pilot of an airplane rolling down the runway at ten mph suddenly retracted the landing gear? that's right, the plane would fall to the ground, because it is not moving through the air fast enough for lift to overcome gravity.

i can't believe i just took that much time to explain something so ridiculously simple. bottom line is this: the airplane starting at rest must accelerate to a high enough speed THROUGH THE AIR for it to lift off the ground (or treadmill). just because the wheels spin makes no difference: a force equal the mass of the plane X the acceleration of gravity (roughly 9.8 m/s/s) is still keeping the plane on the ground and must be overcome in order to lift it. just like a runner, the horizontal component of motion is balanced by a treadmill going the same speed, and thus the runner and the plane DO NOT MOVE relative to their surroundings, including the air. without this horizontal movement, the upward component of lift cannot be developed.

once again, THE AIRPLANE WILL NOT FLY.