Physics Question

This is a college physics question.

An airplane taxi out and parks on a movable runway (something like an enormous treadmill). As the airplane moves the treadmill moves in the opposite direction. The treadmill has a system that tracks the speed of the airplane and matches it exactly in the opposite direction. The pilot is trying to take off, engine is running full speed, airplane is pointed into the wind, brakes are not locked, airplane is not over loaded, airplane is not tied to a tree, airplane is not pulling a parachute, airplane has plenty of fuel, the wheels are free to roll, all conditions are good.

Will the airplane take off or not?

>An airplane is standing on a movable runway (something like
>an enormous treadmill). As the airplane moves the treadmill
>moves in the opposite direction. The treadmill has a system
>that tracks the speed of the airplane and matches it exactly
>in the opposite direction.
>
>Will the airplane take off or not? [photo not recovered: sipcof.gif]

Do we assume that the pilot is tring to take off? Engine running full bore, etc.?

If it is a helicopter, yes. If it is facing a wind with enough speed, yes. If it has enough thrust to take it straight up, yes.

What's the question?

The unspoken assumption is that the treadmill generates enough braking effect on the wheels to, maybe, keep the airplane in place. If the thrust is more than this braking effect, the airplane will move forward. The airplane, into the wind, at full power, generates some lift, even standing still. So, If enough that it is sitting "lightly" on the treadmill, then the breaking effect of the treadmill will decrease and the airplane will move forward.

All the treadmill does is make the wheels of the jet turn twice as fast at lift off. Air speed and distance traveled is the usual for a normal take off.

>All the treadmill does is make the wheels of the jet turn
>twice as fast at lift off. Air speed and distance traveled is
>the usual for a normal take off.

You are correct, mein herr, but where did we get the assumption that it is a jet? It could be a piston powered airplane. It could be a helicopter. It could be a VTO. It could be...

What is the question?

Kind of similar to taking off down wind, but without the danger of the wind dumping you onto your prop (uh, assuming it is a prop powered airplane.) You will just eat up a lot more runway before ascending into the wild blue (unless it is a helicopter, or...What wa the question again?)

I knew the people on this forum were smarter than the people on the other forum. The other forum has 112 replies and only about 30% of the people had the correct answer. The treadmill has absolutely nothing to do with the airplane taking off.

It is funny to read some of the things people said on the other forum. Check some of the crazy replies.

The plane is moving very very fast even if it looks like it's sitting still to us, as we are flying along on this big chunk of rock with it. So it might go off the back end of the treadmill for all we know.

Of course not. There's no airflow over the wings. It may be moving relative to the treadmill, but it's not moving relative to the Earth.

the answer is no because the plain isn't "moving". All the energy is being transmitted to the runway therefore the plane is technically not moving and not generating any lift.

He said the treadmill matches the speed of the plane exactly, so the plane is motionless in relation to a fixed object on earth. No fly.

The moving treadmill can increase the airspeed seen by the wings because of boundary layer effects which will allow the plane to take off sooner. unless it increases the airspeed only on the bottom of the wings and not on the top, in which case it will need more groundspeed to take off.

The plane's groundspeed and airspeed are zero by the definition of the puzzle. There's a feedback loop that maintains the groundspeed zero, it says so in the initial conditions. There's no machine to bring airflow to the wings because the jet engines are blowing air from the rear of the plane, it's a lear jet. Airspeed zero, ground speed zero. Not gonna fly. Jim

The way I read the initial conditions, the treadmill did whatever it took to make the groundspeed zero. If that is the case then the plane won't take off.

As soon as you fire up the engines, you no longer have "still air" (i.e. air speed increases), But your ground speed remains zero. It won't fly.

I'm not even going to comment on the treadmill, there's just so many reasons that wouldn't work.

I don't think any plane has enough thrust or pull from the props to take off WITHOUT the benefit of air flow over the wings - am I wrong?

If the angle of thrust is fixed like most aircraft, it doesn't matter how much air it moves. The lift still has to come from the wings, which means it has to be moving. Or the motor, and prop would have to be INSANELY oversize to both catch enough surface area of the wing, and enough airspeed to get the lift necessary FROM THE WING! Unless the runway was vertical, but nobody said anything about that.

A Harrier will take off but not a normal airplane.

As the airplane's engines deliver more thrust, or forward motion, the treadmill automatically adjusts itself faster to compensate. The net effect is zero. Zero speed relative to the Earth, Zero Lift, Zero Takeoff.

Thanks for posting this topic. The many answers made me realize the collective ignorance (on this site, anyway) about something that I had assumed (obviously incorrectly) to be common educated knowledge.

This thread has only been alive for 7hrs and it already has all but 6 pages of posts. I just had to throw that in for interests sake.

I'm astounded at how many people don't even understand the original question. for example, JST - there is NO movement of the airplane relative to the ground. Zero. If you had a giant treadmill, with a giant fan in front of it, then the plane would take off.
But, as Confucious noted over 500 years ago, no windee, no takeoffee.

There is NO movement of the airplane relative to the ground. Zero. It can not take off.

If all this makes sense to you why do we need a catapult on aircraft carrier?

You have no idea how amusing this is to read. People I regard as intelligent tripped up by a silly riddle.

Come on guys, the only thing that dictates the speed of the "treadmill" is the speed of the plane. Not the THRUST generated by the prop, so the treadmill would never move any faster than the plane can be propelled by it's own thrust.

I especially like the posts where someone goes to great lengths to explain their theory but never states their position on whether it will fly or not.

Can me see the video.

Not sufficient information.

You guys can certainly waste a lot of space on silly questions . If there is no high speed air moving over the wings,how can the plane take off ?

This is hilarious

You can't possibly still think, after clarifications, grabbing by the ear and pointing to the very pertinent criteria in the original question. A PILOT, by god, says it can't possibly fly.

Physics problems like this just make me terrified that I may someday be unlucky enough to be on a plane that happens to stray into the airspace over a treadmill, instantly causing the craft to drop from the sky, killing everyone.

If the airplane were a car, then it would just sit in one place on the treadmill and go nowhere, but since the airplane doesn't generate forward motion via its wheels, it will move forward no differently than if it were on a normal runway, and take off if the pilot so desires.

Reminds me of my dad's favorite question for us when I was a kid.
"If a chicken and a half laid an egg and a half, how much would a pound of cheese weigh?"

Jim

>Reminds me of my dad's favorite question for us when I was a
>kid.
>"If a chicken and a half laid an egg and a half, how much
>would a pound of cheese weigh?"
>
>Jim

"If a chicken and a half laid an egg and a half in a day and a half..." Gotta get it all in.

It reminds me also of the one, "Which weighs more, a pound of gold or a pound of feathers?" Actually the feathers would if you were using a 16 ounce pound for feathers and a 12 ounce pound to weigh the gold.