Did a quick search on Google and found this nifty link:

http://avweb.com/news/columns/191034-1.html

 

78f- good link!! I just couldn't come up with the words to explain it...that's why I never joined the debate team, I guess.

That was a pretty cool exercise

 

What happened to people in this argument is what my first college physics professor used to call the collision between sense and common sense. Common sense is what we believe is going to happen based on our every day experiences in life. Sense is what the application of the Laws of Physics tell us will happen. We spent a lot of time doing problems similar to this one to try and overcome that common sense bias.

 

I think the plane is already in the air. If it has to have a ground speed of 150MPH to take off, the necessary power needed to attain this speed is not connected to the wheels and the conveyor is moving at 150 MPH in the opposite direction that means that if a speedometer was connected to the wheels it would register 300 MPH. Fast enough for takeoff. ....????...... JMHO....

 

Well, here' a little of my common sense bias

4wd...I don't think you should've backed down so easily...that's me personally, and I still haven't figured this out if we're wrong though.

78f-150Supercab and everyone else...I understand how the plane will take off in that hovercraft example, but that's different (from the link). The way I understand this is, the wheels are assumed to be able to spin like no other wheel bearing can, a small, tiny amount of friction, and the conveyer belt can spin at any speed instantly. Both are stopped, and the jet fires up. It's engines begin to produce thrust against air, and inches the jet forward TO WHICH the belt answers immediately and counters the moverment the wheels just made exactly. As the plane thrusts forward more, the power is moving air harder behind the jet and is in turn inching the plane forward using that tiny bit of friction, BUT the conveyor always answers. This is what makes me say it can't take off.

Now, if this conveyor is on the ground, and it can achieve lift off (like some here agree it can do) of a stable-to-the-people-standing-next-to-it conveyor........where's the airspeed coming from???? I just don't get this yet.

 

JD...I'm just outside Philly also, in fact I'm still at work at the Airport!

Let's go to the Tinicum Inn and talk about it over a beer!

 

I think it is pretty much assumed that the wheelbearings are pretty tuff and that friction in them is negligible. If you read that entire link, it gives this exact same example.

Point of the thing being, as you said, the air around the plane is not moving. The thrust of the engines is pushing against that air, just like it would without any conveyor at all. The belt could spin backward twice as fast if it wanted to, as long as the air around the plane is what the engines are pushing against, it'll just take off as if the ground beneath her was solid.

EDIT: Put on a pair of skates and stand on an electric treadmill that is running... now pull yourself toward the front of the treadmill by using your arms to pull the stationary handles... I think you'll find it's very easy.

 

The hovercraft was only part of the article. Did you read the rest of it? It doesn’t rely on the hovercraft example to prove it.

The engines turn on, and produce thrust, inching the plane forward. The Conveyer immediately starts to move the same speed but in the opposite direction. The thing is, the conveyer's only force is on the wheels, which rotate with negligible rotational friction. The amount of force it takes for the engines to push the plane forward is far greater than the rotational friction of the wheels. If the wheels were locked it would be a different story, but they aren't.

Your last paragraph doesn't make sense to me. Sorry. I've had a couple drinks, but not that many :-P

 

It OK 78 150 supercab...none of it is making sense to me yet. Let me try this...this is what I'm picturing from all of this...

I'm standing at Philly International, and in front of me a couple hundred feet away, is a 747 parked on this conveyor belt. The plane fires up, the plane inches first, since the runway answers it is stopped, but then moves to counter the wheels that began to move forward on what was a stationary surface just seconds ago. Now, we have the thrust, and the wheels moving (which I understand, wheels have nothing to do with making a plane take-off.

The way I see it from this point from everyone's explanation is I'm still standing in front of this stationary runway that's now moving and the plane is just roaring. Does it just float here? Does it move forward in the air from the stop without actually running ther distance of the runway?

Maybe it's just because it's late, but I feel like I'm missing something.

 

If I only had a dollar for every time I've said that in relation to a physics problem, I wouldn't have to think. I'd be able to afford to pay NASA to explain it to me. And if my lab partner's in all those physics classes had a dollar for every time I said it to them, they'd have been able to afford that dream of having me killed come true. On the bonus side, I kept at it and I did eventually get it.

 

Hey, I was just trying to help other people understand it. You said you didn't understand, so I tried to help. Am I out of line? Should I not try to further explain it?

EDIT: The conveyer belt runway is the length of a normal runway, but it's a belt. The plane sits at one end. The belt moves like a treadmill. The thrust from the plane moved the plane forward, down the treadmill, and takes off like a plane normally would by catching enough speed (relative to the ground, not the moving runway) until it has enough lift for takeoff.

We're not saying that the plane is standing still relative to the ground, we are saying that the moving runway doesn't affect the plane's movements.

Is that what you are questioning?

 

The plane keeps moving forward, right at you, and never slows its acceleration. It's getting closer to you and its acceleration is unchanged. The WHEELS just doubled their speed, but the plane, keeps right on movin'.

 

I boo booed; it'll fly. Neglecting friction, wind drag, etc, there is no opposing external force to the engine thrust.

 

I’m glad everyone enjoyed the topic. I see many were hung up on the wheels touching the conveyor. The wheels, conveyor, and the ground are completely irrelevant to the problem. A jet engine is a reaction engine. It works by throwing mass (hot air) out of the exhaust-end of the engine to create thrust. Movement is achieved by the reaction of the thrown exhaust mass as described by Newton's Third Law of Motion. This is the same principle that allows rockets to move in space where there is nothing to push against. The only way it will not move is if the mass is equal to or greater than the reaction (thrust) or there is an equal opposing force. There is nothing in the original statement that says the plane remains stationary relative to the ground. (As many assume.) To the contrary, the statements says the motion of the converyor is relative to the "planes speed", not its thrust. Thus, if the plane isn't moving, neither is the conveyor.

The jet will take off.

 

No, the plane won't fly, the belt cancels any forward motion, no ground speed=no airspeed=no lift produced. Hovercrafts generate an air cushion to ride on and then depend on some sort of thrust to propel them in any one direction. Helicopters generate lift buy "spinning their wings over their head" and then tilt their wing the way they need to go... The smaller rotor makes up for the torsional twist created by "spinning the propellers over your head". In order to have a 'sound' someone has to hear it. So if you're not in the woods when the tree falls, You don't hear it, No sound... Now the more important qusetion....whats the airspeed velocity of a swallow?