A harmless little riddle

wadsworth wrote:

there are different interpretations because speed is a RELATIVE value and the reference frame is not defined in the riddle. is the riddle referring to the planes speed relative to the surface of the treadmill? or is it referring to the planes speed relative to the ground?

Speed is relative to what? It doesn't have to be referenced in the riddle because an aircraft's speed is always relative to the air. If an aircraft moves relative to the air then it develops lift. The engines create the forward motion; the surface the aircraft is resting on is immaterial and can be ice, water or solid ground. Or a moving runway, such as an aircraft carrier.

IT LIVES

but what if there are snakes on the treadmill? and they get in the plane because it cannot move?

No, the snakes would be thrown off the back of the treadmill, they would no be able to board the plane.

Seriously thouh, a few months ago I asked a guy at the Smithsonian about this, and he just gave me an odd look and asked where I would find a treadmill that could support a plane.

He never gave me a straight answer...

Hmmmmm...

I only read the first 2 pages of posts. I have NOT read the next 20 pages. So whaterver was said in pages 3-22, I have not read. Sorry if this was solved somewhere in between.

At first I thought it was simple, the plane would NOT take off and would just spin its wheels on the treadmill. But then I realized, no, IT WOULD INDEED TAKEOFF.

Somebody made a good point (easy 10, I think) It's wheels aren't what makes an airplane move on the runway like a car. The wheels are as if they were a car set to neutral. But it is the jet engines (or propellars if you don't want to make it a jet) that "push" an airplane down the runway. Imagine, YOU are running on a treadmill. In theory, the treadmill is capable of going more than 30 mph. Lets say YOU are capable of running 20 mph for a brief moment. You are running at 10 m.p.h. on the treadmill. A friend comes up behind you and PUSHES YOU in the back. He continues to apply pressure to your back as HE moves forward on the ground! No matter how fast you move your legs, and how fast the treadmill is capable of going, YOU will go forward RELATIVE TO THE GROUND AND AIR, not stationary, because your friend is pushing you.

The same principle applies to the airplane!

The jet engines (or propellars if you prefer) are the friend that is pushing you in the back. Yes, the treadmill will turn, and the wheels on the plane will turn, but in addition to that, the plane will move forward because of the jets thrusting against the air. Eventually, the theoretical treadmill could be going as fast as 150 mph but the wheels of the plane will spin at a rate of 300 mph, thus moving the jet RELATIVE TO THE AIR AND GROUND at 150 mph, thus THE JET WILL GET AIRBORNE!

track-vol wrote:

I only read the first 2 pages of posts. I have NOT read the next 20 pages. So whaterver was said in pages 3-22, I have not read. Sorry if this was solved somewhere in between...

Not this again, what you are failing to remember is that the treadmill can go as fast as the thrust of the jets, meaning that in your little scenario, when the kid is pushed from behind, the treadmill speeds up to match the kid's push.

In other words, the plane's wheels do not make it go, but they keep the plane from dragging on the ground... The plane needs its wheels to achieve enough speed to get airborne. The plane's thrust does not work like a Harrier jet, if it did, you would be right. Unfortunately the wheels are needed to get airborne. As we all know, we need speed AND thrustto equal lift, not just thrust.

Sorry, we covered this somewhere around page three...

track-vol wrote:

the theoretical treadmill could be going as fast as 150 mph but the wheels of the plane will spin at a rate of 300 mph

NO NO NO NO No, the mythical treadmill ALWAYS goes as fast as the wheels.

Wait wait... it looks like we've all come to the correct conclusion that the plane will not take off via treadmill. However...

What if we had a fan large enough to provide the airspeed required to lift a stationary plane? Could it then hover and "take-off"

Every last one of you should be shot for this thread...

It would take off.

Are you saying, that if you "Not again" were running on a treadmill, and I am standing next to you. I push you in the back, that you would not go forward relative to the ground AND the treadmill?

Of course you would go forward. Even if the treadmill sped up, you would still go forward, because I pushed you and I am on the ground, NOT the treadmill. Do you think that you could just stone wall me and that you would stay stationary. PLEASE think that through.....

THe plane would take off because the jet engines are the same as me pushing you. The jet engines push off of the air.

Calvert County wrote:

Every last one of you should be shot for this thread...

If I were shot while on a treadmill that moves as fast as a bullet, would I be able to turn around and watch the bullet in flight?

I say yes.

Yeah, this is right. The plane takes off. Peple think of the wheels like the wheels on a car, but they're not. They just hold the thing up. The plane would thrust itself through the air and the wheels just go a bit faster to compensate for the speed of the treadmill.

This has been discussed to death all over the internet.

More Productive wrote:

Seriously thouh, a few months ago I asked a guy at the Smithsonian about this, and he just gave me an odd look and asked where I would find a treadmill that could support a plane.

He never gave me a straight answer...

Hmmmmm...

Dude, why are you asking people in DC about this?

They're part of the conspiracy.

Sh*t, I've said too much already.

hugefan wrote:[/b}

What if we had a fan large enough to provide the airspeed required to lift a stationary plane? Could it then hover and "take-off"

Perhaps if we built a large wooden badger.....

Not even a 100 Rexings could get that plane in the air.

this picture explains it perfectly.

http://tinypic.com/jg3atd.jpg

Just like this guy is going to get pulled off of the treadmill by the motorcycle, the airplane will fly(motorcycle represents jet engine attached to plane).

again:

http://tinypic.com/jg3atd.jpg

just in case you missed it:

http://tinypic.com/jg3atd.jpg

what the hell is that drawing of and why do you think that is the same as the airplane on a treadmill. now you put that motorcycle on the same treadmill as the rollerblader and you have something a little more similar to what we are talking about. i'm tired of this thread, why doesnt someone make a prototpe and test out the theory.

what th hell is that wrote:

now you put that motorcycle on the same treadmill as the rollerblader and you have something a little more similar to what we are talking about. .

NO, NO, No, no. . . .

that is the exact thinking that the "no flyers" can't get their thick skulls through. The motorcycle on the treadmill is what you guys think of the jet engines and the treadmill, but the jet engines don't have anything to do with the treadmill, the jet engines move AIR, not the wheels.

As the motorcycle pulls the rollerblader off of the treadmill no matter how fast the treadmill goes; so goes the airplane. The jet engine WILL push/pull the plane forward no matter how fast the treadmill goes.

Until you realize that fundamental fact, you will never understand this. Why can't some of you figure that out?

Even the airplane people on that airplane message board got it right (although there were a few who didn't get it at first either)

don't worry track-vol. it's probably the same guy posting over and over again that it won't take off just so that he can get a rise out of you.

please read the entire thread before you even think about posting that the plane will not take off.

the issue here is that the question was poorly worded. anyone remember monty hall problem? hundreds of people, math professors included, wrote to inform Marilyn vos Savant that she was an idiot and not qualified to do mathematics, with various insults included like "women can't do math" and the like. In the end, Savant was right, but the majority of the confusion comes from stating the question ambiguously.

Hence 23 pages of people arguing about the problem and calling each other idiots without considering that despite being physics and engineering majors, oh horrors of horrors, they might be wrong. everyone is thinking, "if the plane is stationary, of course it won't take off." and they are perfectly correct. but this isn't the riddle! give me a plane sitting stationary with respect to the air with its engines running full blast, and i'll show you a huge accelerating hunk of metal.

so how can we state the problem more clearly? is it even worth it? perhaps a treadmill the length of 20 runways moves backwards at 100 per hour. the plane is held in place on the runway so that its wheels start spinning. when the plane is released, the runway exerts a small amount of force backwards, but the plane is at rest relative to the air so the engines produce lots of thrust. the engines only stop thrusting when the speed of the air relative to the plane reaches a certain level, regardless of the runway.