A harmless little riddle

KnowItAll wrote:

No engineer, you're wrong. If the riddle says the treadmill is matching the plane's wheel speed, then you can throw out any physics you want because if the treadmill is matching the plane's wheel speed, the plane is not moving. Yes, it's ridiculous and you can argue the complications of it in the real world, but if that's what is meant by the riddle, then the plane won't move forward and will not take off. There are two answers to this riddle depending on what is meant by matching the airplane's speed.

Yes, exactly, and this is what "Engineer" refuses to see. Engineer - I agree that under one interpretation of this riddle the plane will no doubt fly.

However, in order to fly, the plane must be moving relative to the ground, correct? And if the plane is moving relative to the ground, then the TANGENTIAL velocity of the wheels is GREATER than the TANGENTIAL velocity of the treadmill. With me so far? While this is perfectly feasible and possible and the plane will no doubt take off under this scenerio, IF YOU INTERPRET THE RIDDLE AS CONSTRAINING THE TANGENTIAL SPEED OF THE WHEELS TO BE EQUAL TO THE TANGENTIAL SPEED OF THE TREADMILL, THEN THIS SCENERIO VIOLATES THE CONDITIONS OF THE ORIGINAL RIDDLE!

THIS IS THE POINT THAT DK, KNOWITALL, AND OTHERS HAVE TRIED TO POINT OUT TO YOU, BUT THAT YOU FAIL TO RECOGNIZE!

you know, aside from being an asshole, you're not very smart, are you? yes it's a serious question. tell me why when a plane is on a normal runway its wheels go as fast as it is moving, yet when it's on the treadmill, somehow its wheels go twice as fast.

an airplane's engines, by the way, DON'T PUSH ON AIR. combustion of fuel and air in the engines creates rapidly expanding hot gas. the gas goes one way, the plane, due to equal and opposite actions, goes the other way.

the engines do NOT function like a rope that the plane slides along. they do not hold the plane off the ground, not during flight, and especially not sitting on a runway. the only thing they do is create and maintain enough horizontal speed to keep the lift forces in excess of the force of gravity.

therefore, the only spinning of the airplane's wheels, whether on a runway or a moving runway, is due to the airplane's own thrust. the fact that the airplane is being moved (as a whole, not just its wheels) by the treadmill does not change this.

i dare you to refute anything i just said with scientific facts.

Engineer wrote:

i believe it says at the same speed that the PLANE is moving,

MOVING relative to WHAT?

Relative to the treadmill? That means stationary relative to the rest of the world, no air flow and no liftoff.

Relative to the rest of the world? Then if you go fast enough, you will have enough air flow to generate enough lift to get airborne.

Please make yourself clear in your engineering work. They teach that at school.

Asterix wrote:

MOVING relative to WHAT?

A plane moves relative to the airflow. It is assumed that anyone who tackles this riddle knows the basics of aerodynamics or the physics of flight.

Pherekydes wrote:

A plane moves relative to the airflow. It is assumed that anyone who tackles this riddle knows the basics of aerodynamics or the physics of flight.

"Tackles" the riddle? Like it's some big mystery? You're just embarrased that you finally see the alternate interpretation of the riddle and you're trying to save face.

Like the "Engineer" who has yet to post again... My guess is we'll never hear from him again.

yessiree wrote:

IF YOU INTERPRET THE RIDDLE AS CONSTRAINING THE TANGENTIAL SPEED OF THE WHEELS TO BE EQUAL TO THE TANGENTIAL SPEED OF THE TREADMILL, THEN THIS SCENERIO VIOLATES THE CONDITIONS OF THE ORIGINAL RIDDLE!

THIS IS THE POINT THAT DK, KNOWITALL, AND OTHERS HAVE TRIED TO POINT OUT TO YOU, BUT THAT YOU FAIL TO RECOGNIZE!

But the original question talks about the speed the plane is moving at, not the tangential speed of the wheels. These are two entirely different concepts. Excepting the negligible force of friction, the tangential speed of the wheels and the speed of the plane are essentially independent of each other.

Pherekydes wrote:

A plane moves relative to the airflow. It is assumed that anyone who tackles this riddle knows the basics of aerodynamics or the physics of flight.

Why is it assumed to be relative to the airflow in this case? What is preventing it from being relative to the treadmill (and therefore stationary relative to the airflow)? If there is a fan in front of the treadmill, is the treadmill moving at the same speed as the plane relative to the airflow or the plane's speed relative to the rest of the world?

I'm aware of aerodynamics and the requirement for airflow over the wing in order to achieve lift. If you have a strong enough headwind, you could take off with a speed of only 10km/hr. But there is no mention of airflow in the original post.

has yet to post again? He posted less than 1/2 hour ago.

Think of it like the airplane trying to take off on ice.

The plane can take off, it needs the same amount of space (probably a little more) to take off as a traditional runway (the treadmill would have to be that big).

The people that are saying it can't take off are imagining the airplane will be standing still on the treadmill (as I did as well when I first read the riddle) which WOULD be impossible for the plane to take off. The plane WILL continue to move foward no matter how fast the treadmill is going and will eventually gain the speed it needs to take off.

Get it?

like nicotene wrote:

But the original question talks about the speed the plane is moving at, not the tangential speed of the wheels. These are two entirely different concepts. Excepting the negligible force of friction, the tangential speed of the wheels and the speed of the plane are essentially independent of each other.

The tangential speed of the wheels is equal to the tangential speed of the treadmill plus the linear speed of the airplane relative to the ground, or

Vw = Vt + Va

Hardly "esentially independant."

As others have pointed out, "speed the plane is moving at" is ambiguous. What if we assume the "speed the plane is moving at" is relative to the treadmill? Under this assumption, which is perfectly valid given the language of the riddle, Vw MUST ALWAYS EQUAL Vt, since the interface between the wheels and the treadmill is what determines the "speed the plane is moving at relative to the treadmill." If Vw = Vt, then Va must be 0 AS A CONDITION OF THE RIDDLE.

As someone else pointed out, under this interpretation, you could essentially rewrite the riddle to read "if an airplane is held stationary on a moving treadmill, will it take off?" Of course the answer is no.

yessiree wrote:

"Tackles" the riddle? Like it's some big mystery? You're just embarrased that you finally see the alternate interpretation of the riddle and you're trying to save face.

The riddle is a problem and as such can be tackled. The embarrassment is all down to the "no-fly" boys who want a get-out.

"Oh sorry, I didn't understand the riddle. I didn't think that he meant that!"

As I've stated earlier in this thread, the "riddle" was proposed as a mental exercise for Aeronautical engineers, mechanics and pilots, to see if they were alert.

It wasn't meant for people who have no aeronautical training, whatsoever.

Asterix wrote:

Why is it assumed to be relative to the airflow in this case?

You have to assume some basic rules of aerodynamics are a "given", or understood.

Aircraft speed is measured relative to the air. To measure it relative to anything else, or to assume so, is a nonsense.

okay, the only way that the plane moves forward and takes off is if it's going faster than the treadmill. that can't be that hard to comprehend. the riddle said same speed. it seems pretty open and shut. i don't know why people are trying to make up impossible scenarios and say the plane is actually going 200mph when it's going 100, and all this bs about how no treadmill can be as powerful as a jet engine. that has nothing to do with this riddle. you know, the only reason it's even a "riddle" is because the person who wrote it used language just ambiguous enough to give some people cause to say "it only says SPEED, it doesn't say speed of what," and blah blah blah.

Pherekydes wrote:

As I've stated earlier in this thread, the "riddle" was proposed as a mental exercise for Aeronautical engineers, mechanics and pilots, to see if they were alert.

Then it wasn't a very good mental exercise if it did not make clear what the plane was moving relative to. The treadmill? The airflow? The rest of the world?

Anyone who knows aerodynamics knows that lift is determined by plane speed relative to airflow so it is imperative that the original problem clarify what is the frame of reference for the motion of the plane.

It doesn't do that and so is a poor riddle and the writer was not very alert.

Pherekydes wrote:

As I've stated earlier in this thread, the "riddle" was proposed as a mental exercise for Aeronautical engineers, mechanics and pilots, to see if they were alert.

It wasn't meant for people who have no aeronautical training, whatsoever.

Are you serious? You think this riddle is a mental exercise for aeronautical engineers? This is a mental exercise for people who do crossword puzzles and word finds in their spare time, not people who use Bernoulli's equation on a daily basis.

While I agree that the "right" answer to the riddle under its intended assumptions is that the plane will fly, I think the "no fly" camp has a valid point as well. I was just trying to point that out to those who refuse to admit there is a second interpretation.

Asterix wrote:

Then it wasn't a very good mental exercise if it did not make clear what the plane was moving relative to.

As stated, if the exercise was for aeronautical types, certain knowledge would be assumed.

If you were giving mathematicians a "simple little riddle" you wouldn't have to teach them the basics of addition or multiplication.

From an aeronautical point of view, the riddle is quite basic, but includes a red herring to throw people off.

Jag is correct.

wow. ok i will help you out. wanna know how the wheel spins twice as fast as the plane. ill tell you. it is safe to assume if you are looking at a cross section of this plane and treadmill and the wheels that the plane is on top of the wheels, and the treadmills track is under them... correct, good. now find a wheel in your house( and bike will do) lift it off the ground STICK IT in front of your face so you cant miss it. now take your hand and put it on top of the wheel and spin it in one direction. next put your hand on the bottom of the wheel and spin it in the other direction. IT SPINS THE SAME WAY. congratulations this is how the wheel works. feel good. great now as to your post where you dare me to refute anything you said. change the word GAS in your post to AIR and you have the SAME DAMN THING that i said before. jet engines push air out one way pushing the plane forward. good job buddy you look smart now. as to the thought process behind the fact that the treadmill going at 150 mph in one direction would cancel out the air planes thurst at 150mph in the oppisite direction is just plain dumb. find a treadmill and a pair of roller blades :-D. now stand on your treadmill with your sneakers on and turn the treadmill on. notice how you will be sent flying off the end of the treadmill. now but on the rollerblades. the same thing will happen, except to a much less degree(especially once the wheels start spinning) and you will not fly off as fast as you did before. this is because the treadmill although going at the same speed as before cannot exert its full force on you with the free spinning wheels on your feet, as it can when you are grounded by your sneakers. this has to do with friction, which is what i said all along. so we have our plane going in the positive direction at 150 mph and the treadmill going in the opposite direction at 150mph. but heres what happens, the treadmill cannot completly negate the force that the plane creates even though they are going at the same velocity. so what happens. the plane still goes forward creating lift and taking off. you all think it would not be able to take off because there is no lift because the plane is being counteracted by the treadmill. this is not the case because although the treadmill is going 150mph in the othe direction it is not exerting that much force onto the plane because of the wheels. i basically just told you the same thing twice but whatever because it was needed. wow. you are a dumba$$ period the end.

:-D

and you are welcome.

yessiree wrote:

I was just trying to point that out to those who refuse to admit there is a second interpretation.

There is no second interpretation. That's just an excuse for those who don't understand the answer.

lol sorry that i didnt respond fast enough mr. areonoughtical engineer but i was at lunch :-P. and wow. the plane could probally take off even if the treadmill is going faster than the plane, that much is obvious as well so of course if the plane is going faster it would take off silly :-P

Pherekydes wrote:

As I've stated earlier in this thread, the "riddle" was proposed as a mental exercise for Aeronautical engineers, mechanics and pilots, to see if they were alert.

It wasn't meant for people who have no aeronautical training, whatsoever.

The more I think about your comment, the funnier it becomes.

Like there's a bunch of engineers sitting around at Lockheed Martin and their boss runs up and says -

"I want an answer quick!! Will a plane take off if its sitting on a treadmill and the treadmill spins at exactly the same speed the plane is travelling?"

(Engineers in deep thought with hands upon their chins...)

"No it won't! The force from the treadmill will counteract the engine's thrust!"

"WRONG!! Of course the plane will take off! The treadmill only imparts rolling resistance and friction on the plane! What, did you think I meant speed relative to the treadmill, you idiot?! I've got my eye on you, Johnson..."

(Points two fingers at his own eyes, and then to Johnson...)