Physics question

[sperry]

Here's my answer:

1) The plane needs airspeed to generate lift and take off.
2) The plane's engines generate thrust by pushing against the air.
3) The plane's wheels (because the brakes are off) cannot enact a significant force upon the plane against the thrust vector of the engine.

Therefore, the plane will take off. The treadmill has nothing to do with the solution.

I dare you guys to prove me wrong without getting into rediculous agruments like "well if the conveyor is going 1 million mph the friction of the wheel bearings is so high it can't take off". The frictions/inertia of the wheels is insignificant with regards to the magnitude of the engine thrust.

[sperry]

So far, it looks like Austin is the only person in this thread that has a clue. Cal Poly Physics department FTW.

And JC... man, I'm surprised you missed it!

[JC]

You guys are thinking in terms of physics, start thinking in terms of constraints. If the wheel never moves faster than the conveyer belt, how are you creating acceleration? I wish I was there to explain this to you.

[JC]

Originally Posted by sperry

The treadmill has nothing to do with the solution.

There is why you don't get the right answer. The problem imposed the constraint that the wheels on the plane have to move at the same speed as the treadmill. Therefore the treadmill IS part of the solution. You have no way of translating your motion forward.

[MattR]

Oh! JC, if the treadmill HAS to match the wheel speed of the planes tires, then I guess it can't move forward to create lift. So, that makes sense.

[Kevin M]

What if the treadmill does the opposite? What if it matches the speed of the accelerating aircraft exactly, so the the wheel speed is zero? For that to be true, if airspeed = X, then the conveyor belt = X also, yes? This is why I think the plane will take off. Airspeed will reach X, conveyor speed will be -X and wheel speed will be 2X.

[Kevin M]

Okay now I get it. The conditions of the question cannot be met with an aircraft. If the plane lights up its engines on this theoretical conveyor, it would EITHER remain motionless, or the conveyor could not be the exact inverse of wheel speed. At least not in this universe.

[AtomicLabMonkey]

Hmm, after drawing a diagram I think JC wins assuming no slip between tire and belt. The two angular velocities have to be opposite and equal, and the only condition that satisfies that equation is when they are zero, which means no airspeed.

[Nick Koan]

Originally Posted by BAN SUVS

Okay now I get it. The conditions of the question cannot be met with an aircraft. If the plane lights up its engines on this theoretical conveyor, it would EITHER remain motionless, or the conveyor could not be the exact inverse of wheel speed. At least not in this universe.

Technically, the wheels if they moved even the slightest bit, would eventually get up to infinite speed (as does the treadmill, due to the constraint).

The problem *is* poorly posed.

[Kevin M]

Yeah that's my conclusion. It's the one-sided die.

[Kevin M]

The sound of one hand clapping makes a 13 page (and counting) thread in nabisco OT.

[sperry]

Okay, JC actually called my cell phone to "discuss" this with me. He raises a good point, that due to the constraints of the problem and the use of "wheel speed" instead of "ground speed" in the problem definition, the plane can't take off, simply because the problem says so.

IMO, that's a bit of a cop out... I realize the question is poorly written, but that's because I copied it verbatim from the original NASIOC thread. I think the intent of the question is obvious, "what happens if a plane trys to take off on a conveyor belt that attempt to counteract the plane's forward speed".

So, if you want to argue symatics about the wording of the question, then the plane can't take off because the words in the question prevent it. If you'd rather use the laws of physics, as I think the original author of the question intended, the plane will take off because there's no force to counteract the thrust of the engines.

[sperry]

Originally Posted by nKoan

Technically, the wheels if they moved even the slightest bit, would eventually get up to infinite speed (as does the treadmill, due to the constraint).

The problem *is* poorly posed.

That's only true if we assume a feedback loop control system. In a purely theoretical world, the wheels could never get even a tiny bit "ahead" of the belt that would require the belt to catch up.

[MattR]

Wow. I really enjoyed this discussion.

[sperry]

Originally Posted by AtomicLabMonkey

Hmm, after drawing a diagram I think JC wins assuming no slip between tire and belt. The two angular velocities have to be opposite and equal, and the only condition that satisfies that equation is when they are zero, which means no airspeed.

Basically, if you turned on the jet's engines, the world would be unmade because the laws of physics are violated..

[sperry]

Originally Posted by MattR

Wow. I really enjoyed this discussion.

Sarcasm?

[MikeK]

Please tell me that everyone who said no in that thread (and this one) was joking. Otherwise I won't know whether to laugh or cry (or start mocking bitches).

It reminds me of that thread about what a jet (thrust) powered car would put down on a dyno


Here is another problem:

A lake is 10 kilometres wide x 10 kilometres long with a dam at one end. The water depth at the wall of the dam is 100 metres.

A soda straw is 100 metres long. It is suspended vertically and filled with water.

Is the pressure at the bottom of the dam wall more than, less than or equal to the pressure at the bottom of the soda straw?

(anyone who mentions metric gets a complimentary punch to the nuts)

[MattR]

Originally Posted by sperry

Sarcasm?

No, I was serious, I don;t use my brain very often anymore, so It was nice to dust the fuckin thing off

[AtomicLabMonkey]

Originally Posted by sperry

Okay, JC actually called my cell phone to "discuss" this with me. He raises a good point, that due to the constraints of the problem and the use of "wheel speed" instead of "ground speed" in the problem definition, the plane can't take off, simply because the problem says so.

IMO, that's a bit of a cop out... I realize the question is poorly written, but that's because I copied it verbatim from the original NASIOC thread. I think the intent of the question is obvious, "what happens if a plane trys to take off on a conveyor belt that attempt to counteract the plane's forward speed".

So, if you want to argue symatics about the wording of the question, then the plane can't take off because the words in the question prevent it. If you'd rather use the laws of physics, as I think the original author of the question intended, the plane will take off because there's no force to counteract the thrust of the engines.

If the question wording is correct, then it sounds like it was written by a grad student with too much time on his hands that wanted to trip people up.

[MikeK]

Originally Posted by sperry

So, if you want to argue symatics about the wording of the question, then the plane can't take off because the words in the question prevent it.

I disconcur with this ... this sentence:

Originally Posted by world's simplest physics problem

The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation

... implies that the wheels on the plane are free to move at any speed, and the conveyor belt will automatically change speed to match.

[sperry]

Damn Mike, not only are you using those mythical "meter" things again, you're also spelling 'em wrong!

Also, I'm not sure how to solve that problem... at 100 meters deep, the *water pressure* is the same in both... but I'm not sure how that translates into pressure against a dam wall.

[AtomicLabMonkey]

Originally Posted by MikeK

The water depth at the wall of the dam is 100 metres.


(anyone who mentions metric gets a complimentary punch to the nuts)

Haha, you even spelled meters the fancy-pants way.

[MikeK]

Originally Posted by sperry

Damn Mike, not only are you using those mythical "meter" things again, you're also spelling 'em wrong!

Also, I'm not sure how to solve that problem... at 100 meters deep, the *water pressure* is the same in both... but I'm not sure how that translates into pressure against a dam wall.

Water pressure == wall pressure. You win.

[JC]

Originally Posted by MikeK

... implies that the wheels on the plane are free to move at any speed, and the conveyor belt will automatically change speed to match.

It means the plane can never move. Draw a diagram Mike and explain to me how you are moving without sliding or having the wheels be different speeds. Both my friend and I got this pretty fast guys, he even laughed when he read it. C'mon now, always read the problem before you start doing it. And quit your bitchin' Scott.

[sperry]

Originally Posted by MikeK

I disconcur with this ... this sentence:



... implies that the wheels on the plane are free to move at any speed, and the conveyor belt will automatically change speed to match.

Yeah, but there's no way to get the wheels moving to begin with. Think of it this way: if the wheels *have* to spin at the same speed as the belt, there's no way for the wheel's axles to *ever* move relative to the ground around the belt, and therefore there's no way for the plane to ever move.

'Course, there are no physics behind that solution... it's just taking the problem definition to a new level...