Airplane / Treadmill Voting Booth

[orlowskij]

pay attention, orlowski. That illustration is not what's going on, here. In that illustration the rollerblade wheels are spinning, and the guy isn't moving forwards. Impossible in terms of this question.

Read DJ's post above yours.

Except that the illustration is what is happening here. To make the illustration the same as what is going on here, set the treadmill's velocity equal to that of the motorcycle. The rider of the motorcycle moves forward at 1 mi/hr. The treadmill spins the opposite direction at 1 mi/hr. The guy on rollerblades moves forward at 1 mi/hr. The wheels spin at a relative velocity of 2 mi/hr to the treadmill. The motorcycle is the same as the engines of the plane because it's ability to create a force is not affected by the treadmill.

[The AD]

The problem is quite clear that the speed is not measured by a speedometer at the wheel.

I don't really think it is. Your take is that the treadmill's control system measures the linear speed of the airplane (or car) with respect to a stationary reference. You could equally assert the treadmill is using the rotational speed of the airplane's (car's) wheels. This would be the speedo measurement.

[Big Blue]

Wow, focus really is the only one that gets it. Even the other "take-off" people have problems understanding what's happening.

[summit]

Doesn't matter... plane flies

[dbp]

You're in the car, you hit the gas. The car moves forwards, the treadmill moves backwards at the same time the car moves forwards, at the same speed. If the car doesn't move forwards in relation to its point in space on the treadmill, the treadmill cannot move backwards!

The forward speed of the car, in relation to its point in space on the treadmill, will always be 1/2 of the rotational speed of its wheels. The car's acceleration, in relation to its point in space on the treadmill, will always be 1/2 of that of its wheels. But the car WILL accelerate. It WILL move forwards (in relation to its point in space).

IMHO, the problem with looking at the problem that literally is it makes the question boring and about semantics. Yes, if the plane/car/rollerblades have a velocity for the treadmill to match... then it HAS a velocity. It IS moving. I think the "spirit" of the question is talking about the "potential" velocity the thing would have if it wasn't on a treadmill. The questioner wants you to think the plane won't move (our most common experience with treadmills). People know that a car/jogger/anything-using-the-surface-of-the-treadmill-to-propel-itself CAN be kept in place by speeding up the treadmill. People take this mistaken assumption about the plane, add it to their Bernouli knowledge, and say it won't take off. If one thinks about the problem this way, knowing the plane takes off becomes dependent on understanding the difference between a car/jogger and a plane. That is more interesting to me than "Hey the problem says it has a forward velocity." In your interpretation, the discussion lasts about 15 seconds.

[f2f]

time for a group photo of everybody posting in this thread:

[summit]

cave girl is sooooo hot

[DJSapp]

I don't really think it is. Your take is that the treadmill's control system measures the linear speed of the airplane (or car) with respect to a stationary reference. You could equally assert the treadmill is using the rotational speed of the airplane's (car's) wheels. This would be the speedo measurement.

You are creating a paradox. The question states that the object moves. Let's say you are driving the car, and the treadmill is hooked up to the speedo. You step on the gas, hit 1 mph (the whole car, since the problem states the whole thing moves) and put it in neutral. The treadmill begins to move backward at 1 mph, but the speedo reads 2 mph. The treadmill cannot stasify both conditions of matching the speedo reading and allowing the whole car to move. This violates the rules as stated in the question.

[focus]

IMHO, the problem with looking at the problem that literally is it makes the question boring and about semantics. I think the "spirit" of the question is talking about ... The questioner wants you to think .... People know that ... If one thinks about the problem this way, knowing the plane takes off becomes dependent on understanding the difference between a car/jogger and a plane. That is more interesting to me ...

That post isn't really complete until you toss something in about your feelings.

People take this mistaken assumption about the plane, add it to their Bernouli knowledge, and say it won't take off.

I think this is the point of the question. And it's an interesting point about assumptions, reading comprehension, etc.

In your interpretation, the discussion lasts about 15 seconds.

No, the discussion has lasted for about 500 posts.

I won't deny that the question and the controversy surrounding it does raise some interesting discussions about physics/etc. that really have nothing to do w/ the premise of the question. But substituting what you would like for the question to be about with what it really is about as an argument for interpreting it differently is...well...silly.

[The AD]

OK, true, it can't match the car's speedometer reading, but it can match the tire's rotation speed. You seem to think "moving" implies moving in a Cartesian coordinate system, but the question doesn't state what kind of movement is implied.

[focus]

OK, true, it can't match the car's speedometer reading, but it can match the tire's rotation speed. You seem to think "moving" implies moving in a Cartesian coordinate system, but the question doesn't state what kind of movement is implied.

"The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction)."

speed = distance traveled divided by the time in which it traveled.

We also know the plane's movement is directional.

The plane travels a distance in a direction over a period of time -- tough to interpret that in any other way than a cartesian coordinate system without getting super picky.

Do you honestly interpret the plane moving in a direction as just the tires spinning, while the rest of the plane remains at rest? And we know nothing about wheels or tires. We assume that this 747 has wheels. It doesn't actually matter.

The discussion is finally descending into semantics.... The hell of it is that the question really wasn't that poorly or vaguely worded in the first place.

[The AD]

Finally descended into semantics? I'd say that bridge was crossed a long time ago.

I do think the plane will take off, I just take issue with some of the statements about what or what not the question stated. Speed doesn't necessarily mean linear speed. It could mean rotational speed of the tires.

[Brock Landers]

I took it to mean linear speed and I think most others did too.

Speed of the tires doesnt even matter.

[focus]

Finally descended into semantics? I'd say that bridge was crossed a long time ago.

Not for some people.

I do think the plane will take off, I just take issue with some of the statements about what or what not the question stated. Speed doesn't necessarily mean linear speed. It could mean rotational speed of the tires.

Who said anything about tires, man? I don't remember reading anything about wheels, or tires.... You're just adding your own stuff to the problem.

I understand what you're saying re: linear speed. I think we can safely assume that when we're talking about speed, directions, and things like planes, we're talking about linear speed unless there's something that indicates we shouldn't. Anything else is kinda disingenuous, don't you think?

[Tippster]

pay attention, orlowski. That illustration is not what's going on, here. In that illustration the rollerblade wheels are spinning, and the guy isn't moving forwards. Impossible in terms of this question.

Read DJ's post above yours.

It is what's going on here. As soon as the motorcycle moves (the jet engines fire up) the guy on the treadmill will move forward regardless of the treadmill's speed in the opposite direction.

[The AD]

Speed of the tires doesnt even matter.

au contraire. If at all times the rotational speed of the airplane's tires and the treadmill are equal and opposite the airplane cannot move forward. Am I correct in this? If not, why?

[focus]

It is what's going on here. As soon as the motorcycle moves (the jet engines fire up) the guy on the treadmill will move forward regardless of the treadmill's speed in the opposite direction.

You're right. I'd interpreted the illustration to mean the wheels were spinning as the guy was stationary.

I do take some issue with the comment in the illustration that expressly states that the force from the motorcycle is not affected by the treadmill -- simply because it doesn't matter and is misleading (and is, technically, wrong). Illustrations should help, not send us off on the wrong track -- right?

[focus]

au contraire. If at all times the rotational speed of the airplane's tires and the treadmill are equal and opposite the airplane cannot move forward. Am I correct in this? If not, why?

god.

yeah, you're correct, but that isn't what's going on in the original question.

"Rotational speed" and "airplane's tires" have no place in this discussion.

[Brock Landers]

au contraire. If at all times the rotational speed of the airplane's tires and the treadmill are equal and opposite the airplane cannot move forward. Am I correct in this? If not, why?

youre right but it doesnt matter in this case

EDIT- what focus said too

[dbp]

Huh? The treadmill can't keep the plane from moving forward. I don't care if the treadmill is moving in the opposite direction at 100x the rotational speed of the tires. I thought all of the "it takes off" people had agreed on that.

I do take some issue with the comment in the illustration that expressly states that the force from the motorcycle is not affected by the treadmill -- simply because it doesn't matter and is misleading (and is, technically, wrong). Illustrations should help, not send us off on the wrong track -- right?

For all intents and purposes, the motorcycle is not affected by the treadmill - not to the extent it would be if it were actually on the treadmill. This is certainly analogous to the airplane engines which aren't affected either. And it does matter. In fact to the takeoff people who aren't thinking along your lines, it is one of the only things that matters.

Though I disagree with your interpretation of the question, I at least understand it now. I think the fact that so many people believe the treadmill can keep the plane stationary under any circumstances lends credence to the authors intent of a physics question rather than a reading comprehension question.

[DJSapp]

au contraire. If at all times the rotational speed of the airplane's tires and the treadmill are equal and opposite the airplane cannot move forward. Am I correct in this? If not, why?

No. The airplane cannot make it's tires rotate without moving the airframe, as they are free spinning (we've been here before). The engines push the airframe which pushes the tires. According to you, the rotational speed of the plane's tires and treadmill are equal and opposite.

Wheelspeed = Airframe speed + Treadmill speed

The only solution to this equation is for the airframe to be zero, but then the system cannot begin moving, since the wheels require a non-zero airframe speed to begin moving. This is the paradox I attempted to describe earlyer.

[Art Shirk]

Ok dammit. I need to see some dickwaving. Post up your respective qualifications. I want to see them. If your only qualification is common sense then say that too. It isn't dickwaving if requested.

[focus]

Huh? The treadmill can't keep the plane from moving forward. I don't care if the treadmill is moving in the opposite direction at 100x the rotational speed of the tires. I thought all of the "it takes off" people had agreed on that.



For all intents and purposes, the motorcycle is not affected by the treadmill - not to the extent it would be if it were actually on the treadmill. This is certainly analogous to the airplane engines which aren't affected either. And it does matter. In fact to the takeoff people who aren't thinking along your lines, it is one of the only things that matters.

Though I disagree with your interpretation of the question, I at least understand it now. I think the fact that so many people believe the treadmill can keep the plane stationary under any circumstances lends credence to the authors intent of a physics question rather than a reading comprehension question.

Just because you arrive at the correct answer doesn't mean you understand the problem. That's why math teachers made us show our work. And yeah, I think how you got there is every bit as important, if not more important. If you aren't thinking along my lines, I assert that you're thinking along the wrong lines..... Take that with a grain of salt.

I don't see how that can lend any credence to the author's intent, but whatever. I've moved on to a new crusade: http://tetongravity.com/forums/showthread.php?t=77425

[DJSapp]

Ok dammit. I need to see some dickwaving. Post up your respective qualifications. I want to see them. If your only qualification is common sense then say that too. It isn't dickwaving if requested.

2 years high school physics (got a 5 on the AP test), 1 year in college
B.S. in Civil Engineering from UC Davis
CA Engineer in Training
3 years work as a heavy construction engineer (i.e. the engineers that actually know how to get shit done)
Many friends who are pilots
Dabbeled in internet dentistry

[focus]

Ok dammit. I need to see some dickwaving. Post up your respective qualifications. I want to see them. If your only qualification is common sense then say that too. It isn't dickwaving if requested.

I stayed at a Holiday Inn Express last night.