This is insterpretation #1 above for the airplane and under that interpreation the airplane takes off. :fmicon:Quote:
Originally Posted by focus
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This is insterpretation #1 above for the airplane and under that interpreation the airplane takes off. :fmicon:Quote:
Originally Posted by focus
This is where so many of you are so confused. It accelerates along with the car. It could give a shit about the wheels.Quote:
Originally Posted by Summit
It takes off.
until the treadmill activates the bomb placed in the forward cargo hold, end result, the plane crashes
+1 to the treadmill.
I thought you were an interpretation #2 type of guy? (either way the airplane flies)Quote:
Originally Posted by focus
Under interpreation #1 the car should move (and the airplane flies)
Under interpretation #2 the car should not move (and the airplane flies)
I'm a #1. I can't figure out what the hell you are.Quote:
Originally Posted by Summit
Unless you've recanted these and other related statements..... ;)Quote:
In reality, the car would probably move a couple of mm.
Remember, the track is not at a fixed velocity, it accelerates along with the wheels.
It doesn't matter! The airplane takes off anyway! :DQuote:
Originally Posted by focus
(unless the tires blow out)
but I interpret the question under #1
The problem is quite clear that the speed is not measured by a speedometer at the wheel. It would be measured by a radar gun set on the ground. The problem states that the car (yes, the whole fucking thing) MOVES in one direction. At a dyno shop, the car as a whole does not move. This is the difference.Quote:
Originally Posted by original question, modified for the car scenario
I wasn't going to post, but I think this is a clear way to illustrate what is going on.
http://www.tetongravity.com/forums/a...1&d=1134762033
It simplifies the problem a lot if you draw a FBD of the plane and the wheels. The only forces that act on the body of the plane (the part that determines whether or not it takes off) are the normal force upward from the treadmill, the force of gravity downward, and the thrust of the engines forward. Assuming that there is friction between the wheels of the plane and the treadmill, and that the friction in the wheel bearings/rolling resistance of the plane is negligible, the plane will take off. When the pilot throttles up the engines, the plane will start to move forward, the treadmill will spin, the wheels will spin, and the plane will continue to accelerate relative to the ground until it takes off. The treadmill just means that the wheels will be spinning faster when it does take off.
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.
pretty sure that the illustration is meant to show that dumbass exhibit #1 is about to get his ass dragged off the treadmill ;)
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.Quote:
Originally Posted by focus
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.Quote:
Originally Posted by DJSapp
Wow, focus really is the only one that gets it. Even the other "take-off" people have problems understanding what's happening.
Doesn't matter... plane flies
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.Quote:
Originally Posted by focus
time for a group photo of everybody posting in this thread:
http://tetongravity.com/forums/attac...chmentid=22114
cave girl is sooooo hot
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.Quote:
Originally Posted by The AD
That post isn't really complete until you toss something in about your feelings.Quote:
Originally Posted by dbp
I think this is the point of the question. And it's an interesting point about assumptions, reading comprehension, etc.Quote:
People take this mistaken assumption about the plane, add it to their Bernouli knowledge, and say it won't take off.
No, the discussion has lasted for about 500 posts.Quote:
In your interpretation, the discussion lasts about 15 seconds.
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.
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)."Quote:
Originally Posted by The AD
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.
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.
I took it to mean linear speed and I think most others did too.
Speed of the tires doesnt even matter.
Not for some people.Quote:
Originally Posted by The AD
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.Quote:
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.
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?
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.Quote:
Originally Posted by 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?Quote:
Originally Posted by Brock Landers
You're right. I'd interpreted the illustration to mean the wheels were spinning as the guy was stationary.Quote:
Originally Posted by Tippster
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?
god.Quote:
Originally Posted by The AD
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.
youre right but it doesnt matter in this caseQuote:
Originally Posted by The AD
EDIT- what focus said too
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.Quote:
Originally Posted by focus
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.
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.Quote:
Originally Posted by The AD
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.
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.
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.Quote:
Originally Posted by dbp
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
2 years high school physics (got a 5 on the AP test), 1 year in collegeQuote:
Originally Posted by commonlaw
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
I stayed at a Holiday Inn Express last night.Quote:
Originally Posted by commonlaw
So far, my vote is with you...but DJSapp has the whole dentistry thing. I just need to pick a side here without using my own brain. Funny thing is, I wouldn't spend a minute trying to figure out if the plane will take off, but I would spend hours wondering whether the plane actually exists.Quote:
Originally Posted by focus
I knew I should have taken my commitment to internet dentisty more seriously :(Quote:
Originally Posted by commonlaw
Were you a philosophy minor?Quote:
Originally Posted by commonlaw
edit- I majored and got a degree in physics, but I hated mechanics
How about a slight rewording of the problem to hopefully sidestep the difference between the two take-off camps' interpretations...
- Plane is on huge treadmill
- Plane starts accelerating forward while treadmill stays stationary
- When the plane reaches 10 mph, the treadmill starts moving in the opposite direction at 750 mph
I would contend that the plane will continue to accelerate and take off. In fact, the treadmill will still have almost no affect in this scenario.
B.S. with high honors in mechanical engineering from Michigan State
M.S. in mech. eng. (thesis dealt with fluid mechanics)
10 years experience in aero engineering
...and obviously none of these things helped me one iota in figuring out whether the airplane would fly or not.