Have I ever suggested that it wouldn't take off?

Except, of course, for the obvious concerns about overexceeding the specifications and capabilities of the tires from spinning too fast.

[This message has been edited by eric76 (edited 12/8/2006 12:07a).]

The conveyor belt is moot. The plane takes off.

If there were no wheels, then the conveyor belt would be moot. As it is, the wheels turning prevents the plane from moving forward over the rotating conveyor unless the thrust of the plane causes the plane to move faster in which case the wheels would spin faster as well but if the conveyor compensates by speeding up then the plane still doesn't take off unless the wheels break and then the thrust of the plane would be more than enough power to overcome the friction between the broken wheels and the conveyor belt and then the plane would take off but by then you would have much less fuel than expected and you would have to stop in Toledo to re-fuel and since Toledo is nowhere you have by definition gone nowhere which can also be defined as not having taken off.

[This message has been edited by Dad-O-Lot (edited 12/8/2006 2:01p).]

You know, it's abundantly clear that there are two entirely different situations being discussed here.

The conveyor belt is moot because of the wheels.

dad-o-lot,

consider this. When a question is asked to a large group of people, and an overwhelming amount of people pick one answer and you and a few others pick the other answer, you can assume that you are amoung the absolute most intellegent people in the room or you can assume you may have missed something. Now, in this room of highly educated, intelligent people(you included), are you smarter than everyone else or maybe missing something.

This is a takeoff of the old adage--if you cant identify the sucker in the room, its you

Thanks traveler.

Truth is not subject to the whims of the democratic process.

Since you fail to see the humor in the above post, I will give (another) serious reply.

Whether or not the plane takes off depends on how you define the "speed" that the conveyor goes and the existence or absence of friction between the wheels and the conveyor and the friction of the wheel bearings themselves.

Bottom line, it all depends on the assumptions one is working with. There are valid arguments either way. As for the "sucker" that would be anyone who takes this crap seriously enough to be bothered by it and begin to demean others about it.

dad-o-lot, so you're saying your a sucker?

wasn't trying to demean. the sucker part wasnt for you. i understand your point about the friction, but i think you are still missing the point. yes you could create a scenario where the friction is so great it would overcome the thrust of the engine however you are going to the ultimates of physical demensions to do so. This is a theoretical problem only. Thats why they only give you that kind of information(just what you need to address this in theory). If they truly wanted you to address all the physical properties involved they would have including things like that in the problem. that was what i meant by missing something. i think you are getting bogged down by the details and missing the original intent of the exercise.

Let's rephrase the question.

A Bell Jet Ranger (yes, it is an aircraft) is standing on a movable runway(something like a conveyor) As the aircraft moves, the conveyor moves but in the opposite direction. The conveyor has a system that tracks the speed of the aircraft and matches it exactly in the opposite direction.

The question is, will the aircraft take off or not?

Enjoy.

Am I missing something?

No that looks correct to me in simplistic terms.

Don't remember which side you are on...but that doesnt really prove anything. The -v of the conveyor does not cancel out the velocity of the plane....just makes the wheels turn faster.

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dad-o-lot, so you're saying your a sucker?

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Nah, I'm having fun. Not demeaning anyone.

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wasn't trying to demean

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Then I apologize for the assertion.

The key to this issue is how you define the "speed" of the plane and what regulates the "speed" of the conveyor.

I read the original problem to mean that whatever thrust was applied to the plane would be compensated for by the movement of the conveyor in the opposite direction. I also assumed that the wheels would not "slide" across the conveyor but would only spin due to the rotation of the conveyor. Yes, the wheels would spin faster, but then, in my assumption, the conveyor would as well. I also assumed that there were no limits on what speed the conveyor could reach.

In the other thread I posted an analogy of a guy on a skateboard using a powerful fan for propulsion. If he had the fan set to move him across the floor at a constant 5 mph (let's say going east) and then he was placed on a people mover conveyor that was moving 5 mph to the west. Assuming the wheels of the skateboard are free spinning, does the guy on the skateboard move in relation to the area outside of the conveyor? If so, at what relative velocity?

Dad-o-lot : haha, I know, i think this is funny to! Just frustrates me how silly some people make themselves look.

1876: Um, yeah you have no thrust in that picture by the way. There is no such thing as Conservation of Velocity, so don't try using a cancellation of velocity to prove the plane won't take off. There is, however, Conservation of Momentum which you have not accounted for. Draw a thrust vector and you'll see no equal/opposite force.

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I read the original problem to mean that whatever thrust was applied to the plane would be compensated for by the movement of the conveyor in the opposite direction.

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I don't understand how you can interpret it that way.

The original statement was:

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The conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

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It matches the speed exactly in the opposite direction. That's not at all ambiguous.

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There is no such thing as Conservation of Velocity

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I think that we have demonstrated a new law, that of Conservation of Reading Comprehension.

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The conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

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It matches the speed exactly in the opposite direction. That's not at all ambiguous.

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The ambiguity is in how the speed is measured. If it's measured in relation to an outside point, then by definition the plane moves and thus takes off. Then, if the speed of the conveyor equals the speed of the plane relative to an outside point, then by definition the plane and the conveyor are both moving in relation to an outside point so again, the plane moves by definition.

If the speed of the plane is measured in terms of rpms of the wheels, (when on the ground) then the conveyor keeps up and the plane does not take off.

As I have said several times, it depends on how you define the "speed" of the airplane and what drives the "speed" of the conveyor.

Even if the speed was measured by the rpm's of the wheels, how would this apply a force great enough to oppose the thrust of the jet engines?

As I have stated before and as Dad-O-Lot has stated, you must specify how you measure the planes velocity. If it is relative to the ground then it takes off. If it is measured relative to the conveyor then it does not.

When velocity is measured relative to the conveyor the velocity of the plane as measured from a point not on the conveyor is 0. No matter what the speed of the plane is the conveyor matches it. Therefore, if the speed of the plane is 0 it never accelerates. If it never accelerates, then the summation of forces MUST equal zero. This is only true when the thrust force equals the friction force. What this means is by defining the velocity of the conveyor equal to the plane velocity, the forces are automatically defined. Any assumption that the thrust force is greater than friction force violates the velocity constraint. Realistically, the plane could easily increase thrust and take off. But the problem doesn’t give info on forces, only velocity.

The other way to look at it is, if you are the pilot and must operate the plane such that the velocity of the plane (as measured from the conveyor) matches the velocity of the conveyor at all times, would you be able to takeoff? The answer is no.

that just ain't so. If the conveyor goes the same speed in the negative direction as the plane is travelling in the positive direction, the wheels will just spin twice that speed and that's all. The conveyor spinning at any certain speed has very, very little impact on the speed of the plane. In simple mechanical physics, the conveyor is almost a non-event. The opposing force to thrust generated by friction and rolling resistance would never be enough to counter a jet engine. In fact, I would wager that you could lock the brakes on a plane on the conveyor, and the power of the engine would still launch the plane.

I was just about to post that yet again Whoop91. How can you people not see this?

You’re assumption is based on velocity being measured from an observer not on the conveyor. In which case the plane takes off as I stated above but you apparently didn’t read.

Now look at it when velocity is measured from the conveyor.

Draw a free body diagram of the situation of the thrust force being greater than the friction force. Now, draw a velocity vector of the axle going 100 mph (relative to the conveyor) to the left and a velocity vector of the conveyor moving 100 mph to the right. That equals a velocity of 0 mph as measured from someone not on the conveyor. You cannot have an imbalance of force and no acceleration at the same time. That is the whole point.

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Draw a free body diagram of the situation of the thrust force being greater than the friction force. Now, draw a velocity vector of the axle going 100 mph (relative to the conveyor) to the left and a velocity vector of the conveyor moving 100 mph to the right. That equals a velocity of 0 mph as measured from someone not on the conveyor. You cannot have an imbalance of force and no acceleration at the same time. That is the whole point.

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The velocities are only relative, they do not directly cancel each other out. If the plane was flying past a car at 100 mph, and the car was going -100 mph, would the plane and the car stop and not go anywhere? No....

The point is that although the axle is moving with the plane at 100 mph, and the conveyor is moving at -100 mph, the two don't stop each others motion. Only the forces affecting the plane can stop its acceleration.

There is always acceleration because the thrust force is greater than the friction force.

Where in the question does it say the thrust force is greater than the friction force? It does not. That is an assumption you are making. It does however tell you its velocity. It does not tell you where velocity is measured from. You must state this when answering the question. You have not. Depending on where velocity is measured from the plane will either accelerate or it will not.

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As I have stated before and as Dad-O-Lot has stated, you must specify how you measure the planes velocity. If it is relative to the ground then it takes off. If it is measured relative to the conveyor then it does not.

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We don't have to specify how we measure the plane's velocity.

If you put an aircraft on a conveyor belt, as described, and apply the usual throttle necessary to take off, that aircraft is going to move forward with respect to both the conveyor belt and the ground. What the conveyor belt does makes a negligible difference, at best.

the theory of being able to maintain just enough thrust to counter the friction and rolling resistance at pretty much any speed is a fact and would result in 0 forward motion to the observer off of the conveyor. To the observer on the conveyor, the plane would appear to be moving twice its actual speed, since your velocity would be in the negative direction equal to the positive velocity of the plane.

However, there's a fundamental problem with this line of reasoning: the problem states that the conveyor matches the speed of the plane, not the other way around. Therefore, when the plane moves forward, the conveyor moves in the opposite direction. That doesn't hinder the forward progress of the plane (much), but the wheels spin 2*the speed of the plane to satisfy the laws of physics. Playing by the rules of the stated problem, not applying more thrust than needed to counter the friction and rolling resistance is not an option, since those forces won't come into play until thrust has been applied. I suppose you could only apply enough thrust to counter the friction and rolling resistance that will be generated when the plane tries to start rolling.

In the end, the only relavent persective is that of the ambient air, which has to create lift to launch the plane. Since the negative force of the friction and rolling resistance cannot counter the force of the jet engine, the plane will move forward - almost completely unhindered by the conveyor - and take off.

It is a valid assumption to make. Do you actually think that a plane would even be called a flying machine if the force of friction in the axles would prevent it from taking off? NO. Nobody would get inside a plane that doesn't move because the axle friction is too great.

And the place at which you measure the velocity is a moot point. The relative velocities of objects that have no effect on the forces on the plane do not matter.

I'm not saying that a plane cannot overcome the friction force. I know that. We all know that. But the ONLY time the planes velocity and the conveyors velocity are equal is when the thrust force and friction forces ARE equal AND ONLY when the planes velocity is measured by someone on the conveyor.

Now when velocity is measured by someone not on the conveyor the plane can do whatever it wants. The planes wheels, as everyone agrees, will spin twice as fast. The pilot could speed up, slow down, do whatever he wanted.

The plane can do whatever it wants no matter where the velocity is measured. The velocity of the conveyor does not create a force that stops the plane.

Correct. The plane can always do whatever it wants and yes velocity is not a force. But the situation as defined when the plane and conveyor speeds are equal automatically defines the forces when velocity is measured on the conveyor. As soon as you increase thrust then you are no longer following the velocity constraint.

A system could never be developed where a conveyor could prevent a plane from taking off. As soon as the planes thrust overcomes friction in the wheels the conveyor doesn’t matter.

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Yes, we know this.