Author Topic: Airplane on a Conveyor Belt...  (Read 31099 times)

Offline SaburoS

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Airplane on a Conveyor Belt...
« Reply #270 on: February 02, 2008, 12:31:55 PM »
Quote
Originally posted by AKIron
Just a little tangent to give us a break from this heated battle.

You're standing in boxcar that is sitting on a rail that is virtually frictionless. You're standing at one end of the boxcar and you throw a baseball at the far wall. What is the net result of this action?


Well if the boxcar is sitting on an electromagnetic zero friction cushion, the boxcar moves ever so slightly in the opposite direction. The ball then transfers the force on the opposite wall and reverses the progress (canceling it out). The boxcar then returns to its original position. This is of course if no energy is lost from the transfer points (kinetic-heat).

Kind of like trying to use a boat mounted fan to power the sails. The boat stays.
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Offline VERTEX

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Airplane on a Conveyor Belt...
« Reply #271 on: February 02, 2008, 12:36:37 PM »
Heres one for you to ponder.

An airplane sits on a runway, with a huge fan behind it. As the airplane engine is sped up the fan blows air towards the airplane. The fan matches the velocity and volume of air moving over the wings exactly.

What happens to the airplane as the engine is revved up?

Offline AKIron

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Airplane on a Conveyor Belt...
« Reply #272 on: February 02, 2008, 12:37:43 PM »
Quote
Originally posted by SaburoS
Well if the boxcar is sitting on an electromagnetic zero friction cushion, the boxcar moves ever so slightly in the opposite direction. The ball then transfers the force on the opposite wall and reverses the progress (canceling it out). The boxcar then returns to its original position. This is of course if no energy is lost from the transfer points (kinetic-heat).

Kind of like trying to use a boat mounted fan to power the sails. The boat stays.


The ball only stops the boxcar when it hit's the far wall. If it moved it back in the opposite direction why would it stop in the original place with no friction?

Assume that my statement about the car moving, stopping, and then moving back with the walking is correct. Is work being done? Not the sort that your boss would be happy with but the sort defined in a physics 101 book.
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Offline eskimo2

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Airplane on a Conveyor Belt...
« Reply #273 on: February 02, 2008, 12:40:15 PM »
Quote
Originally posted by SaburoS
No. This isn't about giving up.

At no point does the question allow a greater force to be used. It is to match the wheel's speed. Unless the aircraft moves forward the wheel's speed is zero and the conveyor's counter speed is zero.
Unless the aircraft is accelerating the conveyor won't accelerate. Once you grasp that, you'll see where I'm coming from and why I see your reasoning as flawed.
We're not talking about a car that gets its forward movement from its wheels.

The plane flies.
The conveyor fights the losing battle for it has to react to the plane's forward movement for its speed. No forward movement/acceleration, no conveyor speed.

You just haven't grasped the fact of all the forces involved.
You keep confusing ground speed to wheel rpm speed.

Plane flies.


You are getting hung up on the semantics and precision of the word exactly and how the control mechanism could work.  Maybe by exactly it anticipates the power output of the engine/prop and responds at the same instant.  Maybe it has a tolerance of one mm.  

The point is that the conveyor could push the plane back, even if the plane is at full power.  The point is that the question forces a solution that is counter intuitive and hard to imagine, even by very bright people.

In the version of the question where the conveyor matches the plane’s speed, I think most children would have trouble understanding why the plane would fly.  It’s still a good question though.  This question appears to put a twist on that one; it appears to be geared to be understood by some adults.  It really is a higher level question.  Rabbidrabbit indicated that he intentionally put the twist on it.  If he did so with an understanding of this solution; I’d say it was a moment of brilliance.

Offline eskimo2

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Airplane on a Conveyor Belt...
« Reply #274 on: February 02, 2008, 12:43:45 PM »
Quote
Originally posted by VERTEX
The conveyor cannot match the planes speed, the free wheeling nature of the wheels prevents this, the conveyor can only act on the wheels.


In the version of the question where the conveyor matches the plane’s speed, the plane takes off at 50 mph, its wheels are spinning at 100 mph and the conveyor is moving at 50 mph in the opposite direction.

Offline SaburoS

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Airplane on a Conveyor Belt...
« Reply #275 on: February 02, 2008, 12:45:26 PM »
Quote
Originally posted by AKIron
The ball only stops the boxcar when it hit's the far wall. If it moved it back in the opposite direction why would it stop in the original place with no friction?

Assume that my statement about the car moving, stopping, and then moving back with the walking is correct. Is work being done? Not the sort that your boss would be happy with but the sort defined in a physics 101 book.


Bounce collision?
I was taking into account that there would be zero energy lost in this magic example.

Conservation of energy?
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Offline eskimo2

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Re: Re: Re: Re: Re: Re: Re: Aircraft on a conveyor
« Reply #276 on: February 02, 2008, 12:46:30 PM »
Quote
Originally posted by VERTEX
What can I say, the wheels rpm is indepandant from the aircrafts forward motion, the wheels free wheel on an axle, if you cant get that part its no use continuing.


Imagine the wheel is only rolling 2 mph on the treadmill, and its center/axle is moving down the runway at 1 mph.  The treadmill must be going only 1 mph…  In this case the treadmill is not matching the wheel’s speed.

Do you agree?

Offline AKIron

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« Reply #277 on: February 02, 2008, 12:48:55 PM »
Quote
Originally posted by SaburoS
Bounce collision?
I was taking into account that there would be zero energy lost in this magic example.

Conservation of energy?


It's not really a magic situation. I could move the car far out into space where the universe's gravitational effect is balanced or negligible.




To avoid a hijack I'll just answer. Yes, even though there is no net progress in a specific direction when a cycle was completed, there was work performed since mass was moved over time.
« Last Edit: February 02, 2008, 12:53:07 PM by AKIron »
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Offline eskimo2

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Airplane on a Conveyor Belt...
« Reply #278 on: February 02, 2008, 12:49:45 PM »
SaburoS,
Back to the beginning, suppose we take the wheel out of the question:

A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.

What happens?

Offline SaburoS

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Airplane on a Conveyor Belt...
« Reply #279 on: February 02, 2008, 12:58:33 PM »
Quote
Originally posted by AKIron
The ball only stops the boxcar when it hit's the far wall. If it moved it back in the opposite direction why would it stop in the original place with no friction?

Assume that my statement about the car moving, stopping, and then moving back with the walking is correct. Is work being done? Not the sort that your boss would be happy with but the sort defined in a physics 101 book.


I maybe wrong since it's been some (cough) decades(cough) since I've opened a physics book.

I throw the ball to the far wall. The boxcar moves in the opposite direction. The ball impacts the far wall (bounce collision) and double the force is transferred and forces the car to not only stop but to go in the opposite direction. When I catch the ball, the boxcar stops.

Purely a guess and provided this is with zero energy lost at each impact/transfer point. In a vacuum too so none lost to drag.
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Offline AKIron

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Airplane on a Conveyor Belt...
« Reply #280 on: February 02, 2008, 01:00:11 PM »
Here's another way to look at it.

A plane is sitting on a conveyor belt without it's engine running. The belt starts to move and the plane moves with it. The faster the belt moves the faster the plane goes. By definition, the faster the plane goes the more energy it has. This energy came from the belt. Suppose that the belt is accelerating fast enough so that the plane can't keep up and the wheels start spinning. Does that mean there is energy being lost or is it being transferrred to the rolling wheels?
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Offline SaburoS

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Airplane on a Conveyor Belt...
« Reply #281 on: February 02, 2008, 01:08:11 PM »
Quote
Originally posted by eskimo2
SaburoS,
Back to the beginning, suppose we take the wheel out of the question:

A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.

What happens?


Eskimo,

Example one:
A plane is on the runway that has a conveyor system that is designed to counter the forward movement of the plane using greater force as necessary where the conveyor controls the speed where it can maintain an acceleration so great that the plane's wheel's rotational inertia overcomes the thrust of the plane's engines to keep it at zero ground speed. Is this likely?

Yes. I've understood this from the beginning.

Notice that the conveyor is dictating the force here.

I just don't see this coming from the original question.
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Offline AKIron

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Airplane on a Conveyor Belt...
« Reply #282 on: February 02, 2008, 01:10:00 PM »
Quote
Originally posted by SaburoS
Eskimo,

Example one:
A plane is on the runway that has a conveyor system that is designed to counter the forward movement of the plane using greater force as necessary where the conveyor controls the speed where it can maintain an acceleration so great that the plane's wheel's rotational inertia overcomes the thrust of the plane's engines to keep it at zero ground speed. Is this likely?

Yes. I've understood this from the beginning.

Notice that the conveyor is dictating the force here.

I just don't see this coming from the original question.


Are you not seeing the energy being transferred by an accelerating belt or are you not seeing the reason why the belt would accelerate?
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Offline SaburoS

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« Reply #283 on: February 02, 2008, 01:12:07 PM »
Quote
Originally posted by AKIron
Here's another way to look at it.

A plane is sitting on a conveyor belt without it's engine running. The belt starts to move and the plane moves with it. The faster the belt moves the faster the plane goes. By definition, the faster the plane goes the more energy it has. This energy came from the belt. Suppose that the belt is accelerating fast enough so that the plane can't keep up and the wheels start spinning. Does that mean there is energy being lost or is it being transferrred to the rolling wheels?


No energy is lost. But that's not the question. Kind of like saying you are in a small dingy and you blow on the sail. Do you move forward based on that action alone?

What dictates wheel speed in our original question?
The plane's forward movement or the conveyor belt?
Men fear thought as they fear nothing else on earth -- more than ruin -- more even than death.... Thought is subversive and revolutionary, destructive and terrible, thought is merciless to privilege, established institutions, and comfortable habit. ... Bertrand Russell

Offline SaburoS

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Airplane on a Conveyor Belt...
« Reply #284 on: February 02, 2008, 01:16:04 PM »
Quote
Originally posted by AKIron
Are you not seeing the energy being transferred by an accelerating belt or are you not seeing the reason why the belt would accelerate?


Why does the belt accelerate? What is it monitoring?
Based on the original question, the belt is to match the wheels speed, not impart more force to increase the wheel speed.
It is no longer matching wheel speed but increasing it.

Do you see that?
Men fear thought as they fear nothing else on earth -- more than ruin -- more even than death.... Thought is subversive and revolutionary, destructive and terrible, thought is merciless to privilege, established institutions, and comfortable habit. ... Bertrand Russell