quote:

Do you really think that if the plane was travelling at 25mph in relation to the conveyor and the conveyor would be travelling at 25mph to the opposite direction that the plane would move in relation to the ground? Wow, you just don't get it, huh?

What you don't get is that the plane *can't* move the way you think it does, the way the question is described.

The tested solution, with the plane relative to the GROUND, and not relative to the belt surface, is the ONLY way that the setup can be done as described.

Think carefully... what makes the plane's wheels spin, for the belt to be matching? They are not driven by the engine. The only time they spin is when the plane produces thrusts and *moves forwards*. Once the plane is producing thrust and moving, the belt can't hold it back by "matching speeds."

Trying to measure wheelspeed and not ground speed, for the plane, leads to a physical impossibility.

...As 217 pages have repeatedly pointed out. ...And all the hundreds and hundreds of pages before the episode aired.

I know how and why it works, that isn't the point. The point is that it was incorrectly said that the plane and the belt were both travelling at the same speed.

quote:

The point is that it was incorrectly said that the plane and the belt were both travelling at the same speed.

What part of 25=25 was "incorrect"?

well the whole myth loses it's point if the speed of the plane is measured in relation to the groung. If a car would move at 25mph in relation to the ground and the belt would move 25mph in the opposite direction then ofcourse the car would move forward aswell.

quote:

If a car would move at 25mph in relation to the ground and the belt would move 25mph in the opposite direction then ofcourse the car would move forward aswell.

Yes... exactly.

But again, to get the wheels to spin you need forward motion, or else the plane and belt just sit there with all speeds at zero.

So, since the speed of the plane relative to the belt is the speed of the plane relative to the ground minus the speed of the belt relative to the ground, and since the you want the relative speed to equal belt speed, there's an impossibility:

Vplane/belt = Vplane/ground - Vbelt/ground
Vbelt/ground = - Vplane/belt (by your interpretation)
Vbelt/ground = Vbelt/ground - Vplane/ground, which is physically impossible for non-zero Vplane/ground, and yet Vplane/ground *must* be non-zero if the plane has a net thrust.

... and if the plane doesn't have a net thrust the wheels don't spin.



Sure, you can set the wheelspeed of an idling plane by moving the belt first, but that isn't what the question describes.

quote:

Originally posted by roofingguy:
Vplane/belt = Vplane/ground - Vbelt/ground
Vbelt/ground = - Vplane/belt (by your interpretation)
Vbelt/ground = Vbelt/ground - Vplane/ground, which is physically impossible for non-zero Vplane/ground, and yet Vplane/ground *must* be non-zero if the plane has a net thrust.

I agree with the first two lines but I would like to see how you got to that last line

quote:

Originally posted by roofingguy:
... and if the plane doesn't have a net thrust the wheels don't spin.

But ofcource the wheels spin even if the plane is going like 10mph backwards in relation to the ground. Because it is still moving forward on the belt.

quote:

I agree with the first two lines but I would like to see how you got to that last line

Substitution of #1 into #2.

You agree with:
1) Vplane/belt = Vplane/ground - Vbelt/ground
2) Vbelt/ground = - Vplane/belt

So:
2) Vbelt/ground = -Vplane/belt
Vbelt/ground = -(Vplane/ground - Vbelt/ground) from 1.
Vbelt/ground = -Vplane/ground + Vbelt/ground
3)Vbelt/ground = Vbelt/ground - Vplane/ground

quote:

But ofcource the wheels spin even if the plane is going like 10mph backwards in relation to the ground. Because it is still moving forward on the belt.

But what gets the belt moving in the first place? According to the question, the speed of the plane. If you want to go with wheelspeed, and not true speed, then what gets the wheels spinning for the belt to react to? They won't spin on their own. The plane needs to be moving forwards for the wheels to begin spinning for the belt to "match". But if the plane moves forward, than it's physically impossible for the belt to match the wheel speed.

Line 1 and line 2 are kinda opposite opinions so you can't just unite them as they don't work together. Belt is moving because of the motor moving the belt? Not the plane. If the wheelspeed is 25mph then the belt plus the force required from the propellers of the plane to equal the friction made is spinning the wheels. When the wheelspeed is 25mph the plane isn't moving in relation to the ground. Period

quote:

Line 1 and line 2 are kinda opposite opinions so you can't just unite them as they don't work together.

How so? Line one is from the definition of relative motion... it's immutable, not an "opinion". It's simply the Galilean transformation from one coordinate system to the other. It is valid for any situation.

So, if there's a problem somewhere, it must be the Vbelt/ground = -Vbelt/plane interpretation of the problem's wording... that we've been saying is wrong all along.

The point is, if you insist it's a valid interpretation, then equation 3 is a direct result of 1 and 2. The only way 3 can be "wrong" is for 2 to be wrong.

quote:

Belt is moving because of the motor moving the belt? Not the plane.

You're misinterpreting me. I'm not saying the plane's wheels physically drive the belt. But the original question says:

quote:

"A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?"

The control system tunes the belt speed to match the plane speed. If you want "plane speed" to equal wheel speed, then from a stopped initial condition, how do you propose to get the wheels spinning without moving the plane? If the wheels don't spin, the belt won't move. Not because the wheels drive the belt directly, but because the belt is reactive to the plane "speed".

Because the wheels are not driven, the plane *must* move for there to be a speed (either absolute, or wheelspeed) for the belt to track and match. The plane must move. Period.

The fact is that you can set up the plane to be stationary on a moving belt... sure you can set it up that way, but that is *not* what the question is decribing.

The question describes a moving plane. The plane must move. Period.

:O i had never seen that version of the question, I had just heard the version on mythbusters, which on my opinion is a different scenario in itself. Sorry for the trouble, I undestand it if that is the real question...

Er... that was the version the Mythbusters were testing.

The plane won't fly because this debate has been going on so long that the fuel went stale and it is to expensive to replace.

Other then that, the treadmill has nearly zero factor on the plane's ability to take off.

This episode was shown on Discovery Nordic last night.

In my opinion, Adam and Jamie made just the correct base assumptions to visit this myth.

While the motor(s) of the plane drive it forward through the surrounding _air_, the free-rolling wheels and the threadmill's speed are irrelevant.

Once the plane reaches the minimum takeoff speed (and once again, relative to surrounding air that the motor is whopping about), the plane will take off.

Thus setting the threadmill speed to the plane's minimum takeoff speed was correct and appropriate for the test. Doubling or even tenfolding the threadmill speed would not have had any significant effect on the takeoff.

Thanks Adam and Jamie (+ Tory, Grant and Kari) for your good work and entertaining show!

--Koo from Finland

This issue with this myth is that either answer (fly or no-fly) can be correct depending on how you interpret the question. While I can understand the explanation given (it is true given the producer's interpretation of the myth) I must say I am in the "no-fly" group based on my interpretation of the myth. The underlying issue is how you define speed. Anyone who took a basic dynamics course knows that speed is a relative term and can only be describe when expressed in a proper field of reference. I believe that the myth is meant to read that the plane is moving at take-off speed RELATIVE TO the treadmill. But since the treadmill is moving backwards at the same speed it makes the plane stationary relative to the ground (or air). In this case there would be no air over the wings and thus no lift. To those who would reference the show and say "obviously you are wrong because it worked", I would argue that the myth that I (as well as most "no-fly" believers) interpret was not tested on the show. In their test, the conveyor belt is moving at take-off speed in one direction and the plane is moving at take-off speed in the other direction. Therefore, the plane's speed relative to the belt is 2 times take-off speed. I reiterate, the problem here is not an understanding of the results. It is a discrepancy in the interpretation of the question that has led to all this confusion.

[quote]I believe that the myth is meant to read that the plane is moving at take-off speed RELATIVE TO the treadmill.[/quote]


Now read why that's impossible given the conditions of the original question.


[quote]I would argue that the myth that I (as well as most "no-fly" believers) interpret was not tested on the show.[/quote]


And it wasn't tested because it's a physical impossibility. The only physically possible interpretation, was the one tested.

[quote]I believe that the myth is meant to read that the plane is moving at take-off speed RELATIVE TO the treadmill. But since the treadmill is moving backwards at the same speed it makes the plane stationary relative to the ground (or air). In this case there would be no air over the wings and thus no lift.[/quote]

I believe the phrasing was to the effect of "the conveyor belt moves backwards as fast as the plane is moving forwards." if you attempt to make the conveyor belt move backwards in relation to the gorund at the same rate the plane is moving forwards in relation to the conveyor belt, then as soon as the pilot applies throttle, and the plane inches forwards, the conveyor belt must be thrown into an infinite acceleration spiral, resulting in a quantum paradox that will rip apart the space time continuum like the "brand X" paper towel in a Brawny commercial. therefore, the speed of the plane and the speed of the conveyor belt MUST be measured in relation of a common reference.

the fact of the matter is that POAT is a trick question intended to trick people into thinking that the treadmill could affect the plane. (other than potential tracking/steering problems)

[quote]The plane moves in one direction, while the conveyor moves in the opposite direction.[/quote]

oh, and what part of "the plane moves" implies that the plane does not move?

[quote]This issue with this myth is that either answer (fly or no-fly) can be correct depending on how you interpret the question. While I can understand the explanation given (it is true given the producer's interpretation of the myth) I must say I am in the "no-fly" group based on my interpretation of the myth. The underlying issue is how you define speed. Anyone who took a basic dynamics course knows that speed is a relative term and can only be describe when expressed in a proper field of reference. I believe that the myth is meant to read that the plane is moving at take-off speed RELATIVE TO the treadmill. But since the treadmill is moving backwards at the same speed it makes the plane stationary relative to the ground (or air). In this case there would be no air over the wings and thus no lift. To those who would reference the show and say "obviously you are wrong because it worked", I would argue that the myth that I (as well as most "no-fly" believers) interpret was not tested on the show. In their test, the conveyor belt is moving at take-off speed in one direction and the plane is moving at take-off speed in the other direction. Therefore, the plane's speed relative to the belt is 2 times take-off speed. I reiterate, the problem here is not an understanding of the results. It is a discrepancy in the interpretation of the question that has led to all this confusion.[/quote]


bwhitt12,
You are correct that if you set up the situation where the plane is at take-off speed relative to the treadmill and at a speed of zero relative to the ground then the plane will not take off, but there is a major problem with that situation. To make that happen the plane would have the throttle set so low it could not take off on pavement either. When the treadmill is moving back at the takeoff speed, the plain must overcome the rolling resistance that is pulling it back to stay in the same position relative to the ground, but not the wind resistance that it would have to overcome when on the ground. That rolling resistance is so small the plane would only need a little throttle to stay in place. It does not use the same amount of power as it would to get to takeoff speed on the ground. This means that in your situation you are changing things so that the plain can not take off even if the treadmill is stopped.

The problem is one of perception of the situation, but in any reasonable case the plane will always take off.