The "Plane on a Treadmill" Question - Page 9

Happy Monkey · 12-07-2006, 03:41 PM

Another way to explain it:

There are two sources of force in the system. The plane engine, and the treadmill. The plane engine pushes forward, and the treadmill pushes backward. The wheels of the plane are the interaction between the two forces, and by rotating they allow the engines to move the plane forward and the treadmill to move itself backward without canceling, by rotating at the sum of the two speeds.

Under interpretation #1, the speed of the treadmill is automatically set to equal the speed of the wheels. This is only true if the engines supply no force to the system, but the question precludes that.

Shawnee123 · 12-07-2006, 03:42 PM

Quote:

Originally Posted by LabRat

This has been fun, but I didn't get a damn thing done this afternoon. We need to add a NSFP* warning on these.

*Not Safe For Productivity.

hehehee...I was in meetings most of the day but I checked on this thread a few times. I must say, my head exploded a long time ago. You people be too damn smart for me! :p

Elspode · 12-07-2006, 03:46 PM

Quote:

Originally Posted by Flint

It can, because this question says it can. Start by establishing what is being discussed.

This has ceased being a physics discussion and become a semantics issue, hasn't it?

Flint · 12-07-2006, 03:55 PM

Quote:

Originally Posted by Happy Monkey

No, because wheel speed is identical to the speed of the plane relative to the treadmill. They are the same thing.

The question does not reference wheel speed, but plane speed. The question is what the plane speed is measured relative to. You can get two different answers, depending on what plane speed is measure relative to.

Quote:

Originally Posted by Happy Monkey

But the speed of the plane relative to the treadmill does not affect the speed of the plane relative to the ground.

The speed of the plane can be desribed two different ways. They aren't exchangable.

Quote:

Originally Posted by Happy Monkey

If the engine is going, then the plane moves forward relative to the ground.

The plane isn't on the ground. It's on the treadmill. The treadmill either cancels it’s relative motion, or has no effect whatsoever. See above.

Quote:

Originally Posted by 9th Engineer

Flint, you are saying we need to forget HOW a plane works and just assume that because of the word 'but' the plane cannot move forward, even though the question then goes on to say that all the treadmill is doing is matching its speed with the forward motion of the aircraft.

No, I’m not saying that. (The forward motion of the aircraft relative to what?)

Quote:

Originally Posted by 9th Engineer

Asking to forget everything about how the objects in question work and make an assumption based on a conjunction in one of the sentences is ridiculous [sic]…

I'm not asking you to do that.

Quote:

Originally Posted by glatt

He's saying this is a special treadmill that somehow holds the plane back.

It doesn't hold the plane back, the plane is moving at exactly the speed the treadmill is moving, in the opposite direction.

Quote:

Originally Posted by LabRat

The treadmill does not move the axle, it moves the wheel around the axle. Thus, the planes forward thrust has no opposite force, and moves the plane (attached to the axle) forward until lift overcomes gravity and the plane takes off.

The question doesn’t include wheel speed as a variable, only plane speed and treadmill speed.

Quote:

Originally Posted by LabRat

We need to add a NSFP* warning on these.

*Not Safe For Productivity.

No kidding.

Quote:

Originally Posted by Kitsune

Alright, mission accomplished!

Quote:

Originally Posted by Happy Monkey

The wheels of the plane are the interaction between the two forces, and by rotating they allow the engines to move the plane forward and the treadmill to move itself backward without canceling, by rotating at the sum of the two speeds.

The question doesn’t include wheel speed as a variable, only plane speed and treadmill speed.

Quote:

Originally Posted by Happy Monkey

Under interpretation #1, the speed of the treadmill is automatically set to equal the speed of the wheels. This is only true if the engines supply no force to the system, but the question precludes that.

No, the treadmill stated in the question only cares about the plane speed, never the wheel speed.

Quote:

…the treadmill is made to match the forward speed of the plane, only in the opposite direction.

9th Engineer · 12-07-2006, 04:13 PM

Flint, think about the rotation of the Earth being the same as a giant treadmill, can a plane taking off to the west leave the ground?

Flint · 12-07-2006, 04:15 PM

Quote:

Originally Posted by 9th Engineer

Flint, think about the rotation of the Earth being the same as a giant treadmill, can a plane taking off to the west leave the ground?

Q: Does the treadmill have it's own atmosphere?
A: Not as stated in this question.

glatt · 12-07-2006, 04:26 PM

Good point. The question doesn't clearly state that there is an atmosphere, so a plane wouldn't work anyway. No lift in a vacuum.

Flint · 12-07-2006, 04:27 PM

The Earth has an atmosphere, the treadmill doesn't, IE, the atmosphere isn't relative to the treadmill, as in 9ths "Earth treadmill" question.

glatt · 12-07-2006, 04:34 PM

Yes, but the question doesn't state this treadmill is on the Earth, does it?

orthodoc · 12-07-2006, 04:44 PM

Umm ... isn't this solely about the action of the plane's engines and the fact that they push air through at great speed (science pushes, but it never, ever sucks

), creating air flow over and under the wings, and then the whole Bernoulli effect thing happens, and you get lift? So the treadmill plane should lift off ...

Am I remembering Physics 100 correctly?

otoh, will just the movement of air through the engines provide enough airflow to create sufficient lift for the plane to take off? Normally there's the engine push-through of air plus air passing over/under the wings because the plane is moving relative to the atmosphere - on the treadmill it's not. Will there be enough air moving over the wings to create the needed lift?

Now if we could just get planes to land on little bitty treadmills, we wouldn't have to build long runways anymore!

12-07-2006, 04:51 PM

Of course not... the air moving across the wing gives lift (a very simplistic description, at-best). The treadmill negates that.

9th Engineer · 12-07-2006, 05:26 PM

We all have been assuming that the air does not accelerate with the treadmill, which is correct. It is also why the treadmill has no effect. The jet engines push against the AIR, nothing else, so if the air does not accelerate with the treatmill then there must be a constant force pushing back on the aircraft (basic physics). Since the treadmill is only in contact with the wheels, and the wheels cannot transmit a force back onto the aircraft (no brakes being applied), there is an unballenced force acting on the body of the plain. Therefore, the plane must accelerate.

Happy Monkey · 12-07-2006, 06:23 PM

Quote:

Originally Posted by Flint

Quote:

Originally Posted by Happy Monkey
No, because wheel speed is identical to the speed of the plane relative to the treadmill. They are the same thing.

The question does not reference wheel speed, but plane speed. The question is what the plane speed is measured relative to. You can get two different answers, depending on what plane speed is measure relative to.

I didn't say it did. I said that wheel speed is identical to the speed of the plane relative to the treadmill.

Quote:

Originally Posted by Happy Monkey
But the speed of the plane relative to the treadmill does not affect the speed of the plane relative to the ground.

The speed of the plane can be desribed two different ways. They aren't exchangable.

Again, I didn't say they were. I said that they were different, and that the latter doesn't affect the former.

Quote:

Originally Posted by Happy Monkey
If the engine is going, then the plane moves forward relative to the ground.

The plane isn't on the ground. It's on the treadmill. The treadmill either cancels it’s relative motion, or has no effect whatsoever.

It cannot cancel the relative motion, because it doesn't apply any force to the plane, just the wheel. The only force pushing the plane is the engine, the engine acts relative to the air, which is not affected by the treadmill. Therefore the plane moves forward relative to the ground.

Quote:

The question doesn’t include wheel speed as a variable, only plane speed and treadmill speed. No kidding.
...
The question doesn’t include wheel speed as a variable, only plane speed and treadmill speed.
...
No, the treadmill stated in the question only cares about the plane speed, never the wheel speed.

You say that a lot, but it is irrelevant. Wheel speed is equal to the speed of the plane relative to the treadmill. They are identical.

Happy Monkey · 12-07-2006, 06:25 PM

Quote:

Originally Posted by rkzenrage

Of course not... the air moving across the wing gives lift (a very simplistic description, at-best). The treadmill negates that.

It would if it kept the plane stationary relative to the ground, which it can't.

xoxoxoBruce · 12-07-2006, 06:42 PM

Jet engines don't push against the air, they push against the plane. The plane will be pushed forward until it reaches takeoff velocity, no matter what speed the wheels are turning. All they do is keep the plane from dragging on the ground. Planes take off from runways that are moving at 1000 mph, under them, every day.

Quote:

Originally Posted by Undertoad

The answer to why manholes are round? Because men are round. Duh

So a man can tip open a lid that would be to heavy to lift.

12-07-2006, 03:41 PM	#121
Happy Monkey I think this line's mostly filler. Join Date: Jan 2003 Location: DC Posts: 13,575	Another way to explain it: There are two sources of force in the system. The plane engine, and the treadmill. The plane engine pushes forward, and the treadmill pushes backward. The wheels of the plane are the interaction between the two forces, and by rotating they allow the engines to move the plane forward and the treadmill to move itself backward without canceling, by rotating at the sum of the two speeds. Under interpretation #1, the speed of the treadmill is automatically set to equal the speed of the wheels. This is only true if the engines supply no force to the system, but the question precludes that. __________________ _________________ \|...............\| We live in the nick of times. \| Len 17, Wid 3 \| \|_______________\| [pics]

12-07-2006, 04:13 PM	#125
9th Engineer Bioengineer and aspiring lawer Join Date: Apr 2006 Location: Pittsburgh Posts: 872	Flint, think about the rotation of the Earth being the same as a giant treadmill, can a plane taking off to the west leave the ground? __________________ The most valuable renewable resource is stupidity.

12-07-2006, 04:27 PM	#128
Flint Snowflake Join Date: Mar 2006 Location: Dystopia Posts: 13,136	The Earth has an atmosphere, the treadmill doesn't, IE, the atmosphere isn't relative to the treadmill, as in 9ths "Earth treadmill" question. __________________ **************** There's a level of facility that everyone needs to accomplish, and from there it's a matter of deciding for yourself how important ultra-facility is to your expression. ... I found, like Joseph Campbell said, if you just follow whatever gives you a little joy or excitement or awe, then you're on the right track. . . . . . . . . . . . . . . . . . . . . . . . . . . Terry Bozzio**

12-07-2006, 04:44 PM	#130
orthodoc Not Suspicious, Merely Canadian Join Date: Oct 2006 Posts: 3,774	Umm ... isn't this solely about the action of the plane's engines and the fact that they push air through at great speed (science pushes, but it never, ever sucks ), creating air flow over and under the wings, and then the whole Bernoulli effect thing happens, and you get lift? So the treadmill plane should lift off ... Am I remembering Physics 100 correctly? otoh, will just the movement of air through the engines provide enough airflow to create sufficient lift for the plane to take off? Normally there's the engine push-through of air plus air passing over/under the wings because the plane is moving relative to the atmosphere - on the treadmill it's not. Will there be enough air moving over the wings to create the needed lift? Now if we could just get planes to land on little bitty treadmills, we wouldn't have to build long runways anymore! __________________ The greatness of a nation and its moral progress can be judged by the way its animals are treated. - Ghandi

12-07-2006, 05:26 PM	#132
9th Engineer Bioengineer and aspiring lawer Join Date: Apr 2006 Location: Pittsburgh Posts: 872	We all have been assuming that the air does not accelerate with the treadmill, which is correct. It is also why the treadmill has no effect. The jet engines push against the AIR, nothing else, so if the air does not accelerate with the treatmill then there must be a constant force pushing back on the aircraft (basic physics). Since the treadmill is only in contact with the wheels, and the wheels cannot transmit a force back onto the aircraft (no brakes being applied), there is an unballenced force acting on the body of the plain. Therefore, the plane must accelerate. __________________ The most valuable renewable resource is stupidity.

12-07-2006, 04:26 PM	#127
glatt ™ Join Date: Jul 2003 Location: Arlington, VA Posts: 27,717	Good point. The question doesn't clearly state that there is an atmosphere, so a plane wouldn't work anyway. No lift in a vacuum.

12-07-2006, 04:34 PM	#129
glatt ™ Join Date: Jul 2003 Location: Arlington, VA Posts: 27,717	Yes, but the question doesn't state this treadmill is on the Earth, does it?

12-07-2006, 04:51 PM	#131
rkzenrage Guest Posts: n/a	Of course not... the air moving across the wing gives lift (a very simplistic description, at-best). The treadmill negates that.

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