The "Plane on a Treadmill" Question - Page 9

LabRat · 12-07-2006, 03:35 PM

Pie, compare your diagram to mine in post 67. See the difference? This is critical. 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.

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.

Kitsune · 12-07-2006, 03:41 PM

Quote:

Originally Posted by LabRat

This has been fun, but I didn't get a damn thing done this afternoon.

Alright, mission accomplished!

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

Pie · 12-07-2006, 08:34 PM

Quote:

Originally Posted by LabRat

Pie, compare your diagram to mine in post 67. See the difference? This is critical. 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.

Okay, this is the absolute last post I'm going to make on this subject. (promise!) If the wheel experiences a force, that force must be transmitted to the axle. (The other alternative is that the wheel goes that-a-way while the rest of the plane stays put.) The free body diagram holds.

The straight dope misses this point:

Quote:

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.

That "pull yourself forward easily" is impossible. To move yourself forward at all (even a micron per hour) still necessitates a net imbalance in the forces acting on the axle of the wheel; this is patently impossible, given the wording of the problem. F1 == F2 by the very statement of the scenario, so no net motion.

Newton would be scratching his head.

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.

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?

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.

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?

footfootfoot · 12-07-2006, 08:18 PM

Quote:

Originally Posted by glatt

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.

Nor combustion w/o supplied oxygen.

Another example of the unimportance of wheels relative to a plane's taking off.
Ski planes and water planes have no wheels, hell, if a plane were lying on its belly on a greased treadmill it would take off.

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.

12-07-2006, 03:41 PM	#5
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	#7
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	#11
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	#14
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, 03:35 PM	#1
LabRat twatfaced two legged bumhole Join Date: Jun 2004 Posts: 3,143	Pie, compare your diagram to mine in post 67. See the difference? This is critical. 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. 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. __________________ Strength does not come from how much weight you can lift, or how many miles you can run. It comes from knowing that you set a goal, and rose to the challenge. Strength comes from within.

12-07-2006, 04:26 PM	#10
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	#12
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	#15
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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