The "Plane on a Treadmill" Question - Page 3

Kitsune · 12-07-2006, 11:24 AM

That's awesome, UT!

Flint · 12-07-2006, 11:27 AM

Quote:

Originally Posted by glatt

The question doesn't say that.

Yes it does:

Quote:

Originally Posted by Kitsune

A plane is standing on a runway that can move, like a giant treadmill. When the plane's engines throttle up, it begins to move forward, but the treadmill is made to match the forward speed of the plane, only in the opposite direction. So, as the plane moves forward, it moves backwards beneath the aircraft.

It matches the forward speed, cancelling all forward motion. UT's animation repeatedly says "as the plane moves forward" but that is not the scope of what we are discussing. We are discussing a stationary plane.

Fuck the wheels, fuck the treadmill, fuck the thrust. Lift is produced by air moving across the wings. Moving.

Undertoad · 12-07-2006, 11:28 AM

I found it after googling for the problem... turns out it was fought over by folks on a physics board, so I feel a little less foolish...

dar512 · 12-07-2006, 11:34 AM

Flint. The wheels don't matter as far as forward motion is concerned. As the graphic that UT says, the wheels will just spin twice as fast. It doesn't matter how fast the treadmill is going backward, because the plane's pushing against the air, not the ground.

Shawnee123 · 12-07-2006, 11:35 AM

(Not being sarcastic...I find this debate fascinating)

Flint · 12-07-2006, 11:39 AM

Quote:

Originally Posted by dar512

Flint. The wheels don't matter as far as forward motion is concerned. As the graphic that UT says, the wheels will just spin twice as fast. It doesn't matter how fast the treadmill is going backward, because the plane's pushing against the air, not the ground.

Fuck how fast the wheels are spinning. No forward motion = no lift.

The only item of discussion here is how you read the question. Stationary planes (commercial jetliners) do not jump into the air. Ever.

dar512 · 12-07-2006, 11:42 AM

Quote:

Originally Posted by Kitsune

So, as the plane moves forward, it moves backwards beneath the aircraft.

If I understand you, the above part of the problem set means "no forward motion" to you. I don't think it does. It would for a car. It doesn't for a plane.

Kitsune · 12-07-2006, 11:46 AM

Quote:

Originally Posted by dar512

If I understand you, the above part of the problem set means "no forward motion" to you.

It doesn't mean that to me. I would say there is most certainly forward motion in the way I described it. The wheels are free spinning despite the treadmill.

Flint · 12-07-2006, 11:48 AM

Quote:

Originally Posted by dar512

If I understand you, the above part of the problem set means "no forward motion" to you.

No, this part does:

Quote:

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

There aren't two different reasons for the wheels to spin. They only ever spun because the plane was trying to move forward, and the treadmill matches the forward speed, cancelling it. If the plane thrusts harder, the treadmill continues to cancel it's motion. There is no tip-over point, here. The treadmill prevents the plane from moving. The plane cannot take off from a stationary position.

Kitsune · 12-07-2006, 11:52 AM

Quote:

Originally Posted by Flint

If the plane thrusts harder, the treadmill continues to cancel it's motion. There is no tip-over point, here.

Get a motorized treadmill and a bicycle. Hold the bicycle in place and turn the treadmill up as high as it will go. Are you having to fight very much to keep the bike stationary? No -- the friction between the bike frame and wheels is very low. Now, push the bike forward. Is it taking much effort? Not at all.

Flint · 12-07-2006, 11:56 AM

Quote:

Originally Posted by Kitsune

Now, push the bike forward. Is it taking much effort? Not at all.

Quote:

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

It doesn't matter how "hard" it is. In this scenario, you can't do that.

Happy Monkey · 12-07-2006, 11:56 AM

The plane will move forward, no matter how fast the treadmill is going. The treadmill can't cancel the forward motion of the plane, because the plane's speed is not determined by the wheels. If you hold a hotwheel car in your hand, and put it on a treadmill, you can move it forward no matter how fast the treadmill is going. Likewise, the jet engine will move the plane forward no matter what the treadmill and wheels are doing.

Now, if it were in a wind tunnel, and air was blown to cancel the jet engines, that would be a different story.

Flint · 12-07-2006, 11:58 AM

You're right: if you ignore the question you can get whatever answer you want.

Happy Monkey · 12-07-2006, 12:04 PM

Nothing in the question says the plane is stationary. Just that the treadmill moves. But the treadmill can not stop the plane! The assumption that the plane is made stationary by the treadmill is something that you are bringing in, and is the trick part of this trick question.

dar512 · 12-07-2006, 12:06 PM

http://www.straightdope.com/columns/060203.html

12-07-2006, 11:34 AM	#34
dar512 dar512 is now Pete Zicato Join Date: May 2003 Location: Chicago suburb Posts: 4,968	Flint. The wheels don't matter as far as forward motion is concerned. As the graphic that UT says, the wheels will just spin twice as fast. It doesn't matter how fast the treadmill is going backward, because the plane's pushing against the air, not the ground. __________________ "Against stupidity the gods themselves contend in vain." -- Friedrich Schiller

12-07-2006, 11:35 AM	#35
Shawnee123 Why, you're a regular Alfred E Einstein, ain't ya? Join Date: Jun 2006 Posts: 21,206	(Not being sarcastic...I find this debate fascinating) __________________ A word to the wise ain't necessary - it's the stupid ones who need the advice. --Bill Cosby

12-07-2006, 11:56 AM	#42
Happy Monkey I think this line's mostly filler. Join Date: Jan 2003 Location: DC Posts: 13,575	The plane will move forward, no matter how fast the treadmill is going. The treadmill can't cancel the forward motion of the plane, because the plane's speed is not determined by the wheels. If you hold a hotwheel car in your hand, and put it on a treadmill, you can move it forward no matter how fast the treadmill is going. Likewise, the jet engine will move the plane forward no matter what the treadmill and wheels are doing. Now, if it were in a wind tunnel, and air was blown to cancel the jet engines, that would be a different story. __________________ _________________ \|...............\| We live in the nick of times. \| Len 17, Wid 3 \| \|_______________\| [pics]

12-07-2006, 11:58 AM	#43
Flint Snowflake Join Date: Mar 2006 Location: Dystopia Posts: 13,136	You're right: if you ignore the question you can get whatever answer you want. __________________ **************** 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, 12:04 PM	#44
Happy Monkey I think this line's mostly filler. Join Date: Jan 2003 Location: DC Posts: 13,575	Nothing in the question says the plane is stationary. Just that the treadmill moves. But the treadmill can not stop the plane! The assumption that the plane is made stationary by the treadmill is something that you are bringing in, and is the trick part of this trick question. __________________ _________________ \|...............\| We live in the nick of times. \| Len 17, Wid 3 \| \|_______________\| [pics]

12-07-2006, 11:24 AM	#31
Kitsune still eats dirt Join Date: Sep 2003 Location: Tampa, FL Posts: 3,031	That's awesome, UT!

12-07-2006, 11:28 AM	#33
Undertoad Radical Centrist Join Date: Jan 2001 Location: Cottage of Prussia Posts: 31,423	I found it after googling for the problem... turns out it was fought over by folks on a physics board, so I feel a little less foolish...

12-07-2006, 12:06 PM	#45
dar512 dar512 is now Pete Zicato Join Date: May 2003 Location: Chicago suburb Posts: 4,968	http://www.straightdope.com/columns/060203.html __________________ "Against stupidity the gods themselves contend in vain." -- Friedrich Schiller

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