hmmm.... physics question for you guys....

[RobRR]

Re: hmmm.... physics question for you guys....

The engines dont push the wheels, the engines push the plane, which in turn takes the wheels with it. [img]/images/graemlins/laugh.gif[/img]

[Bert]

Re: hmmm.... physics question for you guys....

Maybe we need to repost the link Tim gave:

http://www.avweb.com/news/columns/191034-1.html

Suggestion: Don't post until you've read it!

[toejam]

Re: hmmm.... physics question for you guys....

What happens if the plane does this?

[VitaminG]

Re: hmmm.... physics question for you guys....

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Wheels and friction have nothing to do with either scenario..the bottom line is both are stationary..A guy with a hanglider strapped to him standing on a tarmac will not fly..unless the wind blows really hard. Hard wind won't lift a plane.
No way that thing goes anywhere. [img]/images/graemlins/grin.gif[/img]

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wheels and friction have everything to do with your posted scenario. Are you familiar with how a dyno works?

[dg]

Re: hmmm.... physics question for you guys....

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Now I think the way the original question was worded it was specifically designed to be a bit ambiguous and to create this kind of "debate". But either way, I'm pleasantly surprised at how many closet nerds there are on the JCF.

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+1

In order to believe that the plane can't fly, you have to buy into the premise that a conveyor belt can be built that can stop the plane from moving forward. At first, that is exactly what I did, and what I think everyone in the "can't fly" camp is doing. We are programmed to accept the conditions presented by the problem at face value & it's easy to just accept this point & start reasoning from there. That's what years of doing word problems in school will do to you, & college physics classes only make it worse in this case. It doesn't mean you're stupid, it just means that you are falling into the trap built into the puzzle. The fact that some folks weren't even picturing the conveyor being the length of a runway kind of proves the point that they were already assuming that the plane couldn't move. You've got to at least give the plane a sporting chance to take off just in case it can move! [img]/images/graemlins/laugh.gif[/img]

A conveyor belt that will keep a car in place won't be able to keep a plane in place because the plane's thrust is not applied to the ground. It is applied to the air. The fact that the wheels turn is merely a byproduct of the plane moving forward, not the source of propulsion. This is why a plane with skis or pontoons instead of wheels can take off. All a plane's wheels are there for is to support the aircraft & minimize friction against the ground as it moves forward. Think of the wheels as just a better option than skis or pontoons since the plane is taking off from land, rather than snow or the water.

Why can't the conveyor belt stop the plane from moving forward? As it senses the plane trying to go forward, the conveyor belt instantly adjusts and moves toward the plane at the same speed. This would keep a car, motorcycle, running man, or horse in place because their forward propulsion comes from pushing off against the ground. The plane's propulsion comes from the propeller or jet engine pushing against the air, not from any force applied to the ground (think of the ski or pontoon plane, do they apply force to the snow or water? No.) The plane will move forward despite the best efforts of the conveyor. At any given instant, the wheels of the plane will be turning at twice the forward speed of the plane, overcoming the conveyor. The only hope the conveyor would have of stopping the plane would be if the wheel bearings couldn't handle turning at twice the normal speed required for liftoff.

To sum up: a conveyor belt, even if it's the mythical compensating kind, only has the ability to affect an object whose forward movement is derived via force applied to the ground. A plane's forward movement is derived from force applied to the air, so it will not be stopped by the conveyor. The only effect is that the wheels of the plane will turn faster than normal as moves over the irrelevent conveyor.

If anyone missed the link Tim posted earlier, it explains it better than I did:

http://www.avweb.com/news/columns/191034-1.html

[VitaminG]

Re: hmmm.... physics question for you guys....

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Oh, a hats off to VitaminG as well...an early true believer!

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thanks, man! It just seemed so obvious to me from the start. I expected everyone else to go "oh, trick question. You nearly got me there!" I'm amazed that even after all of the detailed explanation provided here that people are still going with their knee-jerk "of course it will fly" reaction. Are people not reading/understanding the previous posts before responding?

[LouSiffer]

Re: hmmm.... physics question for you guys....

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What happens if the plane does this?


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You use it as a VCR tape cover for one funny ass movie called " Airplane". -Lou

[sworn_enemy]

Re: hmmm.... physics question for you guys....

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So why the fuck does the plane NOT go fucking foreward? What the fuck is keeping the fucking plane from fucking moving fucking foreward? Thats what Im not fucking underfucking standing! For god's fucking sake! [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/tongue.gif[/img]

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Gravity. Say I stand on a conveyer with roller skates on my feet and a jet pack on my back that can generate enough thrust to move me forward 10mph. The jet pack is not yet turned on and the conveyer starts moving @ 10 mph. What happens? I will move backward @ 10 mph...correct? Now I turn the jet pack on. The jet pack is providing 10mph of thrust but the ground I am standing on is equally counteracting that. I will be stationary and the wheels will be turning. That's the way I see it anyway.

The plane is still subject to the force of gravity until it achieves lift. I don't see how it can achieve lift in this scenario.

[lerxstcat]

Re: hmmm.... physics question for you guys....

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I'm sorry, but the plane CAN'T take off. When the thrust is applied to the engines..it moves the WHEELS...if the wheels are going in reverse at the same speed, they cancel each other out and NOTHING happens. The wheels ALLOW the plane to move forward. If it is on a conveyor (sp) belt (I imagine it to be just big enough to hold the plane) and the wheels are essentially NOT moving...no air can pass over the wings to give the plane lift. Hell, I don't know..sounds right to me [img]/images/graemlins/tongue.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/notworthy.gif[/img]

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That's incorrect because the wheels are not connected to the engine by a drivetrain. The plane's jet pushes against the atmosphere to provide propulsion. That would not be offset by the conveyor belt because the conveyor is acting against a passive component, not the source of propulsion, which is the jet engine generating thrust against the air.

The only place the conveyor would stop the plane from flying is in an airless vacuum, and then it wouldn't fly no matter what. The same air that provides lift is the substance the engines push against to gain speed to achieve takeoff.

[RobRR]

Re: hmmm.... physics question for you guys....

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So why the fuck does the plane NOT go fucking foreward? What the fuck is keeping the fucking plane from fucking moving fucking foreward? Thats what Im not fucking underfucking standing! For god's fucking sake! [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/tongue.gif[/img]

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Gravity. Say I stand on a conveyer with roller skates on my feet and a jet pack on my back that can generate enough thrust to move me forward 10mph. The jet pack is not yet turned on and the conveyer starts moving @ 10 mph. What happens? I will move backward @ 10 mph...correct? Now I turn the jet pack on. The jet pack is providing 10mph of thrust but the ground I am standing on is equally counteracting that. I will be stationary and the wheels will be turning. That's the way I see it anyway.

The plane is still subject to the force of gravity until it achieves lift. I don't see how it can achieve lift in this scenario.

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Wrong. You'll move foreward at 10mph but the wheels on your roller skates will be moving at 20mph. The reason youll start moving backwards at first is the work of gravity holding you to the conveyor, with no oposing forces.

[wulfe13]

Re: hmmm.... physics question for you guys....

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I think it will take off. My pilot coworker explained one key thing to me that sort of puts it into perspective. Airspeed and ground speed are completely different. A plane can be flying just fine and be making little or no progress on the ground. So a plane flying 50 miles an hour into a 50 mile an hour wind is making no forward progress so the the ground speed is zippo. So it stands to reason that even if the plane is making no forward progress on the conveyor it can still fly. The props and jets are the things that pull the air thus causing flight.

Oh and Lou, you are still wrong about the skateboard on the mother of all compensating conveyors [img]/images/graemlins/laugh.gif[/img]

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Tim, this doesn't have anything to do with airspeed vs. groundspeed. The plane will achieve sufficient airspeed/groundspeed to take off as if there were no conveyor belt. The same airspeed/groundspeed it would need to achieve to take off from a normal runway under the same weather conditions.

RobRR, Lou, Zeegs, Sandman, et. al., ...boys, you are the genii of the board. [img]/images/graemlins/notworthy.gif[/img]

Newc, Endrik, GOR, free your minds or forever remain the fucktard that I used to be! [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img]

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Uh....geee... thanks for leaving me out there bucko!!! [img]/images/graemlins/poke.gif[/img] [img]/images/graemlins/laugh.gif[/img]

[Endrik]

Re: hmmm.... physics question for you guys....

fuck this shit, the plane lifts off only when somone proves it. You might think it is theoretically possible, well a lot of things are but most of them don't work practically.

[xenophobe]

Re: hmmm.... physics question for you guys....

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Your point of the weels having minimal impact on the planes forward bearing is interesting to say the least. However there is an equally interesting paradox: the plane is reliant on its weels until it reaches sufficient speed = enough airflow = sufficient lift, however since it's still bound to its weels to keep it on the conveyor belt and counteracting gravity and the conveyor belt is moving in the opposite direction at the exact same speed (at no point does the plane move faster or slower than the belt). Now it's necessary to use some sort of archimedal point, let's say the ground, under the conveyor belt or besides the conveyor belt through this we can also consider another archimedal, solid point: the air surrounding the plane. Asides from the minimal airflow created by the belt itself and the engines (Matt's main theory) the air would remain virtually the same. Of course engines are designed nowadays to not only pull in air into themselves for propulsion but also to offset air to above and below the wing to aid in achieving more lift, this however has to be seen as complementary and not sufficient on its own.

The plane remains on the exact same spot (for example it will have the exact same GPS coordinates), no sufficient airflow is created by movement (the main instigator of airflow and lift), the plane does not move forward, the plane does not achieve lift, the plane DOES NOT TAKE OFF!

In other words, I'll repeat the same: the plane won't take off, it'll only be drawing a crowd of people going "what in the name of holy fuck are those dumbasses doing with that Boeing and that gigantic conveyor belt".

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Bullshit.

The only way the plane would not move forward and fly is if the plane was physically held back.... being tethered to the ground...

If that were the case, then the plane wouldn't be moving even without the jets turned on even if the tarmac or treadmill were moving.

I'll make this as simple as possible, though that doesn't seem to help.

1) The plane is on a conveyor belt.

2) Move the conveyor belt and the plane will move backwards.

3) Thrust from the jets are applied... the plane will start moving forward.

4) Since the wheels are not providing movement, only allowing it, they roll effortlessly. THE WHEELS ARE ONLY SUPPORTING THE PLANE, NOT PROPELLING IT LIKE A CAR!

5) Plane moves forward and increases speed, passing it's original starting spot (unless it was tethered to the ground, but as to the original question, no it is not) THE WHEELS ARE SPINNING FURIOUSLY BUT DO NOTHING TO SLOW THE PLANE!

6) Plane reaches flight speed and takes off.


Kiddies, this is not much different than holding a roller skate on a moving treadmill. It will go nowhere if you are holding it down, but if you push it one direction or another, it will follow, REGARDLESS of which direction the treadmill is going.

The average IQ of the posters in this thread would be even more mindbogglingly hillarious...

[buzzsaww]

Re: hmmm.... physics question for you guys....

i love the way insults are lobbed around here.

[sworn_enemy]

Re: hmmm.... physics question for you guys....

After thinking about this for awhile I've changed my mind. The plane will fly. The problem I was having was I was thinking of the force gravity vs. a vertical force. I should have been thinking of a vertical force vs. frictional drag and not gravity. Once the the plane has generated enough thrust to overcome the friction of the wheels the plane will move forward regardless of the conveyer. Also thinking in terms of pounds of force instead of mph helps too.