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

[Endrik]

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

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I know something about planes too, I even planned to go to the pilots school a couple of years ago but there was this math things so I said fuck it [img]/images/graemlins/laugh.gif[/img]

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You were *thinking* about going to pilots school. Why didn't you say so before? I guess I'll go tell my buddy that *is* a pilot that he doesn't know as much about flying as you. After all, he only finished pilots school.

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this is irrelevant in this case.

anyway, I see if I can contact with one physics professor who I know, he is very recognized man among the local sience folks. I'll ask that question from him if I get the chance.

[hippietim]

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

you know, the more i think about this problem the more i wonder - who fucking cares?

[StukaJU87]

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

[Endrik]

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

Myth Busters should do a special episode about this crap

[toejam]

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

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you know, the more i think about this problem the more i wonder - who fucking cares?

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Denny Crane?

[zeegler]

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

To the people who still insist the plane won't take off, I hope your jobs don't involve handling sharp objects! [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img]

[audiophile]

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

For anyone who thinks the plane cannot fly a good quote comes to mind.

It is better to be silent and thought a fool, than to speak and remove all doubt. [img]/images/graemlins/eviltongue.gif[/img] [img]/images/graemlins/grin.gif[/img]

Ima fool anyways, so whatta I care.
Without other variables, there is no accurate answer anyways. If you account the minute friction of the wheels, that may very well be the one variable that prevents it from flying.

I also see it like a car on a dynamo.
A front wheel drive car lol, with its rear wheels (in neutral) on the dynamo. That dynamo can spin to it hearts content in the reverse direction, and the rear wheels roll along with it. Push the accelerator (front wheels+ground=jet engines+air) and it leaps forward

OK, that was gay...

[monk]

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

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anyway, I see if I can contact with one physics professor who I know, he is very recognized man among the local sience folks. I'll ask that question from him if I get the chance.

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You should do that!

A car uses friction to transfer force from the engine to the ground (through the tires). A car placed on a threadmill will remain stationary.

A plane DO NOT use friction in the same way! The force on the plane is generated through the jet engine, who throws mass (air) backwards, thus resulting in a force in the opposite direction (the direction of movement of the plane).

This has nothing to do with the ground, or if the ground moves or not. When the threadmill speeds up, all that happens is that the wheels will spin faster and faster. This will generate more friction, but this friction will be nothing compared to the 1000's of lbs of thrust generated by the engine, because friction is NOT important.

[Newc]

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

Rob - There's you problem: There's no sense in the conveyor being as long as a real runway, because the plane will not travel that distance.
However, if the conveyor was that long, as I said before, it WOULD generate a noticeable amount of airflow over the wings, but ONLY if it was that long.

Q: What causes the plane sitting on a normal runway to move forward from a dead stop?
A: Thrust from the engines which push the plane forward on its wheels.

Now, regardless of thrust amount, a plane MUST have forward motion to build ground speed. Airflow over the wings is generated by ground speed to achieve lift, and ground speed is all about the wheels spining. As soon as the vehicle moves forward along the ground to build ground speed, the conveyor spins in the opposite direction, nullifying any forward momentum.
This is established in the question itself - the conveyor automatically matches the speed of the plane.

You guys keep saying the plane needs only to overcome the inverse velocity of the belt by a fraction, but you're injecting an "IF" that is not an allowed variable.

I truly hope you guys who think it will take off don't make important decisions requiring logic and common sense [img]/images/graemlins/laugh.gif[/img]

[monk]

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

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If a plane moves forward and the runway counteracts that movement there will be NO forward motion...

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True, BUT, the point is there is nothing that is counteracting the plane's forward movement. The problem statement merely says the conveyer belt turns at a speed that is equal to that of the plane's forward motion, only in reverse. The only force counteracting the plane's forward motion is therefore the friction induced by the spinning wheels, which is negligible even given the spinning belt.

No advanced physics here my friends, it's basic classical/Newtonian mechanics.

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+1 That is the best explanation!

Introduce a coordinate system where X is along the runway, Y is across the runway, and Z is vertically aligned. A wheel (perfect, without friction) can only support forces directed along the Y- and Z-direction. The force generated by the engine is directed along the X-axis!

[guitarsjb]

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

I'm with Newc on this one. (I don't believe I just typed that [img]/images/graemlins/tongue.gif[/img] [img]/images/graemlins/laugh.gif[/img])

[RobRR]

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

Newc, the plane WILL travel the entire distance of the conveyor runway, thats apparently what your missing. I didnt bother to read the rest your reply, as apparently it didnt make any sence beings that you still think the plane will sit still. [img]/images/graemlins/laugh.gif[/img]

[Newc]

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

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Introduce a coordinate system where X is along the runway, Y is across the runway, and Z is vertically aligned. A wheel (perfect, without friction) can only support forces directed along the Y- and Z-direction. The force generated by the engine is directed along the X-axis!


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That makes absolutely no sense.

The wheel points in the same direction as the nose of the plane - along the X axis in your scenario. The wheel supports the weight of the plane which is primarily coming from the Z axis, however the MOTION of the wheel is along the X, same as the plane.

Stick stricly to the "givens" in this and it's crystal clear:

The plane needs the wheels to gain ground speed - no matter how much thrust the engines generate, the wheels have to turn over the ground. The plane needs to achieve a predetermined ground speed before it can achieve lift. AFTER it achieves lift, it is dependent on AIR SPEED.
You cannot have AIR SPEED on a grounded vehicle.

It does NOT have AIR SPEED until it is AIR BORNE (read: not touching the ground).

So, to achieve AIR SPEED, the plane must first achieve a predetermined GROUND SPEED, and it can only do that by rolling along on wheels.
In this question, the wheels are on a conveyor which turn in the opposite direction at the same speed with absolutely no variation possible.

Regardless of the amount of thrust coming out of the engines, the plane is still on the ground, and must attain the required GROUND SPEED before it can achieve enough LIFT to have AIR SPEED.

It's just that simple.

[lerxstcat]

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

The wheels on a plane, besides taxiing around the airport, are there to eliminate friction as an inhibiting factor to takeoff.

A car's wheels, via the drivetrain, push off against the ground to generate speed. A plane's jet engines or propellors push off against the Earth's atmosphere to achieve propulsion. The conveyor has no effect on that force and the friction effect of the neutral wheels is minimal.

It's a trick question because the conveyor doesn't act in direct opposition to the actual propulsive force.

[rjohnstone]

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

Actualy... air speed can be achieved while sitting perfectly still on a runway.
Air speed is the speed at which the air is flowing over the wings... hence the reason a pilot will take off into the wind.
If you need 80 knots air speed to take of but point the plane into a 40 knot head wind, you only need 40 knots of ground speed to achieve enough air speed to take off.