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

[Newc]

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

The rope by which you pull yourself forward injects an element into the equation that is not found with the airplane on a treadmill - the physically-grounded object (rope attached to the wall).

The plane would have to have a mechanical device (rope) to pull itself forward to generate lift.

It's simple: the plane has wheels that rotate freely, correct? Correct.
The engines start and the plane moves forward, causing the wheels to rotate, correct? Correct.
As the wheels rotate, they push against the conveyor, correct? Correct.
When the conveyor's speed in the opposite direction matches the rotational speed of the plane's wheels, the plane ceases to move forward, correct? Correct.
You do remember the old "stand on a skateboard and step off of it" trick from school, right? The board rolls back and you don't move forward. Same principle here.
The plane's engines can push wind out the back all they want, but it will never gain enough forward momentum to generate air pressure under the wings, which is why a plane flies, correct? Correct.
Now, as to WHY it cannot generate enough forward momentum to generate air pressure under the wings, we go back to the beginning, where we see that the freely-rotating wheels of the plane are on a conveyor that spins at the same opposing speed.

A plane does not get off the ground just because the engines are running. If that were true, then runways are only needed for landing, not takeoff.

[RobRR]

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

Ok, after explaining this to my girlfriend for 30 seconds, she understands why the plane will move. Ive really lost faith in our education system after this thread. [img]/images/graemlins/laugh.gif[/img]

[buzzsaww]

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

a plane is a ground vehicle until it has enough foward movement, which creates the airflow to make lift. the treadmill would effectively keep it in place by opposing any foward motion the plane makes. this would create no airflow so no lift. unless it could generate a tremendous amount of thrust to take off without lift. that's called a rocket.

[VitaminG]

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

a couple of points:
if the wheels are spinning freely, how are they pushing against the conveyor? Apart from the support they offer the plane against gravity, of course.
"When the conveyor's speed in the opposite direction matches the rotational speed of the plane's wheels, the plane ceases to move forward, correct?" incorrect. The wheels don't drive the plane forward, the jets or props pushing air do.
"The plane's engines can push wind out the back all they want, but it will never gain enough forward momentum to generate air pressure under the wings, which is why a plane flies, correct?" why not? What is your argument that it won't?
a plane will get off the ground just because the engines are running. Seaplanes show you that the wheels have no bearing on the planes ability to move forward. It just needs something to slide forward on, whether that be skis, pontoons or wheels.

The runway is required because the plane needs room to obtain enough speed to lift off, not because it needs the wheels to do the work of getting it up to speed.

[Newc]

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

She's humoring you [img]/images/graemlins/laugh.gif[/img]

And one thing you guys seem to be forgetting is that the conveyor will always rotate at the same speed as the plane's wheels turn, regardless of whether they are powered or not.
If the conveyor's motor was set at 100MPH, then yeah, you push the plane up to 200MPH and the plane will travel forward at 100MPH, but the conveyor is always going as fast as the wheels are turning. The wheels are turning based on the plane's thrust generated by the engines, not by a powered drive system.
Ergo, with the plane generating 100MPH of thrust causing the wheels to rotate at 100MPH, and the conveyor matches that speed in the opposite direction, the plane doesn't move forward.

If the plane boost up to 200MPH, the conveyor also turns at 200MPH, and the plane doesn't move.

If the plane gets a push from the back AND a nitro boost up to 5000MPH, the conveyor still turns in the opposite direction at 5000MPH, and the plane will.not.move.

[VitaminG]

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

[ QUOTE ]
Yup, Zeegs is right.

[/ QUOTE ]hoorah! [ QUOTE ]
After fixxing another drink,

[/ QUOTE ]good idea! I should go to the pub for lunch [ QUOTE ]
I just realized something.... the plane will gain forward momentum. It is pushing off the air which does not move with the conveyor. The wheels would then turn at 200MPH assuming thrust was 100 MPH forward and the conveyor in moving 100 MPH in the opposite direction.

[/ QUOTE ]that's the point I tried to make back on page 1, but zeegs putting it much more eloquently. There might be hope for his teaching career yet! [img]/images/graemlins/grin.gif[/img]

[RobRR]

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

[ QUOTE ]

It's simple: the plane has wheels that rotate freely, correct? Correct.

[/ QUOTE ]
Correct.
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The engines start and the plane moves forward, causing the wheels to rotate, correct? Correct.

[/ QUOTE ]
Correct.

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As the wheels rotate, they push against the conveyor, correct? Correct.

[/ QUOTE ]

False. The wheels arent "pushing" against anything, they are just rolling. The energy to move the plane forward comes from the thrusters, NOT the wheels. The wheels simply move according to how much THRUST the THRUSTERS are putting out. Say the engines are putting out 100,000 lbs/thrust and accellerate the plane to 200mph. So on a normal runway, the wheels are going 200mph. But the conveyor is going in revers 200, so now the wheels are going 400mph. The wheels offer almost a zero drag coeffieient. The thrust is coming from the AIR, NOT the ground.
[ QUOTE ]

When the conveyor's speed in the opposite direction matches the rotational speed of the plane's wheels, the plane ceases to move forward, correct? Correct.


[/ QUOTE ]
False, see above.

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You do remember the old "stand on a skateboard and step off of it" trick from school, right? The board rolls back and you don't move forward. Same principle here.


[/ QUOTE ]
False. If you attach a model rocket engine to that skateboard and ignite it, the skateboard will move forward at the speed equal to the given amount of thrust/weight ratio.

Im too irritated to answer the rest. [img]/images/graemlins/laugh.gif[/img]

[VitaminG]

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

[ QUOTE ]
And one thing you guys seem to be forgetting is that the conveyor will always rotate at the same speed as the plane's wheels turn, regardless of whether they are powered or not.

[/ QUOTE ]nope. The original post stated that the conveyor moved at the same speed as the plane, not the wheels.
[ QUOTE ]
If the conveyor's motor was set at 100MPH, then yeah, you push the plane up to 200MPH and the plane will travel forward at 100MPH, but the conveyor is always going as fast as the wheels are turning. The wheels are turning based on the plane's thrust generated by the engines, not by a powered drive system.

[/ QUOTE ]nope again. The wheels' rotation speed is based on the planes speed relative to the surface it is travelling over. The plane's ability to generate lift is based on it's speed relative to the air around it. So if the plane is travelling at 100 and the belt is travelling at 100 too, the wheels will spin at 200.

[RobRR]

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

Ok, Ill put this in differnt terms you all can understand:

A plane is powerd by air, a car is powerd by ground.

Lets take the same example, and flip it around. Lets put a car into a wind tunnel. For every given MPH the car gains, the wind also gains. So by theory, your saying that in a wind tunnel, the car will not move if equal amounts of wind is applied to its speed? Sounds stupid, right? Well, by theory, thats EXACTLY what you all are suggesting. Sure, wind offers resistance that should be taken into consideration, but nothing drastic at the level were talking about.

[RobRR]

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

Also another example, a plane is moving at 100 mph on the ground. The conveyor is also moving at 100 mph. The wheels will be virtually motionless (in a perfect world) but the plane will still only be doing 100mph. Realize that the wheels are freely rotating, they offer no resistance to the movment of the plane what so ever.

[Number Of The Priest]

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

Okay, since a plane is powered by air, let's try this.

What would happen if the plane is going full throttle on the conveyor belt, and then suddenly the pilot decides to retract the landing gear? Are you saying that the plane will suddenly shoot forward and start flying?

Methinks the plane, after retracting its undercarriage, WILL go forward a little bit, but DIDN'T HAVE THE SPEED RELATIVE TO THE EARTH to be able for its wings to slice the air in such a manner that creates lift. Sure, it's engines are on full blast, but it's stalling and will smack into the ground.

[Newc]

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

[ QUOTE ]
a couple of points:
if the wheels are spinning freely, how are they pushing against the conveyor? Apart from the support they offer the plane against gravity, of course.

[/ QUOTE ]

By your own admission that the engines do all the work [img]/images/graemlins/poke.gif[/img]

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"When the conveyor's speed in the opposite direction matches the rotational speed of the plane's wheels, the plane ceases to move forward, correct?" incorrect. The wheels don't drive the plane forward, the jets or props pushing air do.

[/ QUOTE ]

And what function do the wheels serve? [img]/images/graemlins/eyes.gif[/img]

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"The plane's engines can push wind out the back all they want, but it will never gain enough forward momentum to generate air pressure under the wings, which is why a plane flies, correct?" why not? What is your argument that it won't?

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Because the wheels on which the plane sit, being round, must turn freely in order for the thrust of the engines to propel the plane forward to gain enough momentum to generate air pressure under the wings and achieve lift. If the conveyor is not offering any resistance, the plane does not move forward.

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a plane will get off the ground just because the engines are running. Seaplanes show you that the wheels have no bearing on the planes ability to move forward. It just needs something to slide forward on, whether that be skis, pontoons or wheels.

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Correction: it just needs something to provide RESISTANCE to the support gear (pontoons, wheels, skis, Flintstone's feet) for it to push away from.

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The runway is required because the plane needs room to obtain enough speed to lift off, not because it needs the wheels to do the work of getting it up to speed.

[/ QUOTE ]

YOU are the only one saying that the wheels PROVIDE the speed/power to lift off, not me. The wheels turn, do they not? Why do they turn? Because of the force of engines pushing the plane forward. Supposing that the wheels were on a conveyor that automatically matched the rotational speed of the wheels in the opposite direction, up to infinity, the plane cannot generate enough forward momentum.
If it could, then outgoing runways would be shorter and just have "launch conveyors" on them for a plane to build up speed in one spot.

[zeegler]

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

NOTP, did you completely ignore my treadmill/arms/rope explanation? That is all you need to know to figure this out. The fact that YOU can't figure it out, and the fact that you think it's funny that Rob, VitG and myself don't share you're lack of understanding is frustrating to say the least.

The main thing you're not getting, is that when the pilot opens the throttle on that airplane, and those engines begin to push, what do you think they are pushing against? You have two choices: A)the ground, or B)the air.

Let's hope you said B)the air. Now, forget that air is not a solid substance that you can "feel". The engines act upon the air as if it were solid (like the rope).

So now we have a conveyor belt moving backwards, with the airplane moving backwards at the same speed with the engines off. Don't forget, that the air is NOT moving, so the plane is moving backwards relative to the air.

Now, the pilot engages the engines, initiating thrust. Lets skip back to the rollerskate/rope analogy for a second. You are travelling backwards on the treadmill, and you grab the rope with your arms (initiating thrust). Because the rollerskate wheels are freewheeling, it doesn't take much force to bring yourself to a stop. Now the treadmill is moving, and the wheels on the skates are moving, but you are not. Your arms (engines) are acting upon the rope (air), and holding you stationary. Now let's say that someone increases the speed of the treadmill. In fact, they double the speed of the treadmill. Does it matter? Are you going to start rolling backwards? No, because your arms (engines) are still "thrusting" against the rope (air) which is stationary.

Now, you're not really using much "thrust" to hold yourself in position at this point, because the only force you are acting against is the treadmill which is acting upon your rollerskate wheels, which have very expensive German ball bearings in them, reducing the backwards force to almost nothing.

Now it's time to apply some real thrust. In other words, you pull aginst the rope with your arms, or in the pilot's case, he increases the throttle from 1% to 10%. You will begin to move forward along the rope, toward the end of the treadmill. Similarly, the plane will begin to move forward along the runway, but not in relation to the moving belt, in relation to the ground at the sides of the belt, or the air (which is stationary don't forget).

Finally, if you begin to pull harder and faster on the rope, you will continue to advance along the rope, accelerating toward the end of the treadmill (assuming you have really good balance). In the same way, as the pilot opens up the throttle, the plane accelerates faster and faster until it reaches a speed that will create enough lift for take-off.

The Fucking End!



T

[RobRR]

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

Planes will not take off until they have sufficient lift. So a pilot will not retract the gear until hes at the correct air speed. AIR SPEED. The point your missing in this equasion, is that the conveyor belt has no effect on the speed of the plane.

[zeegler]

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

Don't even bother arguing with NEWC, because even when it is plainly obvious to everyone else that he is wrong, he will not admit it.