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Plane Problem

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#221
Grobut said:
Its says so when it explains that this smart tradmill will match the speed of the plane.. it woulden't be matching the plane if its just maching the wheels and allowing it to roll down the treadmill at will.

That one line forces us to considder that this magic treadmill can somehow match, and counteract the speed of the plane itself.
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erm, no, the line says the treadmill goes as fast as the plane moves, which doesnt have to mean the plane wouldnt move.

The treadmill moves backwards with v, the wheels of the plane spin with 2v.
=>the plane will still move forward and the speedvector of the treadmill is inverted to the planes speedvector.
 
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#222
Grobut said:
Its says so when it explains that this smart tradmill will match the speed of the plane.. it woulden't be matching the plane if its just maching the wheels and allowing it to roll down the treadmill at will.

That one line forces us to considder that this magic treadmill can somehow match, and counteract the speed of the plane itself.
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It's physically impossible for the magic treadmill to counteract the airplane.
The problem says nothing about the treadmill counteracting the airplane, you're inserting that word in there.
The question again (WITHOUT THE STUPID GIF)
A plane equipped with fixed horizontal engines and wheel landing gear is placed on a huge treadmill runway. The treadmill has a clever design and always matches the speed of the plane, but runs in the opposite direction. Will the plane take off and fly or not?
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NOTHING ABOUT COUNTERACTING

The treadmill just moves at the same speed as the airplane in the opposite direction.
 
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#224
Exactly. Let's work this question the other way if it will help the benighted:

The plane takes off - great - this means upward motion - therefore the force upwards is greater than the force of the weight of the plane under gravity.

OK, cool. Where did that force come from? From lift under the wings. This is the only force perpendicular to the horizontal motion and opposite to the weight.

Ok, cool again. So the plane is taking off, now what is causing the lift? The air flowing over (and under) the wings.

OK, why is this air flowing? Because the plane is moving forward through the air.

Is it?

The treadmill has a clever design and always matches the speed of the plane, but runs in the opposite direction.
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If the wheels are moving on the treadmill at 100 mph but the treadmill is moving backwards at 100mph then the plane is NOT moving relative to anything but the treadmill. Certainly not relative to the air. Imagine this...you switch off the engines but the treadmill keeps going - the plane is going to be catapulted backwards at 100mph. This won't happen though cos the problem states that the treadmill always matches and is opposite to the plane's forward motion (speed). Any situation where you have a forwrd thrust on a body that has wheels, unless the wheels slip, the forward motion is going to be relative to the surface it is resting on.

So the whole plane takes off argument is pure specious BS. Again, I repeat, windsocks, wheel friction, undefined areas of pixie magic in nebulous 'sweet spots' behind the wings are all irrelevant.

In any physical problem like this simply take a look at all the forces that exist and decide if they balance out or not.
 
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#225
worluk said:
erm, no, the line says the treadmill goes as fast as the plane moves, which doesnt have to mean the plane wouldnt move.

The treadmill moves backwards with v, the wheels of the plane spin with 2v.
=>the plane will still move forward and the speedvector of the treadmill is inverted to the planes speedvector.
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If that interpretation of the problem is really correct, then what is the point of this little "experiment"? it shows us nothing and it prooves nothing, we might aswell put the plane on a normal fixed runway for all the good it does us.

Ergo, i dont think thats how its supposed to be read, i think we are supposed to assume the plane is not allowed forward movement.
 
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#228
If the wheels are moving on the treadmill at 100 mph but the treadmill is moving backwards at 100mph then the plane is NOT moving relative to anything but the treadmill.
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The problem says nothing about wheels. The problem says everything about the airplane moving.

If the airplane is moving 100 mph (we measure an speed of an airplane by how fast the air moves through a tube, therefore the plane is moving 100 mph forward through the air) and the treadmill is matching that speed.
Since the problem says nothing about wheels, they don't matter. At all. They can be skids, they can be rails, they can be magic fairies for all I care.They don't matter.
The plane will take off
 
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#229
can some one make a gif which shows to Nestor Makhno how my modell would work !

i try to exlpane it one more time:

when you pull the ropes with 100mph or whatever you will pull the plane with the first rope with 100mph! ->
that the treadmill unter the plane is moving <- with 100mph doesnt matters OK!!!
fakt is the f.cking plane moves with 100mph

and now imagin the first rope to be the turbines that push the plane and the second rope is a engine that speeds the treadmill to 100mph
 
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#230
I just read another forum and this analogy is quite interesting -

"Again, you must keep in mind that because the rotation of the wheels is NOT what moves the plane around, the wheels can spin at any speed, totally independent of how fast the plane is moving with respect to the fixed earth.

The jet engine produces a certain amount of force which pushes the plane forward, and this force is completely unrelated to how fast the treadmill is spinning. That's why I used the analogy of a cable pulling the plane forward rather than the exhaust "shoving" it forward, it's a little easier to visualize.

Here's a more familiar example to show why the wheels can spin at any speed you want, so long as they are not providing the force that moves the vehicle.

Get your roller blades (Perfect Physics rollerblades...no friction in the wheel bearings) and go over to the gym. Put on the skates and hop on a level treadmill. Turn on the treadmill.

No matter how fast the treadmill is spinning, you won't go anywhere - the wheels spin at the same speed as the treadmill surface and you stay at 0mph with respect to the fixed floor.

In other words, it's EXACTLY like you were standing on a smooth, frictionless surface. Play with the treadmill speed all that you want, it makes no difference, you don't move. There is NO FORCE causing you to move backward. Set the treadmill to 1mph or 100mph, you don't go anywhere, even though the skate wheels will be spinning like mad.

Now let's apply some forward thrust. Your buddy comes up behind you, standing on the floor, and starts to shove you forward. You begin to move up the treadmill at exactly the speed he is pushing. If he pushes you at 1mph, you move at 1mph, even though you've got the treadmill moving at 100mph.
And that's exactly what happens with the airplane on the conveyor belt - it's just a plane on a frictionless runway. The jet engine provides thrust which causes forward motion regardless of how slippery the runway is. The wheels can revolve at 1mph, 100mph or 1000mph, it makes no difference at all."​
 
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#231
Nestor Makhno said:
So the whole plane takes off argument is pure specious BS. Again, I repeat, windsocks, wheel friction, undefined areas of pixie magic in nebulous 'sweet spots' behind the wings are all irrelevant.

In any physical problem like this simply take a look at all the forces that exist and decide if they balance out or not.
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Did you read my last post?



*breath*


Did you understand it?



*breath*


Sorry, Nestor, you said so yourself, look at the forces. Even if there werent wheels, the movement of the treadmill wouldnt be directly imposed on the plane. Friction would be the keyword there. The wheels only reduce the friction by a huge factor.

btw. i would be interested to see the "calculations" that lead you to believing that the forces cancel out. :)
 
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#233
SnozzFartz said:
In order for a plane to take off it needs air speed, not ground speed. Assuming that the plane at no point goes faster than the treadmill, it's airspeed will be 0 while it's groundspeed will be (for example) 300k/h. If you were to pilot that plane, and were to back on the joystick, the plane would do nothing. It would not go anywhere. There is no lift generated by the wind because there is no wind in the first place.
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the question is not if the plane needs airspeed or not (since we all agree on that), the question is, does it move forward, and yes, it does.
 
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#236
worluk said:
the point is, if it will take off or not. Whats the point in asking if the plane will take of when standing still or not? My little nephew could answer that.
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Atleast, when you read it my way, you have to think about airspeed, so it actually prooves a point about planes, namely that you need airspeed to fly.

If we read it your way, we cant conclude anything usefull from the experiment, as somebody said, its the same as a pair of rollerskates on a treadmill, infact rollerskates would be a better example than a plane as they are simpler, the experiment only prooves that free rolling wheels dont care about treadmills.. so why the plane and the takeoff? and why no mention of wheels in the problem if the whole point of the problem is to show that the wheels wont hinder the plane because they are free-rolling and not powered?

I still think my interpritation of the problem makes much more sense.
 
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#237
Grobut said:
Atleast, when you read it my way, you have to think about airspeed, so it actually prooves a point about planes, namely that you need airspeed to fly.
Everyone knows this.
If we read it your way, we cant conclude anything usefull from the experiment, as somebody said, its the same as a pair of rollerskates on a treadmill, infact rollerskates would be a better example than a plane as they are simpler, the experiment only prooves that free rolling wheels dont care about treadmills.. so why the plane and the takeoff? and why no mention of wheels in the problem if the whole point of the problem is to show that the wheels wont hinder the plane because they are free-rolling and not powered?
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That's the whole point of the problem. Most people, as evidenced by this thread, think that if the treadmill is going the opposite direction of the airplane, it will oppose the airplane. It won't. That's the point.
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#238
The argument has now boiled down to the question of does the polane have any nett motion forward relative to the ground/air. So at least we have weeded out a lot of the idiots and possibly some of them have learnt that wings are simple devices for converting forward motion into lift.

So let's nail down the question of if there actually is any overall forward motion.

The plane is trying to move at a speed relative to a point, P, on the treadmill. Same as if you fire up the engines on the runway...a fixed point (the control tower) will go by the window. However P is not fixed - it is moving backwards relative to the frame of reference. If it did that whilst you were sitting on it in a plane that didn't have the engines on you would go arse first off the end of the treadmill.

The planes's thrust moves it relative to the point P on the treadmill - its forward thrust being transferred through the wheels/skids/pontoons/belly whatever reacting against the surface. However, the point P is moving backwards and the backward motion of the treadmill is also transferred to the plane through the wheels/skids etc. meaning that the overall nett motion of the plane is zero, even though the point P is disappearing back along the treadmill.

O'shannon - I understand perfectly well what you are envisaging. A diagram will not make it any clearer, nor will it make your scenario correct. You have two fundamental errors in your set-up - one is the same hand pulling both ropes, two is imagining that pulling on a rope is exactly analogous to speed - it isn't - it's more closely analogous to force.
 
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#239
Nestor Makhno said:
The plane is trying to move at a speed relative to a point, P, on the treadmill. Same as if you fire up the engines on the runway...a fixed point (the control tower) will go by the window. However P is not fixed - it is moving backwards relative to the plane.

The plane still moves relative to the point P on the treadmill - its forward thrust being transferred through the wheels/skids/pontoons whatever. However, the backward motion of the treadmill is also transferred to the plane through the wheels/skids etc. meaning that the ob
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Not the wheels cause the acceleration, the engines does. The reference point to the engines is the surrounding air. Not the treadmill.

You delink the system with the wheels.
 
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