06 Nov 06
Originally posted by sugiezdOK, you can (bizarrely) use the brake to get that effect in theory. I just think that my interpretation of the question is as correct (for the reasons mentioned earlier), far moe realistic, and almost certainly what was intended in setting the problem.
Partial release of brake, plane moves forward on belt, brakes used to maintain constant speed relative to belt. If both are moving at 5 kph, what happens to the plane relative to a fixed point outside the belt?
Since we're answering different problems, I suppose it's not surprising we give different answers.
06 Nov 06
Originally posted by mtthwNo, you're answering your own perception of the problem - not the one presented:
OK, you can (bizarrely) use the brake to get that effect in theory. I just think that my interpretation of the question is as correct (for the reasons mentioned earlier), far moe realistic, and almost certainly what was intended in setting the problem.
Since we're answering different problems, I suppose it's not surprising we give different answers.
A plane is standing on a runway that can move (some sort of treadmill). The plane moves in one direction, while the conveyer moves at the same speed but in the opposite direction. Can the plane take off?"
Where does it say that the plane is accelerating?
However, thank you for bearing with me.
Now, if the plane accelerates to 50 kph and the belt does the same - there is STILL no movement relative to the fixed point - the wheels are still turning on the belt's surface at 50 kph!
06 Nov 06
Originally posted by sugiezdBut it says "speed" and not "speed relative to the belt". Hence the possible different interpretations.
No, you're answering your own perception of the problem - not the one presented:
A plane is standing on a runway that can move (some sort of treadmill). The plane moves in one direction, while the conveyer moves at the same speed but in the opposite direction. Can the plane take off?"
Where does it say that the plane is accelerating?
However, thank ...[text shortened]... t relative to the fixed point - the wheels are still turning on the belt's surface at 50 kph!
06 Nov 06
Originally posted by Ian68The plane does not move relative to the belt ??????
It is completely illogical to assume it means relative to the belt because the plane does not move relative to the belt.
In other words, it's stationary on the surface of the belt and will move backwards from a fixed point - which problem are YOU answering?
06 Nov 06
Originally posted by sugiezdWhen I say 'does not move relative to the belt' I mean that its method of achieving movement has no relationship with the surface it is contact with and that any movement relative to that surface is a side effect of movement through the air.
The plane does not move relative to the belt ??????
In other words, it's stationary on the surface of the belt and will move backwards from a fixed point - which problem are YOU answering?
06 Nov 06
Originally posted by Ian68Ok, except that it IS IN CONTACT.
When I say 'does not move relative to the belt' I mean that its method of achieving movement has no relationship with the surface it is contact with and that any movement relative to that surface is a side effect of movement through the air.
If by your thinking - it's moving at 50 kph and it's on firm ground, how fast are the wheels turning - 50kph?
Agreed?
Now it's on a belt moving at 50 - how fast are the wheels turning?
06 Nov 06
Originally posted by sugiezdSince when do you measure the speed of an aircraft by how fast the wheels are turning?
Ok, except that it IS IN CONTACT.
If by your thinking - it's moving at 50 kph and it's on firm ground, how fast are the wheels turning - 50kph?
Agreed?
Now it's on a belt moving at 50 - how fast are the wheels turning?
I think we should probably leave it now though :-)
06 Nov 06
2 edits
Originally posted by sugiezdI think the speed of the wheels is not an issue and has no significance.
Ok, except that it IS IN CONTACT.
If by your thinking - it's moving at 50 kph and it's on firm ground, how fast are the wheels turning - 50kph?
Agreed?
Now it's on a belt moving at 50 - how fast are the wheels turning?
The plane is being moved forward by the propeller/engine, and it is moving forward because it is pulling itself through the air. The wheels are allowing it to move with less friction. They are not applying any force that will help or hinder the planes forward movement.
If the engine throttle is fixed at a steady point and the plane is moving at a fixed forward speed, it doesn't matter what you do with that belt as long as the wheels are allowed to roll free. You can speed it up, slow it down, reverse it, or stop it, and it will not change the fact that the plane is moving forward through the air and will continue to do so.
In reality the plane is moving itself along the surface that is supporting its' weight until it generates enough lift to leave the surface, but it is moving through the air.
07 Nov 06
4 edits
A previous post by sugiezd
(Truncated) ... The plane moves in one direction, while the conveyer moves at the same speed but in the opposite direction. Can the plane take off?"
THEY ARE MOVING AT THE SAME SPEED.
This can only mean relative to each other.
The plane cannot, therefore move at a higher speed than the belt BY DEFINITION IN THE PROBLEM.
What is your point here?
Say the plane moves left at 100 km/h relative to a stationary observer.
The belt moves right at 100 km/h relative to a stationary observer.
The resultant situation you have in mind is the plane sitting still relative to the observer, its forward motion negated by the conveyor belt. THIS IS WRONG.
The plane would not be slowed significantly by the conveyor. (How can it? Where is the braking force coming from? It would continue forward at 100 km/h relative to the observer. The wheels on the bottom of the plane would be spinning at 200km/h but the extra friction created by this would not slow the plane.
It has been said many times before. You are thinking of the plane as if it were a car. Yes a car would be slowed to standstill by a conveyor belt. However a plane gets its forward thrust by pulling itself forward through the air, completely independent of the ground.
While appearing simple and practical, this problem is so removed from everyday experience that it becomes quite abstract. Sugie, I think you have a fundamental misundertanding of this scenario and the principles of physics that underpin it.
You still haven't done my experiment have you?
For the sake of the patient people in this forum (myself included) please please please do it. Your constant peddling of cloudy, ignorant (non)thought is getting really tiring and what's worse i see no evidence that you have actually attempted to understand what has been repeated here for you ad nauseum.
So, i beg you, do this experiment.
Take a small toy car with free running wheels. This is our plane.
You as the pusher are the propeller because your forward thrust comes independent of the ground.
Take a long flat plank of wood. This is our conveyor belt.
With the plank motionless, push the toy along at say 1 m/s with respect to some immovable object nearby (say the true ground). Note the force required to achieve this.
Next get a helper to pull the plank in the opposite direction (at roughly the same speed) whilst you maintain the car at a speed of 1 m/s with respect to the true ground, not the conveyor, the speed relative to the conveyor will be 2 m/s.
IMPORTANT BIT
What you will notice (and i noticed) was that pulling the plank back made no difference to the amount of force needed to maintain the toy's velocity. I.E. the plank produced no braking force on the car.
In the same vein the conveyor belt cannot and does not put a braking force on the plane . The ONLY force acting on the plane is forward thrust, therefore, because there is a net force we have a net acceleration. The plane takes off.
Please don't reply until you have done the experiment, you're just wasting our time otherwise.
EDIT: Some spelling edits
07 Nov 06
Originally posted by mtthwWhen it's on the ground how else do you want to do it?
Since when do you measure the speed of an aircraft by how fast the wheels are turning?
I think we should probably leave it now though :-)
Do you understand the difference between air-speed (AS) and ground-speed (GS)?
I doubt that there is anyone out there that would argue that at a steady speed of 5 kph on a belt moving at 5 kph there is no net forward motion (no acceleration). Here the AS is 0kph (assuming still air) yet the plane has a GS of 5kph as its wheels are turning on the surface of the belt.
07 Nov 06
Originally posted by sugiezdDo you? You do know that an aeroplane must reach a minimum air-speed before it will take off don't you? And therefore when determining whether the plane will take off it is only sensible to measure air-speed?
When it's on the ground how else do you want to do it?
Do you understand the difference between air-speed (AS) and ground-speed (GS)?
I don't think planes even have equipment for measuring ground speed (other than via GPS or similar).
07 Nov 06
Originally posted by mwmillerAnd that is exactly where you are going wrong.
I think the speed of the wheels is not an issue and has no significance.
The plane is being moved forward by the propeller/engine, and it is moving forward because it is pulling itself through the air. The wheels are allowing it to move with less friction. They are not applying any force that will help or hinder the planes forward movement.
If the en ...[text shortened]... weight until it generates enough lift to leave the surface, but it is moving through the air.
It doesn't matter whether the forward force is acting through the wheels or not. THE WHEELS MUST STILL TURN.
They are turning on the belt - for the plane to move forward relative to a fixed point then they must be turning faster than the belt is moving in the other direction.
As this expressly contradicts the problem as stated then it is impossible.