The wheel speed and the conveyor belt speed really have nothing to do with whether the plane will take off.
If the plane is exerting the normal amount of thrust needed for take off, it will move forward and accelerate (relative to stationary objects on the ground - not the belt).
Air will flow over the wings and the plane will lift off.
Your argument was exactly the argument I gave on another forum before I could see the correct solution, as I said this question can make you think all kinds of scenarios to try to explain your thinking but once you are on the wrong road it's very hard to be convinced otherwise.
I'm not trying to be cruel or condescending in any way but can I leave the explanation a bit longer so those that doubt it would take off can have a chance to elaborate on their thinking.
OK, well maybe I am misinterpreting the problem statement, since it is a bit vague. It isn't exactly clear what you mean by "matches the speed". Clearly, if you ignore the stuff about wheels and treadmills then the jet can take off under its own thrust. If my interpretation of speed matching is correct, then the jet can't move, and it can't take off. I will be interested in hearing the expanation of the "correct solution".
BTW, I'm assuming our inertial reference point is relative to a point on the surface of the earth. If I use a reference point that is not rotating with the surface of the earth then there is certainly enough speed for a jet to fly.
I think the mythbusters experiment is flawed in that how can they know for certain that the runway was matching the wheel speed?
remember I said hypothetically the runway would match wheel speed instantaneously, the runway in the video wouldn't have been instantly synched to wheel speed.
The plane would have achieved some forward motion beforehand.
OK, well maybe I am misinterpreting the problem statement, since it is a bit vague. It isn't exactly clear what you mean by "matches the speed". Clearly, if you ignore the stuff about wheels and treadmills then the jet can take off under its own thrust. If my interpretation of speed matching is correct, then the jet can't move, and it can't take off. I will be interested in hearing the expanation of the "correct solution".
by matching speed I mean if you had equally spaced dots on the runway that matched the circumfrance of the wheel and you had a dot on the wheel lined up with the first dot of the runway the dots should always meet up together. Hope i'm not giving too much away by saying that.
by matching speed I mean if you had equally spaced dots on the runway that matched the circumfrance of the wheel and you had a dot on the wheel lined up with the first dot of the runway the dots should always meet up together. Hope i'm not giving too much away by saying that.
By that definition you could use a stationary runway.
>wheel bearings they are frictionless!
That would be the same as if the wheels where 1mm off the ground and if the wheels are not touching the ground
it does not matter how much of the ground you move in the opposite direction.
As the engine is pushing/pulling on the relative stationary air around it and not the ground.
You already said the correct answer is that it will take off. One solution would be for the wheels to not turn at all. The conveyer belt just moves at the same speed as the jet, but that seems to be opposite of the way the problem is stated.
So everyone is now convinced that the correct solution is that the plane would take off?
I was hoping for more arguments against it taking off
Phil was spot on very early in the thread and I had to ignore him or the thread would have been dead straight away
Although some did say it wouldn't take off earlier on due to no airflow over the wings as the jet would be stationary
anyway the explanantion that convinced me was that the wheel axle which is fixed to the plane bears no relationship to the ground as the bearings are frictionless and the wheels are not driven like in a car or automobile but freewheeling therefore no matter how fast the wheels would turn and the runway would compensate the plane would be referenced to the air only and the thrusters would indeed push the plane through the air causing flow over the wings and lift.
You guys are too clever by far, was hoping for some answers from non engineer types wrong forum to ask this question I guess
The propellers (or jet impellers) act against the surrounding air mass to produce forward motion - the wheels and belt react with each other but the two action/reaction pairs are separate.
So as long as the engines produce forward motion as a result of thrust against the air, the wing lift will occur...
The propellers (or jet impellers) act against the surrounding air mass to produce forward motion - the wheels and belt react with each other but the two action/reaction pairs are separate.
So as long as the engines produce forward motion as a result of thrust against the air, the wing lift will occur...
Firstly the Mythbusters scenario is that the conveyor speed matches the aircraft speed-over-ground, not its wheel speed.
If the conveyor attempts to match wheel speed there is positive feedback and the tyres/wheels (and conveyor?) will fail - this may or may not prevent the plane from taking off, but it would deter most pilots!
This question as posed isn't physical since you cannot do anything instantaneously.
Firstly the Mythbusters scenario is that the conveyor speed matches the aircraft speed-over-ground, not its wheel speed.
If the conveyor attempts to match wheel speed there is positive feedback and the tyres/wheels (and conveyor?) will fail - this may or may not prevent the plane from taking off, but it would deter most pilots!
This question as posed isn't physical since you cannot do anything instantaneously.
The question poses both physical and hypothetical parts such as no friction
The real part is that as Ron explained excellently is that although you have one physical jet plane the wheels are reacting with the ground totally in isolation to the plane itself is reacting with the air.
As I said you engineers will not allow a bit of creative thinking it's all numbers and formulas to you
Just a quick note about the MythBusters experiment...
No it wasn't flawed, even if they didn't manage to perfectly match the plane's speed.
If an perfectly matched opposite rotation of the conveyor should be able to stop the plane from moving, it follows that ANY movement at all of the coneyor should affect the plane's speed. With enough opposite speed, the plane would then be unable to reach takeoff speed and remain on the conveyor. That didn't happen...
Who is talking about calculus?
This is simple Newtonian mechanics. We were doing experiments with this in high school age 12 or 13. The maths is very simple but not even the main point. You get a feel for how things work.
If it is SITTING on the runway as originally stated, it won't take off.
If you start the engines and open up the throttles and can achieve a speed greater than takeoff velocity, it will take off.
If it is SITTING on the runway and a head wind in excess of takeoff velocity comes along and the plane isn't tied down, it will take off.
I'll go back to my original answer - no wind over the wings, no take-off.
Who is talking about calculus?
This is simple Newtonian mechanics. We were doing experiments with this in high school age 12 or 13. The maths is very simple but not even the main point. You get a feel for how things work.
I didn't make my point well. Physics wasn't a required class in high school or college.
In high school they had a college prep curriculum and a general get-me-a-diploma and get-me-outa-here curriculum. I don't know if any general physics concepts were taught in any other high school classes.
Business, social arts, etc majors certainly did not require physics.
If it is SITTING on the runway as originally stated, it won't take off.
If you start the engines and open up the throttles and can achieve a speed greater than takeoff velocity, it will take off.
If it is SITTING on the runway and a head wind in excess of takeoff velocity comes along and the plane isn't tied down, it will take off.
I'll go back to my original answer - no wind over the wings, no take-off.
As Phil said the runway and wheels are red herrings the plane would move forward as the wheels and runway would have no influence even though they would both be moving there is no friction to influence any motion to the axle
I think that enough physics can be taught qualitatively to develop a working physical intuition. The idea is to reduce the role of magic in people's understanding of the universe. And this can be done via simple demonstrations without bringing a lot of math into the picture.
For example, in this problem, it suffices to say that thrust results in acceleration (you know: for every action ...) and that said acceleration is invariant in any inertial frame of reference. For the jet not to take off, therefore, the air surrounding it would have to be accelerating in the same direction and at the same rate as the plane, which was certainly not among the problem's premises.
For a course in qualitative physics the jet puzzle would make a good test question.
If the conveyor belt could really match the wheel speed, it would soon be running infinitely fast since the plane is moving through the air it will be imposible for the conveyor belt to move fast enough to match the wheel speed. The wheel speed would be the conveyer belt speed plus the airspeed (before take off, assuming no wind). Since the wheel speed whould instantaneously be matching the conveyor belt speed the conveyer belt could never catch up to the wheel's speed.
So basically the question is "What whould happen if the impossible happened?"
Edit: I don't think what I wrote earlier is correct. The conveyor belt would only need to be infinitely fast if the wheels were supposed to stay still (with respect to the earth). The speed of the conveyor belt would have to continue to speed up but not infinitely so.
If the conveyer belt could really match the wheel speed, it would soon be running infinitely fast since the plane is moving through the air it will be imposible for the conveyer belt to move fast enough to match the wheel speed.
I'm not so sure on that as there is a speed when the plane would take off with enough lift would the wheels have reached such a speed by then?
edit- Ah is it because I said the runway reponds instantaneously?
edit- Ah is it because I said the runway reponds instantaneously?
Yes.
The wheel also would also respond instantaneously creating a cycle where you'd soon have infinite speed.
Edit: I think I'm wrong here. This would make a nice calculus problem with instantaneous accellerations and such. I don't think infinite speed would be required.
That's a good argument and kinda spoils the problem perhaps we could say that the runway and wheels are super engineered and could cope at those speeds.
Comments
If the plane is exerting the normal amount of thrust needed for take off, it will move forward and accelerate (relative to stationary objects on the ground - not the belt).
Air will flow over the wings and the plane will lift off.
BTW, I'm assuming our inertial reference point is relative to a point on the surface of the earth. If I use a reference point that is not rotating with the surface of the earth then there is certainly enough speed for a jet to fly.
remember I said hypothetically the runway would match wheel speed instantaneously, the runway in the video wouldn't have been instantly synched to wheel speed.
The plane would have achieved some forward motion beforehand.
That would be the same as if the wheels where 1mm off the ground and if the wheels are not touching the ground
it does not matter how much of the ground you move in the opposite direction.
As the engine is pushing/pulling on the relative stationary air around it and not the ground.
-Phil
I was hoping for more arguments against it taking off
Phil was spot on very early in the thread and I had to ignore him or the thread would have been dead straight away
Although some did say it wouldn't take off earlier on due to no airflow over the wings as the jet would be stationary
anyway the explanantion that convinced me was that the wheel axle which is fixed to the plane bears no relationship to the ground as the bearings are frictionless and the wheels are not driven like in a car or automobile but freewheeling therefore no matter how fast the wheels would turn and the runway would compensate the plane would be referenced to the air only and the thrusters would indeed push the plane through the air causing flow over the wings and lift.
You guys are too clever by far, was hoping for some answers from non engineer types wrong forum to ask this question I guess
So as long as the engines produce forward motion as a result of thrust against the air, the wing lift will occur...
If the conveyor attempts to match wheel speed there is positive feedback and the tyres/wheels (and conveyor?) will fail - this may or may not prevent the plane from taking off, but it would deter most pilots!
This question as posed isn't physical since you cannot do anything instantaneously.
The real part is that as Ron explained excellently is that although you have one physical jet plane the wheels are reacting with the ground totally in isolation to the plane itself is reacting with the air.
As I said you engineers will not allow a bit of creative thinking it's all numbers and formulas to you
Is physics a required course in high school or college?
When I was in college, there were two physics courses - one for math majors (calculus was needed) and another for people with no calculus background.
I was a math major but haven't used calculus in a very longggg time
No it wasn't flawed, even if they didn't manage to perfectly match the plane's speed.
If an perfectly matched opposite rotation of the conveyor should be able to stop the plane from moving, it follows that ANY movement at all of the coneyor should affect the plane's speed. With enough opposite speed, the plane would then be unable to reach takeoff speed and remain on the conveyor. That didn't happen...
This is simple Newtonian mechanics. We were doing experiments with this in high school age 12 or 13. The maths is very simple but not even the main point. You get a feel for how things work.
If you start the engines and open up the throttles and can achieve a speed greater than takeoff velocity, it will take off.
If it is SITTING on the runway and a head wind in excess of takeoff velocity comes along and the plane isn't tied down, it will take off.
I'll go back to my original answer - no wind over the wings, no take-off.
I didn't make my point well. Physics wasn't a required class in high school or college.
In high school they had a college prep curriculum and a general get-me-a-diploma and get-me-outa-here curriculum. I don't know if any general physics concepts were taught in any other high school classes.
Business, social arts, etc majors certainly did not require physics.
Bruce
For example, in this problem, it suffices to say that thrust results in acceleration (you know: for every action ...) and that said acceleration is invariant in any inertial frame of reference. For the jet not to take off, therefore, the air surrounding it would have to be accelerating in the same direction and at the same rate as the plane, which was certainly not among the problem's premises.
For a course in qualitative physics the jet puzzle would make a good test question.
-Phil
If the conveyor belt could really match the wheel speed, it would soon be running infinitely fast since the plane is moving through the air it will be imposible for the conveyor belt to move fast enough to match the wheel speed. The wheel speed would be the conveyer belt speed plus the airspeed (before take off, assuming no wind). Since the wheel speed whould instantaneously be matching the conveyor belt speed the conveyer belt could never catch up to the wheel's speed.
So basically the question is "What whould happen if the impossible happened?"
Edit: I don't think what I wrote earlier is correct. The conveyor belt would only need to be infinitely fast if the wheels were supposed to stay still (with respect to the earth). The speed of the conveyor belt would have to continue to speed up but not infinitely so.
edit- Ah is it because I said the runway reponds instantaneously?
Yes.
The wheel also would also respond instantaneously creating a cycle where you'd soon have infinite speed.
Edit: I think I'm wrong here. This would make a nice calculus problem with instantaneous accellerations and such. I don't think infinite speed would be required.