eVITAERC said:
And the wheels can't roll unless the plane is moving in relation to the air.
So what don't I understand?
Max
maxro said:
You're implying that the wheel rolling has nothing to do with the plane moving forward.
The wheel rolling allows the plane to move forward without having to overocme friction. In order to do this, the plane must transmit 100% of it's moving forward into the mechanics of rolling objects. Therefore to annul the effects of rolling objects moving forwards means to annul the entire plane moving forward. Understand?
maxro said:
It should be:
"And the wheels can't roll unless the plane is moving in relation to the conveyer."
That's what you were missing
eVITAERC said:
Before the plane starts up, all four elements: plane, wheels, air and belt, remain stationary.
If the plane never moves in relation to the air, then the wheels won't roll and the conveyer won't move.
So, what Happens when the propeller starts up? Something's gotta give.
Max
eVITAERC said:
The wheels will roll in response to thrust. If the conveyor moves they'll roll faster but the plane will take off.
If what you posit is true, one couldn't skateboard parallel to the equator nevermind on a travelator.
maxro said:
But the INSTANTANEOUS moment that the body starts moving and the wheel starts turning, the convyer also INSTANTANEOUSLY starts moving equally in the oposite direction.
Your confusion comes from not understanding how finely time can be sliced.
rfbrw said:
The plane will move in relation to the conveyer, but since the conveyer is moving in the opposite direction it cancels out that motion. To a ground observer (including air) the two velocities cancel as required in the question, therefore the plane is effectively stationary.
The only vital factor in this experiment is "Can the wheels' rolling resistance be overcome by the force of the engines?".
If no, the plane would never take off, even from a stationary runway, and if yes, the plane will take off from any runway, including the one proposed in this thought experiment.
All other factors are pretty irrelevant.
If no, the plane would never take off, even from a stationary runway, and if yes, the plane will take off from any runway, including the one proposed in this thought experiment.
All other factors are pretty irrelevant.
Okay, enough is enough
In order for this not to drag on too long and waste everybody's time, I'll just stop the whole argument here:
WHY am I so cocky, insisting that the plane must not move with relation to air, and therefore not take off?
It's because I know the answer. I KNOW the answer because the exact same question was on my 1st year Physics midterm several years ago. It's with rockets instead of planes, but they both operate on the same principle (providing forward momentum by ejecting mass at high velocities). And the conveyer belt isn't even powered, it's just assume to be massless and (internal) frictionless (basically the same thing). The quesiton was whether the rocket will move forward under propulsion.
The instructor, who is the head of Canada's largest particle accelerator lab (TRIUMF), specifically brought this quesiton up in class after the midterm was marked. A lot of people, like here, argued that since the propulsion is provided by the rocket independent of the wheel, the rocket will move forward. However, he noted that all the wheels see is the conveyer belt, it will happily roll along it at whatever the speed propeller would make it go. However, it will also push the conveyer along at the exact same but opposite speed, so the conveyer itself would move in the opposite direction in relation to the ground observer.
And yes, the answer is indeed THE ROCKET WILL NOT MOVE
In order for this not to drag on too long and waste everybody's time, I'll just stop the whole argument here:
WHY am I so cocky, insisting that the plane must not move with relation to air, and therefore not take off?
It's because I know the answer. I KNOW the answer because the exact same question was on my 1st year Physics midterm several years ago. It's with rockets instead of planes, but they both operate on the same principle (providing forward momentum by ejecting mass at high velocities). And the conveyer belt isn't even powered, it's just assume to be massless and (internal) frictionless (basically the same thing). The quesiton was whether the rocket will move forward under propulsion.
The instructor, who is the head of Canada's largest particle accelerator lab (TRIUMF), specifically brought this quesiton up in class after the midterm was marked. A lot of people, like here, argued that since the propulsion is provided by the rocket independent of the wheel, the rocket will move forward. However, he noted that all the wheels see is the conveyer belt, it will happily roll along it at whatever the speed propeller would make it go. However, it will also push the conveyer along at the exact same but opposite speed, so the conveyer itself would move in the opposite direction in relation to the ground observer.
And yes, the answer is indeed THE ROCKET WILL NOT MOVE
eVITAERC said:
okay. Can you post the exact rocket problem you were given? I'm not clear on how it is the same. If the plane were on a frictionless belt, then the belt would just turn instead of the wheels and the plane would take off.
Oh I guess it IS the same - the plane would still take off.
Max
I agree that it would be more convincing if we felt we were getting the exact question rather than a paraphrase. Maybe also the exact explanation of the solution by the instructor! The explaination provided leaves a bit to be desired IMHO.
maxro said:
The similarily is in that Rocket == Plane with engine mounted on fuselage and wings chopped off
It's a question about rocket on wheels on a conveyer belt free to move. It's exactly the same as this quesiton.
eVITAERC said:
In that case, the rolling resistance of the wheels will prevent them from ever turning, and the frictionless belt will be driven directly forward by the engines and the plane will still take off.
eVITAERC said:
OK, what would happen if a plane landed on a conveyer belt that matched the speed of the wheels in the opposite direction?
Would it stop dead in relation to the ground as soon as the belt exactly matched the speed of the wheels? 🙂
Oh come on eVITAERC,
could it be you remember something wrong 😀 ? Otherwise, that physics instructor had a very very bad day when he came to that answer...
Forget the rocket, lets take a simple box with wheels under it. Now we put this box on the mass- and frictionless, non powered conveyor belt. You stand on solid ground beside that belt and look at it. Do you really want to tell me that you will not be able to move that box forward with your hands, just reach for it and push? Of course you WILL be able to push it forward, and as well will the rocket engine. Hell, how easy would it be to construct engine test grounds, if it was so easy to keep tons of generated thrust under control...
btw, i would appreciate a reaction to my answer on your first thought experiment, you do not seem to have noticed it...
Greetings,
Andreas
eVITAERC said:
The wheel simply does what its told. When it is no longer in contact with the ground it stops moving. If it is contact with the ground it moves at a rate determined by the speed of the plane as a whole. If the ground happens to move in concert with the wheel it moves slower, if in opposition, it moves faster but the plane carries on on its merry way.
For the conveyor to function as a rolling-road, the plane has to fastened to a static part of the conveyor mechanism by something strong enough to counter the engines. It isn't so off it goes.
There exactly is the point you're keep on getting wrong, eVITAERC... The plane will NOT move in relation to the conveyor belt, its engines force it to move in relation to the surrounding air. If you want to cancel out THIS movement (which you can, of course) you will have to move the complete atmosphere along with the conveyor belt. Then you reach the situation you described in your first thought experiment, remember, the one with the spacecraft. THIS case works pretty well, but it is not what the question said (no wind).
Greetings,
Andreas
In spite of what eVITAERC says "rockets instead of planes, but they both operate on the same principle", this is unfortunately entirely untrue.
Their methods of propulsion and their means of staying 'aloft' are not the same, and this is why at a certain height (at a few tens of miles above the earth's surface) planes can neither maintain forward momentum, nor can they defy gravity and stay up in the air. Rockets, however can, and indeed go millions of miles where there is no air at all.
Planes require the passage of air past their wings which creates lift (as I said in post 77) and their relationship with the ground is to all practical intents and purposes, completely irrelevant to their ability to fly, or take off.
Consider that planes are frequently tested in wind tunnels, where there is no movement whatsoever compared with either the surface on which they are standing, nor in relation to the earth. Lift is created and is measured etc during these tests, simply because of the artificially-induced passage of wind past their wings.
Racing cars are tested for the opposite to 'lift', also in wind tunnels, where they are measured for downthrust which helps with track adhesion, and yet these cars are also static in relation to the ground.
In the original question, there is nothing out of the ordinary implied here except for the fact that the surface on which the plane stands is not fixed and can roll or whatever, one way or another.
This is simply a 'red herring' and has no effect at all on the plane's ability to move **through the air which surrounds it** and therefore to "take off", which was what the question asked.
Any rolling resistances or wheel-bearing friction will be orders of magnitude below the level of the 'normal' forces involved with planes taking off, and therefore (without taking things to ridiculous extremes, which are not implied in the question) will also have negligible effect on the issue.
Goodness know what a "particle accelerator" has to do with the basic priciples of flight (except perhaps by way of a little name-dropping to add some credance) but if any such head of this lab in Canada said anything like what has been reported here, I am glad I live in the U.K!
Their methods of propulsion and their means of staying 'aloft' are not the same, and this is why at a certain height (at a few tens of miles above the earth's surface) planes can neither maintain forward momentum, nor can they defy gravity and stay up in the air. Rockets, however can, and indeed go millions of miles where there is no air at all.
Planes require the passage of air past their wings which creates lift (as I said in post 77) and their relationship with the ground is to all practical intents and purposes, completely irrelevant to their ability to fly, or take off.
Consider that planes are frequently tested in wind tunnels, where there is no movement whatsoever compared with either the surface on which they are standing, nor in relation to the earth. Lift is created and is measured etc during these tests, simply because of the artificially-induced passage of wind past their wings.
Racing cars are tested for the opposite to 'lift', also in wind tunnels, where they are measured for downthrust which helps with track adhesion, and yet these cars are also static in relation to the ground.
In the original question, there is nothing out of the ordinary implied here except for the fact that the surface on which the plane stands is not fixed and can roll or whatever, one way or another.
This is simply a 'red herring' and has no effect at all on the plane's ability to move **through the air which surrounds it** and therefore to "take off", which was what the question asked.
Any rolling resistances or wheel-bearing friction will be orders of magnitude below the level of the 'normal' forces involved with planes taking off, and therefore (without taking things to ridiculous extremes, which are not implied in the question) will also have negligible effect on the issue.
Goodness know what a "particle accelerator" has to do with the basic priciples of flight (except perhaps by way of a little name-dropping to add some credance) but if any such head of this lab in Canada said anything like what has been reported here, I am glad I live in the U.K!
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