Originally posted by sugiezdAs I said before, relative forces and speeds. Do you not understand them? Planes move forwards by applying a force to the air behind them. As that is the case they gain speed relative to the air.
The plane will accelerate to 150 RELATIVE to the surface of the belt (its groundspeed). As it has an airspeed of only 50 and needs 100 to become airborne, it will remain an exotic form of ground transport.
You understand that much? So why say the the plane will accelerate relative to the belt?
Also, what if we replaced the wheels with skis and covered the belt in snow? Will the plane take off then?
Sugiezd lets take it all down to the simplest possible situation and draw a force vector diagram.
Ignore gravity, it's irrelevant in discussions of horizontal motion.
You have a large forward vector (propeller thrust) pointing horizontally. Yes?
You spoke before of an equilibrium. OK where is the opposing force coming from to counteract the thrust?
My "toy car on plank of wood" experiment described above demonstrates that a conveyor belt cannot apply a braking force to a free wheeling object.
So where is the braking force coming from?
Originally posted by sugiezdBefore, I would be willing to shrug off your misunderstanding of the original problem, even if your interpretation of it was plain ridiculous. But this shows clearly that you just don't know what you're talking about.
The plane will accelerate to 150 RELATIVE to the surface of the belt (its groundspeed). As it has an airspeed of only 50 and needs 100 to become airborne, it will remain an exotic form of ground transport.
Originally posted by XanthosNZBecause it's on the BLOODY BELT !
As I said before, relative forces and speeds. Do you not understand them? Planes move forwards by applying a force to the air behind them. As that is the case they gain speed relative to the air.
You understand that much? So why say the the plane will accelerate relative to the belt?
Also, what if we replaced the wheels with skis and covered the belt in snow? Will the plane take off then?
Originally posted by sugiezdNo, it accelerates to 150 relative to the air and ground, and 250 relative to the belt.
Starting condition:
Plane is stationary on a conveyer belt.
Belt is stationary. (also assume the belt is infinitely long)
Plane has wheel brakes on and engines at idle.
There is no wind, air is stationary relative to the ground.
Plane needs airspeed of 100 to take off.
At full power the plane could normally achieve a speed of 150
Then, in the ...[text shortened]... f only 50 and needs 100 to become airborne, it will remain an exotic form of ground transport.
The propellor doesn't care about the belt, it drags the plane forward relative to the air regardless of what the wheels are doing. All the belt does is make the wheels spin faster, it does not prevent the propellor from pulling the plane through the air.
Originally posted by flyUnityAs stated in the problem.
Wow, I didnt know my puzzle would go this long, The answer is that the plane will take off, but this is a trick question as some might think that the plane is only moving relative to the belt.
If the the plane accelerates and somehow manages to reach take off airspeed then it is moving faster than the belt and the problem becomes a paradox.
Originally posted by sugiezdWhy is it moving faster than the belt?
As stated in the problem.
If the the plane accelerates and somehow manages to reach take off airspeed then it is moving faster than the belt and the problem becomes a paradox.
Whatever speed the plane goes forward, the belt can go the same speed backward. Ignore the wheels.
The barrier seems to be your insistance on measuring the plane's speed relative to the belt, but the belt's speed relative to the ground.
Measure them both relative to the ground.
Originally posted by aging blitzerIndeed.
Why is it moving faster than the belt?
Whatever speed the plane goes forward, the belt can go the same speed backward. Ignore the wheels.
The barrier seems to be your insistance on measuring the plane's speed relative to the belt, but the belt's speed relative to the ground.
Measure them both relative to the ground.
The problem as stated requires that the plane is in contact with the belt via its wheels, I think that we can all agree on that.
Again as stated, if the plane is moving at 10 kph on the belt then the belt moves backwards at 10 kph. The wheels are rotating at 10 kph.
Why can you not see that?
Originally posted by sugiezdOne question...
Indeed.
The problem as stated requires that the plane is in contact with the belt via its wheels, I think that we can all agree on that.
Again as stated, if the plane is moving at 10 kph on the belt then the belt moves backwards at 10 kph. The wheels are rotating at 10 kph.
Why can you not see that?
If you are observing this from the side of the belt and are standing on solid ground (not moving at all), what movement do you see?
Is it the plane moving in one direction, and the belt moving in the opposite direction at exactly the same speed?
Originally posted by mwmillerThe plane is stationary, relative to you.
One question...
If you are observing this from the side of the belt and are standing on solid ground (not moving at all), what movement do you see?
Is it the plane moving in one direction, and the belt moving in the opposite direction at exactly the same speed?
Originally posted by sugiezd"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?"
The plane is stationary, relative to you.
So the plane is stationary and moves in one direction, at the same time?
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?"
This really is staring get me down.
Why is it that you all seem to have a block on what this problem actually means?
So many of you go off into the greenery, talking about avionics, propellers not driving the wheels and a lot more nonsense hardly worth repeating.
It’s all irrelevant.
The two uses of the verb “to move” and the single use of “speed”, in context must mean the movement of the plane is relative to the belt, which is effectively the ground in terms of the plane.
Their relative speed can only be indicated by the speed of rotation of the wheels thus, by definition in the problem, this speed is the same as that of the belt and, although the plane is moving relative to the belt it is not moving with respect to an external observer.
Any other scenario, eg. where the plane accelerates to a speed which makes it move forward in relation to the observer, is obviously possible BUT, as this would require the wheels to be turning faster in one direction than the belt is moving in the other, would be a paradox in relation to the stated problem.