Why the plane takes off in laymans (my) terms

Originally posted by mwmiller
"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?"

So the plane is stationary and moves in one direction, at the same time?

Yes - it's on a moving belt - did that escape your notice?

Originally posted by sugiezd
This really is staring get me down.
Why is it that you all seem to have a block on what this problem actually means?

Doesn't the fact that so many of us have a different view give you pause for thought?

Originally posted by aging blitzer
Doesn't the fact that so many of us have a different view give you pause for thought?

Of course.

If you troll the net - you'll find discussion groups that are weighted the other way. Until somebody actually performs a controlled experiment, I shall remain unconvinced.

Can you refute any statement made in my last post?

Originally posted by sugiezd
Can you refute any statement made in my last post?

There doesn't seem much point trying.
You are making certain assumptions about the problem.
Everyone else is making different ones.

I will admit the problem as stated here does leave room for interpretation.
You don't appear to be about to change the way you interpret it.


http://www.straightdope.com/columns/060203.html
"the question is often worded badly, leading to confusion, arguments, etc"

Originally posted by sugiezd
Yes - it's on a moving belt - did that escape your notice?

Maybe you could explain to me one thing here:

The plane moves in one direction, as stated in the original problem.

How does the moving belt change that? What physical force does the moving belt have that will keep that plane from continuing to move in one direction?

Originally posted by mwmiller
Maybe you could explain to me one thing here:

The plane moves in one direction, as stated in the original problem.

How does the moving belt change that? What physical force does the moving belt have that will keep that plane from continuing to move in one direction?

Very little, but that's not the point.

If the plane is at constant throttle and moving at 5 kph in relation to the belt which is moving at the same speed in the opposite direction, what is the relative motion of the plane to the non-moving surrounds?

Originally posted by sugiezd
Very little, but that's not the point.

If the plane is at constant throttle and moving at 5 kph in relation to the belt which is moving at the same speed in the opposite direction, what is the relative motion of the plane to the non-moving surrounds?

5 kph from a non-moving observation point.

(and that IS the point)

Actually if it is 5 kph in relation to the belt I suppose the plane would only be moving at 2.5 kph from a fixed point. But whatever the speed is, the point is that the plane is moving forward from that fixed observation point.

Originally posted by mwmiller
5 kph from a non-moving observation point.

(and that IS the point)

So, the wheels are turning at 5 kph on the belt which is moving at 5 kph in the other direction yet the plane is moving forward relative to a fixed point at the same speed?

Are you serious?

Originally posted by sugiezd
So, the wheels are turning at 5 kph on the belt which is moving at 5 kph in the other direction yet the plane is moving forward relative to a fixed point at the same speed?

Are you serious?

Absolutely. What is preventing the plane from moving forward from a fixed point?

In your example the belt would be moving at only 2.5 in one direction, the plane at 2.5 in the other direction, and the wheels would turn at 5. If you stand at a fixed point and observe this, the plane will move away from you at 2.5 kph and eventually vanish from your point of view.

Originally posted by mwmiller
Absolutely. What is preventing the plane from moving forward from a fixed point?

In your example the belt would be moving at only 2.5 in one direction, the plane at 2.5 in the other direction, and the wheels would turn at 5. If you stand at a fixed point and observe this, the plane will move away from you at 2.5 kph and eventually vanish from your point of view.

Where on earth do you get this 2.5 from?

You're assuming forward motion and thus using circular logic.

If the wheels are turning at the combined speeds then the original condition of the problem has been violated.

Originally posted by sugiezd
Where on earth do you get this 2.5 from?

You're assuming forward motion and thus using circular logic.

If the wheels are turning at the combined speeds then the original condition of the problem has been violated.

lIt was your scenerio a few posts back. Here is your post:

"Very little, but that's not the point.

If the plane is at constant throttle and moving at 5 kph in relation to the belt which is moving at the same speed in the opposite direction, what is the relative motion of the plane to the non-moving surrounds? "

So that's where I got it from.

You gave the original condition of the plane moving 5 kph in relation to the moving belt. If they are both moving at the same speed but in opposite directions, then each must move at half of their relative speed as observed from a non-moving point.

Originally posted by mwmiller
You gave the original condition of the plane moving 5 kph in relation to the moving belt. If they are both moving at the same speed but in opposite directions, then each must move at half of their relative speed as observed from a non-moving point.

Absolutley not.

They are self-cancelling.

Originally posted by sugiezd
If the wheels are turning at the combined speeds then the original condition of the problem has been violated.

"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 mention anything about the wheels?

Originally posted by sugiezd
Absolutley not.

They are self-cancelling.

Maybe you need to explain what you mean by "self-cancelling". And after that explain what is preventing the plane from moving forward from a fixed observation point.....

Originally posted by mwmiller
Maybe you need to explain what you mean by "self-cancelling". And after that explain what is preventing the plane from moving forward from a fixed observation point.....

If you can't visualise that then I'll try another way.

Imagine that the belt is stationary and the plane is moving at a constant 5 kph to the left.

The belt starts and moves at 1 kph to the right - net motion: plane moves left at 4 kph.

Belt goes to 2 kph, net motion 3 kph to left and so on.

Ever been on a treadmill? You have no net forwrd motion so, if you're walking at 6 kp, how fast is the treadmill going?