Why the plane takes off in laymans (my) terms

Originally posted by sugiezd
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?

So you're saying that the belt is reducing the speed of the plane?

I don't think so. The plane is not exerting force against the belt to move forward. It happens to be rolling on the belt, but it is pulling itself through the air to move forward.

Originally posted by sugiezd
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?

This problem is somewhat ridiculous in that it is self-defeating.

It took me a while to realise that.

The only way that you can measure the forward motion of plane is in relation to the speed at which the wheels are turning on the surface of the belt (for some reason, some people cannot see this).

So if, as the problem states, that speed is matched by the belt, there is no net motion.

Originally posted by mwmiller
So you're saying that the belt is reducing the speed of the plane?

I don't think so. The plane is not exerting force against the belt to move forward. It happens to be rolling on the belt, but it is pulling itself through the air to move forward.

No, the belt is not reducing the speed of the plane, it's still moving at 5 kph to the surface of the belt - that very surface is moving in the opposite direction.

Originally posted by sugiezd
No, the belt is not reducing the speed of the plane, it's still moving at 5 kph to the surface of the belt - that very surface is moving in the opposite direction.

PS - don't fall for this BS about the wheels not being driven - it has nothing to do with it at all.

Remember the plane is not accelerating.

Originally posted by sugiezd
This problem is somewhat ridiculous in that it is self-defeating.

It took me a while to realise that.

The only way that you can measure the forward motion of plane is in relation to the speed at which the wheels are turning on the surface of the belt (for some reason, some people cannot see this).

So if, as the problem states, that speed is matched by the belt, there is no net motion.

Using your logic, if the belt and the plane are moving in the same direction and the same speed, then the wheels would not be turning at all, and therefore nothing would be moving forward.

Originally posted by mwmiller
Using your logic, if the belt and the plane are moving in the same direction and the same speed, then the wheels would not be turning at all, and therefore nothing would be moving forward.

No, if the plane is moving at 5 kph to the left on the surface of the belt, relative to it and the belt is moving at the same speed. also to the left then the 2 speeds are additive in relation to a fixed observer - ie. 10 kp h to the left.

Originally posted by sugiezd
The only way that you can measure the forward motion of plane is in relation to the speed at which the wheels are turning on the surface of the belt

This is just another of your assumptions.
Everyone else thinks it can be measured against other things.

Originally posted by sugiezd
No, if the plane is moving at 5 kph to the left on the surface of the belt, relative to it and the belt is moving at the same speed. also to the left then the 2 speeds are additive in relation to a fixed observer - ie. 10 kp h to the left.

If the plane and the belt are moving at the same speed and in the same direction as you have described, I don't thing there is any relative speed between them.

Observing them from a fixed point would see them both moving at 5 kph.

Originally posted by aging blitzer
This is just another of your assumptions.
Everyone else thinks it can be measured against other things.

Such as what?

Originally posted by mwmiller
If the plane and the belt are moving at the same speed and in the same direction as you have described, I don't thing there is any relative speed between them.

Observing them from a fixed point would see them both moving at 5 kph.

That is not what I said.

Originally posted by sugiezd
Such as what?

The ground.
The air.

Originally posted by sugiezd
That is not what I said.

What you said is:

"No, if the plane is moving at 5 kph to the left on the surface of the belt, relative to it and the belt is moving at the same speed. also to the left then the 2 speeds are additive in relation to a fixed observer - ie. 10 kp h to the left."

I disagree....

A fixed observer would see a plane moving at 5 kph to the left, and a belt moving at the same speed. The speeds would not be additive. Furthermore, if they were moving in opposite directions, the fixed observer would see the plane moving in one direction at 5 kph and the belt moving in the other direction at 5 kph.

Originally posted by aging blitzer
The ground.
The air.

Air - ground the same thing for a plane on the ground in still air.

If the problem had stated that the plane was moving in relation to the ground then eventually, assuming it accelerates, it will take off. Here the wheel speed would be the sum of the 2 speeds.

Unfortunately, the way it's written implies the motion is in relation to the surface on which it finds itself.

NB, there is no mention of acceleration in the problem, nor of a vaiance of speed - in which case, as the plane is not already flying at that speed, it never will.

Originally posted by mwmiller
What you said is:

"No, if the plane is moving at 5 kph to the left on the surface of the belt, relative to it and the belt is moving at the same speed. also to the left then the 2 speeds are additive in relation to a fixed observer - ie. 10 kp h to the left."

I disagree....

A fixed observer would see a plane moving at 5 kph to the left, and a belt ...[text shortened]... he plane moving in one direction at 5 kph and the belt moving in the other direction at 5 kph.

Then we'll have to disagree.

Originally posted by sugiezd
Unfortunately, the way it's written implies the motion is in relation to the surface on which it finds itself.

Well, I'll leave it that then, because I don't think it does imply that.
If you stick to that view, then we are at an impasse.