The "Plane on a Treadmill" Question

[Flint]

Originally Posted by SteveDallas

It says the speed of the treadmill equals the speed of the airplane. It doesn't say the airplane is prevented from moving forward.

If the plane moves at a given speed in one direction, and the treadmill moves at that same speed in the opposite direction, what is the net speed of the plane? The debate is not: "how does the treadmill know what speed the plane is going?" it simply says the speed is matched, thus cancelled.

[SteveDallas]

You're assuming that, in rotating, the treadmill exerts a backward force on the body of the plane. It's only interacting with the wheels.

[LabRat]

Flint, when the treadmill moves, what part of the plane is it moving?

The wheels.

The wheels are free-spinning around their axles.

So, as the plane moves forward (which i thought you said it couldn't do ) the wheels end up spinning faster and faster as the treadmill matches the forward motion of the plane.

[Happy Monkey]

If the plane is moving forward at X mph, and the treadmill is moving backwards at X mph, then the net forward speed of the plane is X mph, but the wheels are spinning at 2X mph. The treadmill is matching the speed of the plane, not the speed that the wheels are turning.

[barefoot serpent]

another way to look at this is to consider the aircraft carrier: the planes are 'launched' with a catapult. But the wheels are on the deck, and the catapult moves much faster than the deck. The deck could be moving backwards and still the catapult could launch the plane.

[LabRat]

Did my bike example make sense to you?

[Flint]

A plane is standing on a runway that can move, like a giant treadmill. When the plane's engines throttle up, it begins to move forward, but the treadmill is made to match the forward speed of the plane, only in the opposite direction. So, as the plane moves forward, it moves backwards beneath the aircraft.

In order for the question not to contradict itself, you have to assume that the plane "moving forward" is only relative to the treadmill. It cannot "begin" to move forward, in any other sense, if the treadmill is moving backward at the same speed, as stated. What the wheels are doing doesn't matter. How the treadmill works doesn't matter. The stated scenario is that the plane cannot ever move forward because the treadmill moves backward at the same speed - not the same speed as the wheels, the same speed (realtive to the treadmill) as the whole plane.

You're assuming that, in rotating, the treadmill exerts a backward force on the body of the plane. It's only interacting with the wheels.

No, I'm taking the question literally and establishing the parameters of discussion, to prevent going on irrelevant tangents.

Did my bike example make sense to you?

Yes it makes perfect sense to me, as a description of a bike, a person, and a treadmill.

another way to look at this...

We don't need "another" way. We need to read the question and not add anything to it.

The treadmill is matching the speed of the plane, not the speed that the wheels are turning.

Right. The treadmill is preventing the plane from moving forward because it matches it's forward speed.

[dar512]

Originally Posted by Flint

Right. The treadmill is preventing the plane from moving forward because it matches it's forward speed.

Why does the treadmill moving backward prevent the plane from moving forward?

[Flint]

Originally Posted by dar512

Why does the treadmill moving backward prevent the plane from moving forward?

Because it matches its forward speed, as stated. X minus X is zero.

[Happy Monkey]

Originally Posted by Flint

In order for the question not to contradict itself, you have to assume that the plane "moving forward" is only relative to the treadmill.

Nope. That is one interpretation, but that interpretation requires you to assume that planes don't work the way they do. It is not possible for the speed of a treadmill to affect the speed of the plane (short of mechanical failure in the wheels). Therefore, your interpretation is incorrect. You must assume that the plane is moving forward relative to the ground, and the treadmill is moving backward at the same speed, relative to the ground. That is the only physically possible interpretation.

Right. The treadmill is preventing the plane from moving forward because it matches it's forward speed.

Nope, the plane keeps going forward, the treadmill moves backward, and the wheels do double duty.

[Flint]

Originally Posted by Happy Monkey

It is not possible for the speed of a treadmill to affect the speed of the plane (short of mechanical failure in the wheels).

It doesn't have to, and I never said it did. Where did you get that?

Originally Posted by Happy Monkey

You must assume that the plane is moving forward relative to the ground, and the treadmill is moving backward at the same speed, relative to the ground.

If the treadmill matches the forward speed of the plane, the plane cannot "begin" to move forward, except relative to the treadmill.

[Happy Monkey]

Originally Posted by Flint

Because it matches its forward speed, as stated. X minus X is zero.

But the treadmill X can't subtract from the plane's X. The plane still goes X, but the wheels go 2X.

[Kitsune]

The guy brushing his teeth knows what he's talking about

[Flint]

Originally Posted by Happy Monkey

But the treadmill X can't subtract from the plane's X.

It can, because this question says it can. Start by establishing what is being discussed.

[Happy Monkey]

Originally Posted by Flint

It doesn't have to, and I never said it did. Where did you get that?

When you said that they could cancel each other out. That is not physically possible.

If the treadmill matches the forward speed of the plane, the plane cannot "begin" to move forward, except relative to the treadmill.

The treadmill is moving backward relative to the ground at speed X. The plane is moving forward, relative to the ground, at speed X. The plane is moving at speed 2X relative to the treadmill. Your interpretation is not physically possible.