2023 LC Thread - It was predetermined that I would change the thread title (Part 1)

Politics & News
8485

The friction between the ground and the runway causes them to move.

8486

Conveniently ignoring his explanation to wage more forum wars.

Yes a plane’s engine turns its wheels indirectly.

8487

Let’s look at this from a purely logical perspective. You have 4 propositions.

  • (1) The plane moves forward relative to the ground if and only if the tangential speed of the wheels (Sw) exceeds the speed of the treadmill (St) at some instant (the ground is the frame of reference for both speeds)
  • (2) Sw equals St at every instant
  • (3) The plane needs to move forward relative to the ground in order to achieve liftoff
  • (4) The plane achieves liftoff from the treadmill as described

All four of them can’t be true, so you have to reject at least one of them to have a coherent story about what happens.

2 Likes
8488

Yeah no shit, I understand that the wheels wouldn’t turn if there was no friction between the tires and the runway. You turn on the engine and the wheels start turning. If there was no friction it would just start skidding forward. What’s causing the plane to move and the wheels to begin to turn? The force of the engines.

8489

I reject (1).

3 Likes
8490

So the wheels are moving anytime the plane is on the ground. Interesting

2 Likes
8491

Pathetic trolling by Shookie when compared to how good he used to be at it. You ok bud?

1 Like
8492

I think that (1) is the most plausible one to fail in a real world scenario because wheels could skip or skid. (That assumes we can’t reject (2) because it’s stipulated by the problem.)

Others seem to reject (3)

8493

Yeah exactly. Great post.

8494

What is Sw?

8495

If it makes a difference you can think of it as RPM where the treadmill is around cylinders with the same circumference as the wheels, and its speed is also measured in RPM

1 Like
8496

8 day weekend, fam

5 Likes
8497

Then I reject #2.

8498

the plane can’t get off the ground without forward movement (or a REALLY strong headwind, I guess)

8499

Trivially, (1) can’t be true once the plane has taken off. That may seem like a flip response, but think carefully about what happens when the plane goes from being on the ground to takeoff.

8500

It’s happened to me a couple times. One was a weird green bug and the other was a fly.

8501

This is the key point. And that’s why you have to assume it’s a magical treadmill because a real treadmill couldn’t act in such a way to make 2 true.

If everything is frictionless then to keep 2 true the wheels just immediately turn infinitely fast the instant the force of the engines is applied. If nothing burns up but the wheel bearings have friction then 2 can be true by increasing the speed of the wheels until the friction in the wheels exactly opposes the thrust of the engine.

8502

I wasn’t specific enough. I should have written “While the wheels are in contact with the treadmill, …”

8503

The force from friction from the wheels is not dependent on the speed of the plane or the thrust of the engine.

8504

Imagine we have some vertical jump jets installed on the plane so it can hop straight up for a few seconds if it wants to. OK, so imagine your plane is on the treadmill going full speed but staying put because the treadmill is matching that speed.

What happens when you fire the jump jets? Does it zoom forward like a rocket?

You can reverse this: imagine the plane flying full blast and then touches down on the conveyor as it is going full blast. Does the plane stop instantly, turning everyone on board into chunky salsa?