MythBusters tackles "plane on a conveyor belt problem"

By The_Decryptor
in Science News & Discussion

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Grand Master

DrunkenMaster

Posted
Posted
Answer me this. While running on treadmill, do you feel a breeze you create from running? No!! No breeze, no lift.

Shame on you science geek! :p

I don't need science, when the Truthiness can explain it all. I wasn't the one on the treadmill when it happened so I don't know what "unexpecting classmate" felt. Trust me, the dude flew off and kept flying there for a few seconds. His only mistake: not extending his arms! I would like to prove you wrong, but my ethical obligations are now quite strong so I refuse to seek further test subjects or experiments.

What would happen If a plane landed on a conveyor belt moving in the opposite direction at the planes landing speed?

Interesting. I think this thought question should be answered in a Spy vs. Spy comic.

Mentor

Post-It Note

Posted
Posted

First, we're not saying the plane will lift off by not having wind going against the wings. We're saying that the plane will be moving forward.

Wheels are designed to reduce friction between the plane and the ground. If you remove the wheels from the equation and just think of a plane with ice skates on ice, then it's about the same situation. Obviously that could lift off.

Just imagine you are pushing a block on a treadmill (You are not on the treadmill). You hand is providing forward motion for the block, while the treadmill is providing backward motion through friction. If you didn't move your hand, then you are counteracting the friction force. All you need is a little bit more force from your hand to have the block move forward.

So, physically, even without wheels, as long as there is enough thrust, the plane will still move forward as long as the thrust force can overcome the friction force.

(To be precise, thrust actually points in the opposite direction of the plane, which, due to newton's third law, is what provides the equal forward force.)

Grand Master

+Xinok Subscriber²

Posted
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Posted
What would happen If a plane landed on a conveyor belt moving in the opposite direction at the planes landing speed?

:D

There are two things which will decelerate the plane - The wheels moving along the ground, and air resistance.

Because the conveyor belt moves at the same speed as the plane, the wheels cover twice as much ground. This means the amount of deceleration applied by the wheels doubles.

However, this doesn't mean the plane will stop twice as fast. As I said, air resistance also slows the plane down. And the air resistance will be the same, regardless of the conveyor belt.

So nobody uses this against me, because the amount of deceleration applied by the wheels doubles, this doesn't make it impossible to take off.

The engine on the plane applies acceleration, and the wheels apply deceleration. In order not to achieve takeoff, deceleration has to be greater than acceleration.

So the question is, is doubling the amount of deceleration caused by the wheels enough to overcome the acceleration?

Grand Master

Astra.Xtreme

Posted
Posted

I think to solve a bit of this dispute, let's clear one thing up:

Are they testing:

1. Will the plane outrun the conveyor belt?

2. Can a stationary plane running the engines at 100% take off?

I think we are pushing different things here.

Grand Master

HawkMan

Posted
Posted
I think to solve a bit of this dispute, let's clear one thing up:

Are they testing:

1. Will the plane outrun the conveyor belt?

2. Can a stationary plane running the engines at 100% take off?

I think we are pushing different things here.

1: the plane has to to be ale to lift of

2: no it cannot. unless you're in a windotunnel, or the wings are moving faster than the body (but you usually don't call it a plane then :p)

The key thing to this test would be wether the forces applied by the conveyor would be able to keep the plane stationary at all. Seeing as they're testign with an ultralight, I don't see how that would even be possible, the thing is so light it's barely touching the ground to start with, and won't by impacing much wight/force down on the conveyor in the first place.

Community Regular

xinary

Posted
Posted

Anyone who thinks it will take off is dillusional, the engines of a plane do not provide lift, they provide thrust, which moves the plane through the air, creating lift. Just because the engines make the plane keep up with the treadmill does not mean it is actually moving anywhere, thus, no lift. It's simple. Anyone who isn't a high school drop out can figure this out.

Mentor

Post-It Note

Posted
Posted
Anyone who thinks it will take off is dillusional, the engines of a plane do not provide lift, they provide thrust, which moves the plane through the air, creating lift. Just because the engines make the plane keep up with the treadmill does not mean it is actually moving anywhere, thus, no lift. It's simple. Anyone who isn't a high school drop out can figure this out.

Did you read my post? It pretty much answers all your theories.

Experienced

Thrawn

Posted
Posted

The plane will take off. It's so obvious that I refuse to explain, but some posters have done it very well already.

I think it's sad that people these days have so many gadgets and are used to so much technology, but they've not developed (in majority) a sufficient physical intuition. :iiam: :busted:

Grand Master

Astra.Xtreme

Posted
Posted
1: the plane has to to be ale to lift of

2: no it cannot. unless you're in a windotunnel, or the wings are moving faster than the body (but you usually don't call it a plane then tongue.gif)

The key thing to this test would be wether the forces applied by the conveyor would be able to keep the plane stationary at all. Seeing as they're testign with an ultralight, I don't see how that would even be possible, the thing is so light it's barely touching the ground to start with, and won't by impacing much wight/force down on the conveyor in the first place.

I wasn't asking those questions. I was asking which one they are testing

Experienced

brilliant naoki

Posted
Posted (edited)

EDIT: After a second glance at this thread, I see the problem most people are running into now (or at least I was). The conveyor belt is as long as a standard runway, and thus its purpose is not to make the plane stand still as the engines are running. The idea behind the conveyor belt isn't to be able to have a plane stand still relative to the ground and take off, as this is impossible.

No one was really saying that the plane will be at a standstill due to the conveyor belt and take flight. It would be more clear to state that the plane will move forward relative to still ground, and thus the plane will fly, rather than just saying the plane will fly.

Edited by brilliant naoki
Grand Master

sundayx Veteran

Posted
  • Veteran
Posted
EDIT: After a second glance at this thread, I see the problem most people are running into now (or at least I was). The conveyor belt is as long as a standard runway, and thus its purpose is not to make the plane stand still as the engines are running. The idea behind the conveyor belt isn't to be able to have a plane stand still relative to the ground and take off, as this is impossible.

No one was really saying that the plane will be at a standstill due to the conveyor belt and take flight. It would be more clear to state that the plane will move forward relative to still ground, and thus the plane will fly, rather than just saying the plane will fly.

Right, so a conveyor belt as long as a standard runway is...

The idea of the conveyor belt is so that the plane is at a standstill (relative to the ground, not the belt).

Experienced

brilliant naoki

Posted
Posted
Right, so a conveyor belt as long as a standard runway is...

The idea of the conveyor belt is so that the plane is at a standstill (relative to the ground, not the belt).

That's the idea of the conveyor belt, but that's not what it will do. The plane moves forward; nowhere in the "plane on a conveyor belt problem" does it state that the plane is at a standstill. I think that's the idea that people are having the most trouble with.

Grand Master

sundayx Veteran

Posted
  • Veteran
Posted
That's the idea of the conveyor belt, but that's not what it will do. The plane moves forward; nowhere in the "plane on a conveyor belt problem" does it state that the plane is at a standstill. I think that's the idea that people are having the most trouble with.
The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

btw... 7k posts yippy.

Experienced

brilliant naoki

Posted
Posted (edited)
The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

That states that the conveyor belt is moving at the same speed as the plane, but it doesn't state that the speed of the conveyor belt must directly affect the speed of the plane. The speed of the conveyor belt affects the speed of the wheels; this is because, as people have mentioned, the wheels are freely spinning.

For those who are having trouble visualizing why the freely spinning wheels have anything to do with it, let's say the plane is moving forward (to the left) and the conveyor belt is OFF.

post-101917-1201593616.jpg

Friction from the ground causes the wheel to turn counter-clockwise.

Now let's say the conveyor belt has been turned on. What will happen to the wheels?

post-101917-1201593707.jpg

The conveyor belt pushes the wheels in the same direction they are already spinning, making the wheels move twice as fast.

Had the conveyor belt been moving in the same direction with the plane, then the wheels would not move at all.

Notice that there has been no effect on the forward motion of the plane. The conveyor belt only affects the speed of the wheels.

All in all, the question is designed to be confusing. Anyone can understand why the conveyor belt has no effect on the forward motion of the airplane once they understand that the wheels are freely moving. The way the problem is stated, however, makes it sound like that the conveyor belt negates the propulsion of the airplane. And so, the real question is, will the plane move forward at its normal rate? Once we conclude that it will, then it is easy to see why the plane will fly.

Edited by brilliant naoki
Collaborator

RealFduch

Posted
Posted
All in all, the question is designed to be confusing. Anyone can understand why the conveyor belt has no effect on the forward motion of the airplane once they understand that the wheels are freely moving. The way the problem is stated, however, makes it sound like that the conveyor belt negates the propulsion of the airplane. And so, the real question is, will the plane move forward at its normal rate? Once we conclude that it will, then it is easy to see why the plane will fly.

Are "freely moving" wheels even theoretically possible?

I think you are saying "2+2 != 4 if 2 is 3". But 2 is not 3.

Experienced

brilliant naoki

Posted
Posted
Are "freely moving" wheels even theoretically possible?

Picture a wheel, attached to a rod as an axis. Now, you and your friend hold loosely a piece of string on both ends parallel to each other, and place the axis on top of the string.

post-101917-1201597479.jpg

Run a conveyor belt underneath this. The wheel spins freely, but notice you and your friend do not feel a force pushing the strings in the direction the conveyor belt is moving. No matter what holds or anchors the strings, the conveyor belt will not effect a force on it; it merely rotates the wheel.

This is essentially how the wheels on an airplane work. They simply sit there and rotate between a piece of metal loosely. When the plane's jet engines propel the airplane forward, the wheels move because the thing holding the axis of the wheel is moving forward with the plane; this is like you and your friend moving the strings forward on a still conveyor belt, the wheel will turn because you are coaxing the axis forward, and there is friction between the wheel and the still conveyor belt. Turn the conveyor belt on, and the wheel only spins faster, but your friend and you do not feel a force from the strings. The airplane similarly will not feel a force by the conveyor belt.

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