Stupid physics question

[Rubber Bullets]

Jupitus,

Go and get a matchbox car, roll it over a piece of paper, pull the paper backwards out from under it whilst it is rolling. Does it stop? Is it's forward momentum affected only by the paper?

RB

 

[Tilda]

I'm going to make a video on my camera to show you how it works. I've typed it so many times now!

Although I think the general confusion is with the question;

If you read the question to be "THE BELT KEEPS THE PLANE STATIONARY IN RELATION TO THE GROUND" then the plane won't take off as it has 0 airspeed.

However, if you read the question to be "THE BELT MATCHES THE PLANES SPEED" (where there is no mention of the plane being still) the plane would take off, just the wheels (which freewheel) move twice as fast.

 

[Furr]

It won't move,

A plane and a rocket work differently in that rockets use overwhelming energy to move the mass it has, while a place uses the culminative effect of constantly adding more and more lift to a planes wings buy continiously increasing its speed in the airflow.

The engines by themselves on most planes in the standard configuration (ie not pointing downwards, and with not enough force to overcome the mass of the place) would not be able to make a plane take off without a run up. And if as the original question stated that the plane was on a conveyor that would match the speed rotation of the wheels (not taking into account the amazing properties of the wheels) then the plane would not be able to take off as it would not be able to gain the initial sufficient airlow to allow the wings to provide life. End of, ITS WONT FLY...

However, if you read the question to be "THE BELT MATCHES THE PLANES SPEED" (where there is no mention of the plane being still) the plane would take off, just the wheels (which freewheel) move twice as fast.

Still wont move, if the plane is moving in relation to a stationary point of 0m/s then the belt is moving at 0m/s at 4m/s the belt moves at 4m/s, however this relies on the belt compenstating exactly to the same speed of the plane, otherwise even if there is a 0.00000000000001% margin of error the plane will start to gain speed and thus move.

 

[Rubber Bullets]

Although I think the general confusion is with the question;

Quite, and although Xane begs to differ, that was the whole point of wording the question that way.

RB

 

[Xgkkp]

So... the wheels are connected to the plane, and the motion of the wheels is 'related directly to the conveyor belt' and yet you argue that although the conveyor is moving at an equal speed but in the opposite direction the plane would still have forward velocity...

As stated above, the wheels of the plane are physically connected to the plane itself, but not mechanically - the wheels can spin at whatever speed they like (within reason), while having minimal effect on the plane above it.

 

[Rubber Bullets]

It won't move,

A plane and a rocket work differently in that rockets use overwhelming energy to move the mass it has, while a place uses the culminative effect of constantly adding more and more lift to a planes wings buy continiously increasing its speed in the airflow.

The engines by themselves on most planes in the standard configuration (ie not pointing downwards, and with not enough force to overcome the mass of the place) would not be able to make a plane take off without a run up. And if as the original question stated that the plane was on a conveyor that would match the speed rotation of the wheels (not taking into account the amazing properties of the wheels) then the plane would not be able to take off as it would not be able to gain the initial sufficient airlow to allow the wings to provide life. End of, ITS WONT FLY...

I'm sorry Furr ut that is incorrect. All plane engines are capable of overcoming the mass of the plane, they'd never move else.

Also rockets and plane engines are more similar than you think.

Rockets create their own gases which are moved into a space behind the aircraft. Airplane engines, jet or propeller move the air from infront, but both rely on the newton principle that for every action there is an equal and opposite reaction.

RB

 

[Xgkkp]

It won't move,

A plane and a rocket work differently in that rockets use overwhelming energy to move the mass it has, while a plane uses the culminative effect of constantly adding more and more lift to a planes wings buy continiously increasing its speed in the airflow.

I'm going to take the 'plane continuously adds lift' to be a mistake in communication, not understanding, but the case of a plane and a rocket are exactly the same.

They both continuously add velocity, the fact that a planes lift is directly related to it's air velocity is irrelevant. If you put a rockets on the back of the plane instead of jet engines the question would still be exactly the same.

Unless of course you mean standard 'space rockets' not 'rocket engines' in which case I have no idea where that came from because it has nothing to do with the problem.

Edit: I think Rubber Bullets explained my point more eloquently

 

[Furr]

I'm sorry Furr ut that is incorrect. All plane engines are capable of overcoming the mass of the plane, they'd never move else.

Also rockets and plane engines are more similar than you think.

Rockets create their own gases which are moved into a space behind the aircraft. Airplane engines, jet or propeller move the air from infront, but both rely on the newton principle that for every action there is an equal and opposite reaction.

RB

Your right... opps

I still say it won't fly... But im just going to MSN a Uni physics student and ask his perpective

What i meant was that a plane engine makes the plane move, but if you turn it off you slow down and crash... however i guess if there wasn't any gravity or air friction it could go on, but then if there was no air friction then the wings won't work anyway.

 

[Jupitus]

As stated above, the wheels of the plane are physically connected to the plane itself, but not mechanically - the wheels can spin at whatever speed they like (within reason), while having minimal effect on the plane above it.

Ahhh - I think I am seeing your POV - in essence, the thrust of the engines is such that the body of the plane would effectively 'overtake' (in speed terms) the speed of the wheels and the body of the plane would hence move forwards in real terms.... yes?

 

[Rubber Bullets]

Ahhh - I think I am seeing your POV - in essence, the thrust of the engines is such that the body of the plane would effectively 'overtake' (in speed terms) the speed of the wheels and the body of the plane would hence move forwards in real terms.... yes?

I think you're overcomplicating the answer slightly Jup, but yes that is it.

RB

 

[Furr]

ANYWAY!!!!

Its all about air speed

http://www.avweb.com/news/columns/191034-1.html

 

[Tilda]

Ahhh - I think I am seeing your POV - in essence, the thrust of the engines is such that the body of the plane would effectively 'overtake' (in speed terms) the speed of the wheels and the body of the plane would hence move forwards in real terms.... yes?

Imagine this Jup:

The plane is turned off, the belt is on, result, the plane moves backwards off the end of the belt.

The plane is turned off, the belt is on, a big hand holds the plane in the same place on the runway.
Result, the plane stays still in relation to the ground, the wheels turn at the same speed as the belt.

The plane turns on its engines (this is like the hand pulling the plane forwards along the belt).
Result, the belt is turning one way, the plane is moving in the other direction and the wheels are moving at the speed of both added together. The plane takes off like a normal take off.

If that dosn't work, try this;

If you have a belt sander and a toy plane and you hold the plane onto the sander (ok imagine that its wheels dont get sanded down!), its wheels turn at the speed of the sander.
If you then drag the plane, while its in contact with the sanding belt, in the opposite direction to the belt movement,
its then going forwards at the speed you drag it (or in the big plane case, the speed the engines pull it),
however the wheels go at this speed: (Speed of belt + Speed your moving it)

Does that help? A flash thing just finished downloading, so i'll start on a flash movie to explain it

 

[Furr]

Personally i give up.. haven't done phyics in years so I've decided that everyone can ignore what i said since Im probably wrong...

back to work


(i still think it won't fly though......)

 

[Jupitus]

Well - if the original question had stated 'the belt is designed to match the speed of the wheels of the plane' I would then be in the yes camp. The original question, however, states that the belt matches the speed of the plane, hence one has to assume they mean the body of it, and therefore the plane would not gain any actual foward velocity, and hence it would not take off.

 

[Xgkkp]

Personally i give up.. haven't done phyics in years so I've decided that everyone can ignore what i said since Im probably wrong...

back to work


(i still think it won't fly though......)

Haha what? Despite the fact that you provided a link saying it would and explaining in no uncertain terms why? (the same terms, I may add, that the yes people have been arguing here).

The link says:
"It was an interesting argument, but as things progressed, more rational heads prevailed, pointing out that the airplanes do not apply their thrust via their wheels, so the conveyor belt is irrelevant to whether the airplane will takeoff."
We can only hope.

 

[Xgkkp]

Well - if the original question had stated 'the belt is designed to match the speed of the wheels of the plane' I would then be in the yes camp. The original question, however, states that the belt matches the speed of the plane, hence one has to assume they mean the body of it, and therefore the plane would not gain any actual foward velocity, and hence it would not take off.

Ugh you so nearly had it. Read the link, that explains it.

It is matching the speed of the plane, but the plane is not mechanically connected to the conveyor belt.

It's like watching a ball roll along, and walking in the other direction. Just because you are walking in the other direction of the ball doesn't mean you affect it, you hav eno mechanical connection to it.

 

[Tilda]

Well - if the original question had stated 'the belt is designed to match the speed of the wheels of the plane' I would then be in the yes camp. The original question, however, states that the belt matches the speed of the plane, hence one has to assume they mean the body of it, and therefore the plane would not gain any actual foward velocity, and hence it would not take off.

Jup you're thinking like it was a car.

Ok, the car example, it goes faster by pushing the ground behind it through the rotation of its wheels if a car is on a treadmill, and the treadmill matches its speed, then it dosnt go anywhere because as hard as it tries to push itself forwards through its wheels, the treadmill matches it and it dosn't go anywhere.

However aeroplanes are fundamentally different, they dont push themselves down the runway with their wheels, they push with the jets, the wheels only purpose is to support the jet and allow stearing when on the ground.

So it dosn't matter if there is a belt matching speed with the plane. The only thing the belt effects is the speed of the wheels, while this would matter in a car, and prevent it from moving, in a plane, it has no effect on the planes efforts to move with its jets.

 

[Chilly]

Sweet jesus I hope this was sarcasm.

Care to explain which bit of my theory is bollocks? Just because the belt would have to be moving at a stupid speed doesnt mean the theory is bollocks, it just means its a bit out of bounds

 

[Will]

Stolen from somewhere

What I learned from Old Hack was that an updated version of a question aimed at confusing folks over relative measurements of airplane motion and the medium in which it operates had shown up on the Internet, and it was causing the fracas in the Lounge.

The question that has been going around is not particularly artfully worded, and I think that has caused some of the disagreements, but I'll repeat it as it is shown: "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"

My comment: Notice that the question does not state that the conveyor's movement keeps the airplane over the starting position relative to the ground, just that it moves in the direction opposite to any movement of the airplane.

Initially, about a third of the folks here said that the airplane could not ever takeoff, because the conveyor would overcome the speed of the airplane and it could never get any airspeed. The rest said the airplane would fly.

The "It won't fly, Rocky" group said that the conveyor would hold back the airplane. They asked us to imagine a person running on a treadmill. As he or she sped up, the treadmill would be programmed to speed up, just as the conveyor in the problem, and the person would remain over the same locus on the earth, while running as fast as possible.

The argument was that if the airplane started to move forward, the conveyor program was set up to move the conveyor at exactly that speed, in the opposite direction, thus, the airplane would never move relative to the ground, and, because the air was calm, it could never get any wind over its wings. One of the analogies presented was the person rowing at three mph upstream in a river on a calm day. However, the current was flowing downstream at three mph, so the resultant speed with reference to the stream bank and air was zero, and thus there was no wind on the rowboat.

I watched and listened to the disagreement for a while and was fascinated to see that the argument seemed to split between those who had some engineering or math background, all of whom said the airplane would takeoff and fly without any problem; and those with some other background, who visualized the airplane as having to push against the conveyor in order to gain speed. Because the conveyor equaled the airplane's push against the conveyor, the airplane stayed in one place over the ground and in the calm air could not get any airspeed and fly.

It was an interesting argument, but as things progressed, more rational heads prevailed, pointing out that the airplanes do not apply their thrust via their wheels, so the conveyor belt is irrelevant to whether the airplane will takeoff. One guy even got one of those rubber band powered wood and plastic airplane that sell for about a buck, put it on the treadmill someone foolishly donated to the Lounge years ago, thinking that pilots might actually exercise. He wound up the rubber band, set the treadmill to be level, and at its highest speed. Then he simultaneously set the airplane on the treadmill and let the prop start to turn. It took off without moving the slightest bit backwards.

 

[Chilly]

Jup you're thinking like it was a car.

Ok, the car example, it goes faster by pushing the ground behind it through the rotation of its wheels if a car is on a treadmill, and the treadmill matches its speed, then it dosnt go anywhere because as hard as it tries to push itself forwards through its wheels, the treadmill matches it and it dosn't go anywhere.

However aeroplanes are fundamentally different, they dont push themselves down the runway with their wheels, they push with the jets, the wheels only purpose is to support the jet and allow stearing when on the ground.

So it dosn't matter if there is a belt matching speed with the plane. The only thing the belt effects is the speed of the wheels, while this would matter in a car, and prevent it from moving, in a plane, it has no effect on the planes efforts to move with its jets.

they are not fundamentally different at all. Its just the point of friction and thrust is different. If the plane is making so forward progress vs a staitonary lump of stuff bolted to teh ground to the side of the conveyor then its not moving. No matter how many stupid arguments you have about it.

 

[Furr]

Yup, after speaking to the aviation guy

I would like to apologise, i was wrong, the plane would still take off, but the wheels would spin twice as fast!

 

[Will]

To summarise that huge amount of text I pasted in, the question is a trick, as some people have tried to point out. Since the aircraft does not apply thrust via its wheels, the treadmill has no effect on its motion. Turn jet engine on, plane moves forwards, no matter what the belt does.

 

[Aithe]

Well it's a good job that on airplanes they do then!

well sorry mr. ass, but I didnt know wether they maybe were also powered to get to the runway etc.

 

[Xgkkp]

Care to explain which bit of my theory is bollocks? Just because the belt would have to be moving at a stupid speed doesnt mean the theory is bollocks, it just means its a bit out of bounds

The part where you ridicule everyones perfectly safe arguments, say that 'There is no flakey maths here' and then put forwards pretty much the 'flakiest' theory yet.

well sorry mr. ass, but I didnt know wether they maybe were also powered to get to the runway etc.

You mean, you didn't notice the multitude of people saying that that they weren't powered?
If you've ever flown on a plane (which there is a chance you haven't) you'll find that (after sometimes being towed backwards out of the terminal dock) they taxi along under their own power. The gearing needed to move the entire plane through it's tiny wheels would also probably be immense, not to mention unpractical.

they are not fundamentally different at all. Its just the point of friction and thrust is different. If the plane is making so forward progress vs a staitonary lump of stuff bolted to teh ground to the side of the conveyor then its not moving. No matter how many stupid arguments you have about it.

(Translation: They are not fundamentally different, they just work in completely different ways!) Yes, they are fundamentally different. In a car the wheels are mechanically coupled to the engine, and are what propels it along. In a plane they are not.

Yup, after speaking to the aviation guy

I would like to apologise, i was wrong, the plane would still take off, but the wheels would spin twice as fast!

I have newfound respect for you.

Seriously No's, read that link Will posted and then come back.

 

[Tilda]

I gave up on the flash video, i got as far as making my "wheel" turn around on the tarmac before I realised it would be too complicated

 

[Xgkkp]

I gave up on the flash video, i got as far as making my "wheel" turn around on the tarmac before I realised it would be too complicated

You don't seriously think that any of the entrenched people would have changed your mind, do you?

 

[Will]

I changed my mind. The answer is supposed to be no, but the question is actually a crock of shit. The way a plane is powered, it wouldn't give a fuck.

 

[TdC]

now that I once again realize that planes' wheels don't care what direction they're turning in I feel like a bit of a heel tbh. my original line of thought was that for the forward motion of the plane the runway would apply an equal motion exactly nullifying the thrust.

I propose a new question:

if you're on a bike with a non-fixed rear axle, and the bike is on a speeding conveyerbelt set to cause the illusion of forward motion...will you fall over?

 

[Will]

No. Gyroscopic effect baby.

 

[rynnor]

Flight (lift) is generated by the movement of air across the wing surface, which because of the design, generates a higher pressure below it than above it, you must have air move across a wing surface to generate lift.

It does not matter what the speed is relative to the ground, it matters what the speed is relative to the air.

A plane will fly in a wind tunnel with absolutely no propulsion or forward movement at all, gliders dont have engines, it is the movement of air across the wings that generates the lift.

No lift, no takeoff.

Aye this is the correct version - planes take off because the wing is shaped so that the air has to travel faster over the top of the wing which generates a kind of suction force - to take off the wind over the wings must be moving sufficiently fast to generate a force sufficient to lift the mass of the plane.

In this case the plane is standing still so no force to lift a plane. Its like being on the machine when you go for an MOT - the wheels are going fast but the cars not moving.