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Moridin920
Nov 15, 2007

by FactsAreUseless
So this was never really answered satisfactorily for me.

If you have a plane on a giant treadmill which goes backward as fast as the plane moves forward will the plane be able to take off? My friend says yes but only with a propeller plane but he's a bit of a fuckwit so I'm not sure.


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Doctor J Off
Dec 28, 2005

There Is
yeah it will

criscodisco
Feb 18, 2004

do it

Of course not, as there's no air movement over the wings that provides lift.

Avalanche
Feb 2, 2007
The plane crashes into the world trade center and the treadmill turns into a beautiful butterfly.

Enfield
May 30, 2011

by Nyc_Tattoo
no but itll get a good cardio workout

Knot My President!
Jan 10, 2005

Unless there's a conveyor belt in each engine, the plane won't move forward

Isaac
Aug 3, 2006

Fun Shoe
Oh great just what i needed. A problem!

Supreme Allah
Oct 6, 2004

everybody relax, i'm here
Nap Ghost
We'll start by making two approximations, and then consider the effects of these effects more closely.

Wheels. First we assume that there is no rolling resistance, that the wheels roll completely freely and that they have negligible mass: therefore it takes no force to turn the wheels, even at high speed. This is obviously approximate: because of their mass and thus their moment of inertia, the wheels do require a force (and thus a torque) to make them rotate; because of friction in bearings they require a force (and thus a torque) to keep them rotating; because of the deformation of the tires there is a rolling resistance and this requires a force as well. In principle, however, we could imagine making this effect very small. We might even use hovercraft or maglev instead of wheels.

Air motion. Second, we assume that the conveyor belt does not create a wind with respect to the ground. In practice, the conveyor has to be very long and probably very wide and so will move air to some extent. In principle, however, we could imagine this to be very small: there could be one very narrow belt for each wheel: these would not affect the air much, though it might require precise steering. (On the other hand, if we make the conveyor belt the size of a planet, we end up with Question 3.)

Let's start the plane's engines and the conveyor belt simultaneously. The moving conveyor belt starts to rotate the wheels but, according to our initial approximation of freely rotating, massless wheels and no rolling resistance, this requires negligible horizontal force from the ground.

So, subject to our approximations, the horizontal forces on the plane are only the thrust of the engines and the drag exerted by the air. Because we've assumed that the conveyor belt doesn't affect the air, these horizontal forces have their normal values. So the plane accelerates at its normal acceleration and, after a normal period, achieves its take-off velocity and then, with the usual values of flaps etc, takes off. In this case, with these approximations, the question and answer are both simple.

sketch of plane on conveyor or runway showing horizontal forcesSo now let's look at the approximations. The sketches at right show the horizontal forces at some point during the acceleration towards take-off: the propellor provides forward thrust to the plane. The drag is the plane passes through the air is in the opposite direction. There is also a small backwards force on the wheels.

Wheels. The runway and the conveyor belt both exert backwards forces on the wheels for three reasons. These effects are all reasonably small compared to the thrust of the engines, but not zero, and not equal for the runway and conveyor cases. What can we say about them?

Bearings. The wheel bearings are not perfect so, just to keep the wheels turning at constant angular velocity on their axles, a torque is required. This torque is small: give a wheel a spin and observe that its angular velocity decreases due to this effect (and due to air resistance on the tires), but does so slowly. Nevertheless, though small, this is one of the key differences between the plane on the conveyor belt and the plane on the runway. This force increases with angular velocity and, at take-off on the conveyor belt, the wheels must turn twice as fast as on the runway. That's why, in the sketch, the force on the wheels is very small in the runway case and not quite so small in the conveyor belt case. So, because of this effect alone, one would expect that, in order to take off, the pilot would require very slightly more throttle on the conveyor belt than on the runway.

Angular accleration. The wheels also have mass, and therefore a non-zero moment of inertia. (See Rotation, torques, moment of inertia.) So a torque is required for the angular acceleration involved in making them rotate. Lift a bicycle and try spinning the wheel: you have to apply a force to get it spinning. For take-off from the conveyor belt, we have to achieve twice the angular velocity that we'd have on the runway. (Plane is going forward at normal take-off speed, conveyor belt is going backwards at the same speed.) Does this mean twice as much accelerating force? In fact no, because the conveyor belt is like an infinitely long runway: we can take as long as we like (fuel permitting) to spin the wheels up to the required angular velocity. So this is not an important effect in our problem.

Rolling resistance. Third, there is rolling resistance. The tire is deformed as it rolls, and the energy required to deform it where it touches the ground is a bit less than the energy you recover when it regains its shape after leaving the ground. This gives rise to rolling resistance. Now the amount of deformation of the tire depends on the downwards force. Initially, with the plane stationary with respect to the air, this downwards force on all wheels equals the plane's weight. As the plane accelerates with respect to the air, however, lift starts to support the weight of the plane, so the downwards force on the wheels becomes successively smaller, and so does the rolling resistance. As we approach take-off speed, rolling resistance becomes very small.

Air motion. We assumed above that the conveyor belt does not move the air. This can never be quite true, of course. The length of our conveyor belt has to be somewhat longer than the normal take-off distance of the plane. It probably has to be tens of metres wide, too. So it will have an effect on the air above it: it will tend to drag air backwards: it will create a headwind. Pilots usually choose to take off into a headwind: it means that, when they have normal take-off speed with respect to the air, they have a lower speed with respect to the ground. So they achieve take-off speed (with respect to the air) more quickly and need less runway. So, if all other effects were zero, this effect would make it easier for the plane to take off from the conveyor belt than from the runway. The effect, however, is small. (but again, see Question 3 in which this effect dominates.)

So, for question 1, the answer is that the plane will take off at approximately the same speed (with respect to the ground) in the two cases. On the conveyor belt, the take-off speed (with respect to the ground) would be slightly higher due to the wheel bearing effect and slightly lower due to the wind created by the conveyor belt. Which is larger is hard to say, because they can be manipulated by say changing the width of the conveyor belt and forgetting to lubricate the wheel bearings.

Supreme Allah
Oct 6, 2004

everybody relax, i'm here
Nap Ghost
Voted 2, could of been a decent thread if you cared more.

Robo Reagan
Feb 12, 2012

by Fluffdaddy
pls help i didnt pass high school physics

Moridin920
Nov 15, 2007

by FactsAreUseless

Robo Reagan posted:

pls help i didnt pass high school physics

hey now no need to get rude

proof of concept
Mar 6, 2005

by FactsAreUseless
this poo poo again?

Moridin920
Nov 15, 2007

by FactsAreUseless

proof of concept posted:

this poo poo again?

my friend the last thread enraged ralp and it was gassed but I just want to know the truth

The Fattest PI
Mar 4, 2008

Moridin920 posted:

So this was never really answered satisfactorily for me.

If you have a plane on a giant treadmill which goes backward as fast as the plane moves forward will the plane be able to take off? My friend says yes but only with a propeller plane but he's a bit of a fuckwit so I'm not sure.




The conveyor can't move backwards as fast as the plane moves forward. The wheels on the airplane aren't connected to a driveshaft or anything, they're essentially freewheeling. The reason people argue over this dumb this is because people see it two different ways.
The first group see the treadmill as just a fill in device for the thought experiment to keep the plane in place. If the plane is kept in place there is no air moving over the wings and it doesn't take off.
The second group recognize that the speed the wheels are moving have little impact on the air speed of the plane, and the airplane takes off, providing the treadmill is long enough because the plane will move forward due to the engines providing thrust

I hope this helps you in your endeavors mr OP. I'm gay

Moridin920
Nov 15, 2007

by FactsAreUseless

The Fattest PI posted:

The conveyor can't move backwards as fast as the plane moves forward.

why?

The Fattest PI
Mar 4, 2008

The wheels aren't what's making the airplane go forward, it's the engines pushing air. The wheels just spin freely (faster on the treadmill I guess) while the engines push the plane forard.

Drunk Nerds
Jan 25, 2011

Just close your eyes
Fun Shoe
It's a trick question:

The plane can not remain stationary, no matter how fast nor powerful the treadmill. This is because it is not the wheels that move the plane but the engines.

Consider this: if the wheels weren't there, and the plane was just magically suspended in midaid, it would move forward when the rocket engines started up. So how would having wheels touching the ground possibly affect this? It wouldn't

Moridin920
Nov 15, 2007

by FactsAreUseless
umm idk I'd prefer not to bring magic into it at this time

Contra Duck
Nov 4, 2004

#1 DAD
Is this one of those moving walkways at the airport? I don't think those go fast enough to stop a plane.

CaptainSarcastic
Jul 6, 2013



Is it a Russian plane? This is important.

Orkin Mang
Nov 1, 2007

by FactsAreUseless
it would get just enough lift to hover

EorayMel
May 30, 2015

WE GET IT. YOU LOVE GUN JESUS. Toujours des fusils Bullpup Français.
mods knew the answer long ago

MeLKoR
Dec 23, 2004

by FactsAreUseless
Only if the plane is circumcised.

Fojar38
Sep 2, 2011


Sorry I meant to say I hope that the police use maximum force and kill or maim a bunch of innocent people, thus paving a way for a proletarian uprising and socialist utopia


also here's a stupid take
---------------------------->

Moridin920 posted:

So this was never really answered satisfactorily for me.

If you have a plane on a giant treadmill which goes backward as fast as the plane moves forward will the plane be able to take off? My friend says yes but only with a propeller plane but he's a bit of a fuckwit so I'm not sure.




im glad you provided a picture otherwise i would not have been able to follow

Enfield
May 30, 2011

by Nyc_Tattoo
its a nice picture

walking around
Jul 29, 2013
in 9th grade this kid had his parents make sure he got into physics class which was only for 10th graders and up, and he would go on at length to other 9th graders about what velocity he would need to throw something out a window at the window's height to hit the taco bell across the street

i thought he made this thread but i guess not

gary oldmans diary
Sep 26, 2005
gonna have a resonance cascade in the warp core for sure

Stick Figure Mafia
Dec 11, 2004

Enfield posted:

its a nice picture

i agree. it is a very nice picture and you should be proud op. don't take no guff

SLICK GOKU BABY
Jun 12, 2001

Hey Hey Let's Go! 喧嘩する
大切な物を protect my balls


More importantly, if the plane would take off, why are airports and the navy not using this space saving lift off technology?

Crash_N_Burn
Apr 19, 2014

here's a physics problem for you: if you put a hotdog in between two slices of bread does it then become a sandwich?

SLICK GOKU BABY
Jun 12, 2001

Hey Hey Let's Go! 喧嘩する
大切な物を protect my balls


Crash_N_Burn posted:

here's a physics problem for you: if you put a hotdog in between two slices of bread does it then become a sandwich?

What the gently caress is this poo poo? Only poors eat a hotdog off of sliced bread. And if you're poor enough to do that, you sure as hell are only going to use 1 slice of bread and fold it around the hotdog to make a poor mans hotdog bun.

Crash_N_Burn
Apr 19, 2014

SLICK GOKU BABY posted:

What the gently caress is this poo poo? Only poors eat a hotdog off of sliced bread. And if you're poor enough to do that, you sure as hell are only going to use 1 slice of bread and fold it around the hotdog to make a poor mans hotdog bun.

open your mind to hypotheticals you cretin

Mr. Meagles
Apr 30, 2004

Out here, everything hurts


nobody posting here is rich by any measure that actually matters. we're all just different degrees of poor.

unless you have enough dough to crush the dreams of future generations of entire cultures/races there's no reason to listen, and people should be free to wrap their hotdogs in tortillas, or whatever

a hole-y ghost
May 10, 2010

Crash_N_Burn posted:

here's a physics problem for you: if you put a hotdog in between two slices of bread does it then become a sandwich?
It becomes a hotdog sandwich. It's a sandwich but I don't think you can give it to someone as just a "sandwich" without mentioning the hotdog without being misleading.

SLICK GOKU BABY
Jun 12, 2001

Hey Hey Let's Go! 喧嘩する
大切な物を protect my balls


Really if you think about it, a hotdog is just a hoagie sandwich.

Gazpacho
Jun 18, 2004

by Fluffdaddy
Slippery Tilde

Drunk Nerds posted:

It's a trick question:

The plane can not remain stationary, no matter how fast nor powerful the treadmill. This is because it is not the wheels that move the plane but the engines.

Consider this: if the wheels weren't there, and the plane was just magically suspended in midaid, it would move forward when the rocket engines started up. So how would having wheels touching the ground possibly affect this? It wouldn't
ok... sounds fake but ok

ballistics statistics
Nov 27, 2003

:shepface:God I fucking love Diablo 3 gold, it even paid for this shitty title:shepface:


The Fattest PI posted:

The conveyor can't move backwards as fast as the plane moves forward. The wheels on the airplane aren't connected to a driveshaft or anything, they're essentially freewheeling. The reason people argue over this dumb this is because people see it two different ways.
The first group see the treadmill as just a fill in device for the thought experiment to keep the plane in place. If the plane is kept in place there is no air moving over the wings and it doesn't take off.
The second group recognize that the speed the wheels are moving have little impact on the air speed of the plane, and the airplane takes off, providing the treadmill is long enough because the plane will move forward due to the engines providing thrust

I hope this helps you in your endeavors mr OP. I'm gay

Thank god someone posted this on the first page this time.

It's not an argument about physics, it's an argument about how people interpret the question.

Justin Tyme
Feb 22, 2011


Supreme Allah posted:

We'll start by making two approximations, and then consider the effects of these effects more closely.

Wheels. First we assume that there is no rolling resistance, that the wheels roll completely freely and that they have negligible mass: therefore it takes no force to turn the wheels, even at high speed. This is obviously approximate: because of their mass and thus their moment of inertia, the wheels do require a force (and thus a torque) to make them rotate; because of friction in bearings they require a force (and thus a torque) to keep them rotating; because of the deformation of the tires there is a rolling resistance and this requires a force as well. In principle, however, we could imagine making this effect very small. We might even use hovercraft or maglev instead of wheels.

Air motion. Second, we assume that the conveyor belt does not create a wind with respect to the ground. In practice, the conveyor has to be very long and probably very wide and so will move air to some extent. In principle, however, we could imagine this to be very small: there could be one very narrow belt for each wheel: these would not affect the air much, though it might require precise steering. (On the other hand, if we make the conveyor belt the size of a planet, we end up with Question 3.)

Let's start the plane's engines and the conveyor belt simultaneously. The moving conveyor belt starts to rotate the wheels but, according to our initial approximation of freely rotating, massless wheels and no rolling resistance, this requires negligible horizontal force from the ground.

So, subject to our approximations, the horizontal forces on the plane are only the thrust of the engines and the drag exerted by the air. Because we've assumed that the conveyor belt doesn't affect the air, these horizontal forces have their normal values. So the plane accelerates at its normal acceleration and, after a normal period, achieves its take-off velocity and then, with the usual values of flaps etc, takes off. In this case, with these approximations, the question and answer are both simple.

sketch of plane on conveyor or runway showing horizontal forcesSo now let's look at the approximations. The sketches at right show the horizontal forces at some point during the acceleration towards take-off: the propellor provides forward thrust to the plane. The drag is the plane passes through the air is in the opposite direction. There is also a small backwards force on the wheels.

Wheels. The runway and the conveyor belt both exert backwards forces on the wheels for three reasons. These effects are all reasonably small compared to the thrust of the engines, but not zero, and not equal for the runway and conveyor cases. What can we say about them?

Bearings. The wheel bearings are not perfect so, just to keep the wheels turning at constant angular velocity on their axles, a torque is required. This torque is small: give a wheel a spin and observe that its angular velocity decreases due to this effect (and due to air resistance on the tires), but does so slowly. Nevertheless, though small, this is one of the key differences between the plane on the conveyor belt and the plane on the runway. This force increases with angular velocity and, at take-off on the conveyor belt, the wheels must turn twice as fast as on the runway. That's why, in the sketch, the force on the wheels is very small in the runway case and not quite so small in the conveyor belt case. So, because of this effect alone, one would expect that, in order to take off, the pilot would require very slightly more throttle on the conveyor belt than on the runway.

Angular accleration. The wheels also have mass, and therefore a non-zero moment of inertia. (See Rotation, torques, moment of inertia.) So a torque is required for the angular acceleration involved in making them rotate. Lift a bicycle and try spinning the wheel: you have to apply a force to get it spinning. For take-off from the conveyor belt, we have to achieve twice the angular velocity that we'd have on the runway. (Plane is going forward at normal take-off speed, conveyor belt is going backwards at the same speed.) Does this mean twice as much accelerating force? In fact no, because the conveyor belt is like an infinitely long runway: we can take as long as we like (fuel permitting) to spin the wheels up to the required angular velocity. So this is not an important effect in our problem.

Rolling resistance. Third, there is rolling resistance. The tire is deformed as it rolls, and the energy required to deform it where it touches the ground is a bit less than the energy you recover when it regains its shape after leaving the ground. This gives rise to rolling resistance. Now the amount of deformation of the tire depends on the downwards force. Initially, with the plane stationary with respect to the air, this downwards force on all wheels equals the plane's weight. As the plane accelerates with respect to the air, however, lift starts to support the weight of the plane, so the downwards force on the wheels becomes successively smaller, and so does the rolling resistance. As we approach take-off speed, rolling resistance becomes very small.

Air motion. We assumed above that the conveyor belt does not move the air. This can never be quite true, of course. The length of our conveyor belt has to be somewhat longer than the normal take-off distance of the plane. It probably has to be tens of metres wide, too. So it will have an effect on the air above it: it will tend to drag air backwards: it will create a headwind. Pilots usually choose to take off into a headwind: it means that, when they have normal take-off speed with respect to the air, they have a lower speed with respect to the ground. So they achieve take-off speed (with respect to the air) more quickly and need less runway. So, if all other effects were zero, this effect would make it easier for the plane to take off from the conveyor belt than from the runway. The effect, however, is small. (but again, see Question 3 in which this effect dominates.)

So, for question 1, the answer is that the plane will take off at approximately the same speed (with respect to the ground) in the two cases. On the conveyor belt, the take-off speed (with respect to the ground) would be slightly higher due to the wheel bearing effect and slightly lower due to the wind created by the conveyor belt. Which is larger is hard to say, because they can be manipulated by say changing the width of the conveyor belt and forgetting to lubricate the wheel bearings.

fake and gay

Orkin Mang
Nov 1, 2007

by FactsAreUseless
i believe it would neither fly nor land, but rather 'hover', as a ufo might, inverting cows and bluring camera lenses due to the socalled antagonism of movements

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ballistics statistics
Nov 27, 2003

:shepface:God I fucking love Diablo 3 gold, it even paid for this shitty title:shepface:


Actually I change my earlier statement, Obama would come out of the white house and say it took off, and all the liberals and democrats would agree with him, but then the majority rebublicans in the House of Reps would come out and say it's the gay agenda and pass a bill saying that it wouldn't take off.

It would fail to pass in the senate because everyone's high as poo poo on that thanksgiving weed (it's super dank man). Obama would be passed a blank piece of paper and he'd veto it because it's white, and black lives matter.

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