Stupid physics question

[Scouse]

think of the wheels purley as propping the plane up.
then imagine the jet engines as having rope wound around their core. then some rope is tied to a pole at the end of the conveyor.

Falls down again. Just like the helicopter argument.

There is no magical force stopping the plane from going backwards with the conveyor belt.

Look at the question this way instead:

A plane is standing on a runway that can move (some sort of band conveyer). 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 opposite direction).

The conveyor belt starts first and the plane starts moving backwards. After ten seconds the plane applys exactly the right amount of thrust to stop it moving backwards on the conveyor belt - but no more than that.

The question is:

Will the plane take off or not?

 

[Wij]

Right, that's G and Jup brought to the light side. Anyone else ?

 

[Tom]

There is no magical force stopping the plane from going backwards with the conveyor belt.

*bangs head against wall*

Inertia.

 

[tris-]

Falls down again. Just like the helicopter argument.

There is no magical force stopping the plane from going backwards with the conveyor belt.

but the wheels are also not magical. the 'drive' doesnt go through the wheel like on a car. their function is to cause the plane to be able to move over the surface.

the wheels are not needed for takeoff in the way that its the engines that thrust teh plane forward.

and

the wheels are needed so the plane can actually move across the ground.

so i hope that highlights the function of the wheels, and shows they have nothing to do with it except cause the plane to be able to move along the ground

 

[Xgkkp]

Okay I'm really starting to get pissed off now.

This doesn't work unless the pulley is also on the conveyor belt.

The pulley would be going backwards at the same speed as the conveyor belt whilst pulling the plane to it.

Unfortunately the pully would pull the plane into itself (whilst the plane was stuck in exactly the same position it started).

You can't use examples of external forces acting on the plane.

What. The. fuck. OKAY THEN THE HELICOPTER IS FLYING LEVEL TO THE CONVEYOR BELT. Jesus the pulley is just a handy way (albeit obviously too complicated for some people) to imagine an entirely vertical force being turned into a horizontal one.

Example I made with a hot air balloon, but scrapped because I thought people wouldn't be able to comprehend the possibility that a balloon would do the trick:

You needn't even think of the problem in 3d (or even 2d, but that's probably way above the heads of people here) - and dismissing stuff like an example of a helicopter because '

I thought it was just misunderstanding but now I'm starting to be convinced that it's just true stupidity stopping people from understanding it.

Please explain to me why I can't use examples of 'external forces being applied to the plane', and

explain how the force from a planes engine is different from it being pulled (or pushed) along. (seriously, I really would like to hear your explanation on this one, so much so that I am going to repeat it in case you skim over this post)

Please explain how the force from a planes engine is different from the force from it being pulled (or pushed) along.

 

[tris-]

it really isnt different. but i think people have mixed ideas of what a jet engine on a plane does.

those who were educated
those who were educated in 1850

 

[Xgkkp]

There is no magical force stopping the plane from going backwards with the conveyor belt.

Here you are correct - there is no magical force, there is a very real force CALLED THE THRUST FROM THE PLANES ENGINES.

also HAHAHAHAHAHAHA your version of the question states 'After ten seconds the plane applys exactly the right amount of thrust to stop it moving backwards on the conveyor belt' - handily, the plane only gives out enough thrust to balance it's position - because if it gave out more it would move forwards and take off!

You can't even fabricate a reworded question that supports your bullshit theories.

it really isnt different. but i think people have mixed ideas of what a jet engine on a plane does.

No it isn't, as most people seem to realize. I'm specifically interested in why the people who say that they are different, think that they are. I think most people seem to be under the illusion that a airplane engine moves the planes wheels.

Also:
Those who were educated,
Those who people attempted to educate.

 

[Xgkkp]

You can't use examples of external forces acting on the plane.

Sorry for another post but this also needs to be ridiculed directly:
So you suggest that we'd sit in the seats in the airplane and push the seats in front of us to move the airplane forwards? Even the flintstones knew better than that.

 

[tris-]

I think most people seem to be under the illusion that a airplane engine moves the planes wheels.

tbh, this is what ive been trying to say in the simplest terms. the wheels are simply their to keep the plane in its correct positioning and make it capable of moving along a surface.

there is no drive shaft going to the wheels, or belts that make them spin. the wheels dont move to make the plane go forward. the wheels move because the plan is going forward.

 

[Earl]

So does it take off or not? Or has nobody decided yet?

 

[tris-]

well earl, its not agreed across the board.

there is people who understand the principle of how a plane works and say it will

people who understand and say it wont

people who dont understand and say it wont

 

[Calaen]

dont know mate, but its worth watching cause I think Xgkkp is getting so mad he might just shit himself

 

[Louster]

So does it take off or not? Or has nobody decided yet?

After all this, I don't think the plane feels like bothering to take off anymore, and will probably go live in a cave for a while on its own.

 

[Tilda]

To the no camp.

Can a water plane (ie one with floating ski's) take off in a river upstream?

Also, jup, edit my result to the yes camp please. My no was a result of misinterpreting the question

 

[tris-]

think tilda should of converted them now.

if we assume the water flows back at the same power that the plane thrusts forward, it will take off.

 

[Xgkkp]

dont know mate, but its worth watching cause I think Xgkkp is getting so mad he might just shit himself

Yes I think I may explode soon

 

[DeShark]

Urgh. So many people are cursing so many people for being wrong. Yet the way I see it, few people have been right (or able to justify themselves)...

The way I see it is...

There are three forces responsible for the plane moving forwards and backwards. These are

a) The propulsion from the jet engines as they shift tonnes of air particles backwards, making full use of Newton's 3rd law (every action has equal and opposite reaction).

b) The friction as a result of the air (air resistance). Basically, unless the plane moves forwards (relative to the air, which is not moving relatively to the ground) this remains nill.

c) The friction which results from the runway. This friction with the surface of the plane's wheels causes the wheel to spin. The friction against the axle restricts the spin of the wheel. This causes them to slow down. We all know that if you slow down the spin of the wheels in a car, then the car slows down too (if you disagree with this, try putting the brakes on in a car). The faster the wheels move against the track (or the track against the wheels), the more the friction will act to slow down the plane (Imagine being pulled along by a rope. The faster someone pulls, the more your arse hurts. Imagine being on a your arse on a treadmill with a rope tied to the wall so that you can't move with the treadmill. The faster the treadmill's going, the more your arse hurts).

Now for the first situation, the plane's wheels cannot spin. As you can see, the conveyor belt moves backwards with the same speed as the plane moves forwards. The plane pushes forwards and gains a speed of 1mph forwards. The conveyor belt has other ideas and rolls backwards at 1mph. The wheels do not spin, there is a huge amount of friction at the bottom of the plane. The top of the plane goes forwards, its wheels go back. Result: The plane topples forwards crashing onto its nose.

The Second situation states that the plane is now on ice skates and the track is made of ice (we'll assume there is VERY little friction). The plane pushes forwards at 1mph, the track moves backwards at 1mph. There is no force against the wheels. The top of the plane moves forwards at 1mph. The bottom of the plane receives no resistance and is dragged by the top of the plane (if the jet engines are right at the top of the plane then it would start to turn onto its nose, but the front wheel is there to stop this at the moment). So the plane is moving at 1mph forwards. The plane now speeds up to 10mph. The track speeds up too, in the opposite direction, but causes no (detectable) increase in friction. Now the plane wants to tip even more, but a new force comes into play, that being the air resistance. The wings are shaped nicely so that when the air moves past the wings, there is high pressure below them and low pressure above. This causes the air to lift the plane enough to stop it falling on its nose. To skip to the end, the plane reaches, say, 100mph which is sufficient for the air flow to cause lift off.

So it's now clear that it depends how much friction exists between the wheels and the track on whether the plane moves forwards or not. I hope that clears it up for everyone. It's quite a complicated situation so there's no need to start moaning at people and calling people thick if they don't understand what's going on. Besides which, I've noticed that those who are the least right start arguing most (possibly the reason that they're so retarded is due to the fact that they don't listen to other people, just bitch at them).

 

[Louster]

So it's now clear that it depends how much friction exists between the wheels and the track on whether the plane moves forwards or not.

I can't really identify what you're referring to with "first situation", exactly (and I have this sneaking suspicion that this whole reply was copy-pasted from some other thread in some other context), but the wheels can spin, so it's not the friction between the wheels and the track that matters, it's the friction between the wheels and the plane - which is as you said earlier on in your post but which you apparently ignored later? This force is relatively insignificant. You don't need to invent iceskates - a normal plane with a normal undercarriage still won't generate anywhere near enough friction to stop it from taking off. It is as if you're assuming throughout that the wheels' brakes are on... which would be kind of silly.

 

[DeShark]

I can't really identify what you're referring to with "first situation", exactly (and I have this sneaking suspicion that this whole reply was copy-pasted from some other thread in some other context), but the wheels can spin, so it's not the friction between the wheels and the track that matters, it's the friction between the wheels and the plane - which is as you said earlier on in your post but which you apparently ignored later? This force is relatively insignificant. You don't need to invent iceskates - a normal plane with a normal undercarriage still won't generate anywhere near enough friction to stop it from taking off. It is as if you're assuming throughout that the wheels' brakes are on... which would be kind of silly.

Not quite. I was assuming that the friction between the wheels and the axle is huge. So they don't spin. Assuming there is an enormous amount of friction between the surface of the wheels and the road, the wheels will not move forwards, but will be dragged back with the track. So the plane will tip on its nose. Sorry I wasn't clearer. And yes, I agree, most wheels will not have this much friction, so *most* planes will take off. But the question doesn't specify. So... yeah.

And I wrote it all myself. So neh.

The question's stupid, I agree. If the question had stated that the track moves back in order to oppose the speed of the plane (i.e. provide the same frictional force), then the plane would not move, and would not take off

 

[Xgkkp]

Not quite. I was assuming that the friction between the wheels and the axle is huge. So they don't spin. Assuming there is an enormous amount of friction between the surface of the wheels and the road, the wheels will not move forwards, but will be dragged back with the track. So the plane will tip on its nose. Sorry I wasn't clearer. And yes, I agree, most wheels will not have this much friction, so *most* planes will take off. But the question doesn't specify. So... yeah.

And I wrote it all myself. So neh.

The question's stupid, I agree. If the question had stated that the track moves back in order to oppose the speed of the plane (i.e. provide the same frictional force), then the plane would not move, and would not take off

Except it doesn't. If the question said 'The plane is made of Ice cream and melts' then it wouldn't. It would be a non-question if said in your way.

anyhow, Thanks for originally replying in a completely misleading and confusing way.

Instead of comparing two completely different methods of contact, let us observe the two you should have looked at:
1) The case where the wheels cannot spin
The airplane can (probably) not move forwards - although depending entirely on the contact area the engines may be able to overcome the frictional force in this case, I am prepared to discount that possibility.

2) The wheels can spin as fast as you can spin them
In this case there is no friction between the axle of the wheels and the undercarriage of the aircraft. not being restricted by the wheel speed, and being disconnected from it, the plane can move at whatever speed it likes .

As aircraft wheels are tested to speeds in excess of normal operating conditions, like everything else on a plane, in the context of the problem this case is the dominant one. We shouldn't even be considering the case where it wouldn't be, as it's clearly people just being picky and grasping at straws when their piss-poor arguments are ground into dust.

In essence you have claimed that few have been right, then put forwards a butchered version of what people have been saying.

Screw this for now, I'm going back to writing my dissertation.

 

[DeShark]

Except it doesn't. If the question said 'The plane is made of Ice cream and melts' then it wouldn't. It would be a non-question if said in your way.

I know it doesn't say that. I'm not stupid. I agree with you. As the question stands, it depends on the friction between the wheels and the runway.

Instead of comparing two completely different methods of contact, let us observe the two you should have looked at...

The cases I looked at are virtually equivalent to those you state. Both of our situations argue that the friction is the only thing that matters. Yours is a little more vague in saying it though.

although depending entirely on the contact area...

Contrary to what you might think, the frictional force doesn't depend on the surface area of contact between the two surfaces. It depends on the co-efficient of friction (usually denoted μ) and the size of the resistive force (R). The resistive force in this case is equal to the mass of the plane multiplied by the Earth's gravitational constant (9.81). F (the frictional force) is less than or equal to μR.

 

[Wij]

Contrary to what you might think, the frictional force doesn't depend on the surface area of contact between the two surfaces. It depends on the co-efficient of friction (usually denoted μ) and the size of the resistive force (R). The resistive force in this case is equal to the mass of the plane multiplied by the Earth's gravitational constant (9.81). F (the frictional force) is less than or equal to μR.

Very good but it is still negligible in this case.

 

[DeShark]

Very good but it is still negligible in this case.

The frictional force between the tyres of the plane and the road is negligible?!

Let's assume our plane is about 2 tonnes (2000kg) (A 747-400 includes 66 tonnes of aluminum, never mind its fuel and passengers and whatnot). The frictional co-efficient between rubber and tarmac is about 0.5 to 0.8 (assuming the wheels start to slide as stated). We'll take 0.6 as a rough guide.

F = 0.6 x 2000 = 1200N.

1200N is a hardly negligible, for a plane of this size...

 

[Xgkkp]

The cases I looked at are virtually equivalent to those you state. Both of our situations argue that the friction is the only thing that matters. Yours is a little more vague in saying it though.

No, they are not, yours implies that the plane would be in direct contact with the ground, a misleading implication since it seems to be a misconception amongst most of the 'No' people. Yours is more vague and off-topic because it compares a non-moving wheel to a non-moving piece of metal. Not to mention the fact that several other people had no idea what you were talking about.

Contrary to what you might think, the frictional force doesn't depend on the surface area of contact between the two surfaces. (and a ton of completely irrelevant stuff)

Contrary to what you might think, the static friction (normally denoted by \mu_s) is dependent on surface area. If you look at what I said before your condescending reply,
"The airplane can ... not move forwards ... depending entirely on the contact area the engines may be able to overcome the frictional force"
Hmm, engines getting the plane to move.. might have to overcome.. Static friction!

Also please don't try to confuse the issue further by going into 'friction numbers' and 'equations'. They serve absolutely no helpful purpose besides making you look like a condescending asshole.

The friction on the planes wheels is negligible in the context of the problem. Period.

 

[Xgkkp]

The frictional force between the tyres of the plane and the road is negligible?!

Let's assume our plane is about 2 tonnes (2000kg) (A 747-400 includes 66 tonnes of aluminum, never mind its fuel and passengers and whatnot). The frictional co-efficient between rubber and tarmac is about 0.5 to 0.8 (assuming the wheels start to slide as stated). We'll take 0.6 as a rough guide.

F = 0.6 x 2000 = 1200N.

1200N is a hardly negligible, for a plane of this size...

OH NO MAYBE THAT MEANS THAT THE PLANE WON'T TA...... oh no wait it just means the wheel's won't slip with a high friction coefficient.

Hang on, does this mean you are saying that the plane wheels slipping will mean it will slide along like ice skates to take off?

Guh the wheel-tarmac friction is irrelevant to the problem, it's the axle-undercarriage connection that matters, and that has negligible friction.

 

[DeShark]

Guh the wheel-tarmac friction is irrelevant to the problem, it's the axle-undercarriage connection that matters, and that has negligible friction.

Ok, It's a combination of the two. If there is no friction between the axle and the undercarriage, the plane will take off, regardless of the wheel-tarmac. And if the Wheel-tarmac is zero, the plane will take off regardless of the axle-undercarriage. They would need both have to be high to prevent the plane from taking off. If that's what you've been trying to explain all along, then I'm sorry for not understanding you. But then, I don't think I've been wrong either. I'm willing to accept that we're both right, after understanding you properly. An average plane (with average axle-undercarriage and average wheel-tarmac friction) would take off.

 

[Frizz]

An average plane (with average axle-undercarriage and average wheel-tarmac friction) would take off.

Though none of this is stated in the question. Talk about making a mountain out of a mole-hill.

 

[Xgkkp]

Ok, It's a combination of the two. If there is no friction between the axle and the undercarriage, the plane will take off, regardless of the wheel-tarmac. And if the Wheel-tarmac is zero, the plane will take off regardless of the axle-undercarriage. They would need both have to be high to prevent the plane from taking off. If that's what you've been trying to explain all along, then I'm sorry for not understanding you. But then, I don't think I've been wrong either. I'm willing to accept that we're both right, after understanding you properly. An average plane (with average axle-undercarriage and average wheel-tarmac friction) would take off.

Yes, it is a combination of the two, however your half of the argument, although right, has been irrelevant. The whole point of a wheel is that it has a high friction coefficient, and sticks to the 'road' - otherwise the plane has no realistic way to turn whilst on the taxiway. The friction coefficient between the wheel and tarmac should always be high, and be assumed to be high unless otherwise specified, which the problem doesn't. If[\i] the wheel had low friction, yes it would allow it to take off, but it never would in the context of the question. This isn't a scientific study where very aspect of a system needs to be investigated in depth, it's trying to explain to people who aren't familiar with physics or mechanics (for the most part) why the plane takes off.

I can appreciate your trying to simplify it, but to be honest it just adds fuel to the fire of the 'No' side because they can take the argument and misinterpret it easily.

 

[Tilda]

wtf?

A wheel with a huge amount of friction is no longer a wheel, its a leg with a wierd cylindrical blob at the bottom, which has no relation to a question.
ofc the fucking wheel turns!

 

[Xgkkp]

wtf?

A wheel with a huge amount of friction is no longer a wheel, its a leg with a wierd cylindrical blob at the bottom, which has no relation to a question.
ofc the fucking wheel turns!

Exactly (however it has high friction between the wheel itself and the ground, so that it 'sticks to the road')