Plane on a conveyor belt

CabbageCat

Sporky said:

ICBM said:

If the conveyor was going fast enough the wheels would actually be turning *backwards* and the plane would still fly.

That can't happen given the constraints of the original puzzle. The wheels of the plane turn at the exact same speed as the conveyor is moving.

Following the original constraints:

Plane's engines start to produce thrust. Plane therefore starts to move forwards - on this I think we are agreed?

However, the conveyor belt is currently not turning. As soon as the plane starts to move forwards the plane's wheels turn - again, I think we are agreed on this - it is after all stated in the opening premise?

As soon as the wheels start moving, the conveyor belt starts turning in the opposite direction at the same speed. Thus the plane does not move forwards. The plane's engines continue to produce thrust, which moves the plane forwards, which makes the wheels turn faster. The conveyor accelerates at the same rate, so the plane does not move forwards.

The question frames an impossible situation. If you say that the plane moves forward then you are saying that the conveyor does not turn, but the question says that the conveyor does turn.

It's all very well talking about basic physics and calling people names, but this is not a question about basic physics. The question itself discounts basic physics. Every time you talk about basic physics you are effectively saying that you have not actually understood the question, and are instead answering a different question that you prefer.

I do indeed have a scientific and engineering education, but I also love logic puzzles and hypothetical questions, particularly ones that don't quite make sense. Such as this one.

Yeah. This.

Though I like to think that it would be a weird quantum situation where the wheels would "know" that there is no point in turning so neither the wheels or the conveyor would move, effective making it the same as using a runway but with a full thrust and no movement of any part of the situation.

Sporky

Ravenous said:

Sporky said:

The question frames an impossible situation. If you say that the plane moves forward then you are saying that the conveyor does not turn, but the question says that the conveyor does turn.

Actually - although I'm sure you're right (and I'm one of the "no fly" people) - I think there is real physics in this question. Because I'm including wheel bearings and rolling resistance as drag forces counteracting the engine thrust.

But I really don't want to explain that here, because it's pretty obvious I won't get taken seriously!

I agree with you, but I also regard the real physics approach as a rabbit-hole that's only worth exploring once you've understood the question.

Gassage

Handsome_Chris said:

Gassage said:

Handsome_Chris said:

Gassage said:

Point of order- on the belt, the wheels would be spinning at c. 480mph- twice their design speed. So they'd fall off.

Not true.  In the example the wheels may be spinning but the speed has not been determined yet.

Chris- same speed as plane in opp direction

We know V2 is 185kts...so that's 430mph at take off, 422 at rotation. (VR)

It does not matter, as I posted earlier, if we have a theoretically indestructible conveyor belt, we have a theoretically indestructible plane.

Assume a can opener...

It clearly says a 747

*FIGHT!*

CabbageCat

ICBM said:

The wheels, their speed of rotation or its direction are IRRELEVANT.

You say that because you don't understand the question. They are a massive constraint. Can you have a go at answering the questions I put to you a few posts ago?

dtr

Sambostar said:

And the majority of people on here voted to stay in the EU and said that the people who voted out were thick?  I give up.

Bring back apprenticeships.

You've spent all day philosophising about and mowing the wrong person's lawn.

I wonder if there is a correlation between voting remain/plane taking off, and voting for brexit/plane staying on the ground?

When the question says the conveyor belt is designed to match the speed of the plane's wheels, some folks stick to believing in a magical conveyor belt, however impossible such a thing would be in reality (even to the point of assuming that the force of friction making the plane's wheels stick to the magical conveyor belt must be greater than the forces of multiple jet engines, so determined are they to insist the magical conveyor belt MUST work because the question says it's designed to).

It's a bit like believing that Brexit is designed to give £350 million to the NHS and stop the entire population of Turkey moving here and return parliamentary sovereignty, when in reality that's all obviously insane magical-thinking bollocks and all it actually achieves is to take 16% of the value of everything each of us own and have worked for and pisses it away into oblivion.

Sambostar

In fairness, normally I'd agree with @IBCM, because the question poses a load of bollocks scenario, states the impossible and requires you to assume it too, it's purely theoretical, it's not practical. 

I struggled with this sort of shit in the 11 plus and at GSCE level, because, being purely theoretical, the questions often were framed to make you assume the impossible and as a practical crusader this reallly grated me.

Although I still remember writing in my GSCE Geography 'Scandinavian Shit Slag' as the answer to what they made paper from and I still got an A.  Muppets.  Still gives me a smile in my heart that one.

But iif you don't read and understand the question it doesn't matter how many degrees and PhD's and bells and whistles and tech you have, you've still just wasted a shit load of time.

You can't just look up and ask teach what kind of radials the plane is riding on.  Stuff like that isn't allowed in the 11 plus.

In terms of psychology though, it's interesting to see the way people vehemently believe they are right, even when they are wrong, often because of the way they have gone down a linear route in their career and life path which just serves to compound their linear thinking.  Such as we saw with the pro EU voters in the referendum.

Gassage

We need to define the altitude that this is taking place.

It's the missing link. Overlooked.

Back to the drawing board....

Handsome_Chris

Gassage said:

Handsome_Chris said:

Gassage said:

Handsome_Chris said:

Gassage said:

Point of order- on the belt, the wheels would be spinning at c. 480mph- twice their design speed. So they'd fall off.

Not true.  In the example the wheels may be spinning but the speed has not been determined yet.

Chris- same speed as plane in opp direction

We know V2 is 185kts...so that's 430mph at take off, 422 at rotation. (VR)

It does not matter, as I posted earlier, if we have a theoretically indestructible conveyor belt, we have a theoretically indestructible plane.

Assume a can opener...

It clearly says a 747

*FIGHT!*

But you are you assuming that it is a real Boeing 747?  I am assuming the 747 is a physically correct as the infinately capable conveyor belt.

CabbageCat

dtr said:

When the question says the conveyor belt is designed to match the speed of the plane's wheels, some folks stick to believing in a magical conveyor belt, however impossible such a thing would be in reality (even to the point of assuming that the force of friction making the plane's wheels stick to the magical conveyor belt must be greater than the forces of multiple jet engines, so determined are they to insist the magical conveyor belt MUST work because the question says it's designed to).

So, because the conveyor that exists in the question doesn't exist you decide to answer a different question?

Sambostar

Anyway @Sporky put it to bed.

Who wants an argument about cheese?

holnrew

http://www.airplaneonatreadmill.com/2008/01/airplane-on-treadmill.html

Ravenous

holnrew said:

http://www.airplaneonatreadmill.com/2008/01/airplane-on-treadmill.html

Already read that, and I don't agree with the way he models friction.

monquixote

I can't believe this is not completely bloody obvious to anyone with GCSE physics.

If you are struggling then  imagine pushing it with your arm

If you are still struggling also consider that the tyres don't have infinite traction.

Emp_Fab

You know what..... @ICBM is right....  the plane would take off !!!   The humble pie is ready for us all to eat !!!

The little drawing in the original post is misleading - it implies a little conveyor belt (just bear this in mind for the moment).

The reason we're all saying the plane wouldn't fly is because we're all assuming the plane remains stationary relative to the ground (and the surrounding air).  The plane doesn't remain stationary - it moves along the (very long) conveyor belt and takes off exactly the same as if it were on a tarmac runway.

Why does it move ?  Because the forward thrust from a plane is generated by its engines against the surrounding air.  The surrounding air has no 'knowledge' of the existence of the conveyor belt.  If the forward thrust was applied through the plane's wheels instead, then yes, the plane would remain stationary and wouldn't take off.  However, a plane with powered wheels is useless - it's just a car with wings.  The reason any plane moves is because of the force it applies against the surrounding air.  What is going on beneath the wheels is irrelevant (unless the brakes are applied).  Discounting the minor effect of friction in the wheel bearings, the wheels have no role to play in any of this other than to keep the plane from hitting the ground.  The ground / conveyor belt could be moving sideways for all it matters (presuming the wheels were on castors !).  The only force that has any relevance here is the thrust of the engines against the surrounding air - which is what makes the plane move forward, regardless of the ground beneath it.

Of course, in the OP's post, it's a tiny conveyor belt, in which case, the plane would just fall off the end.

CabbageCat

monquixote said:

I can't believe this is not completely bloody obvious to anyone with GCSE physics.

If you are struggling then  imagine pushing it with your arm

If you are still struggling also consider that the tyres don't have infinite traction.

Tyres do not have infinite traction but then jets do not have infinite thrust.

One way of simplifying it might be to say, could a 747 take off if someone locked the wheels so they couldn't turn? That would create the situation in the question because a runway is just a conveyor belt moving at 0mph.

monquixote

CabbageCat said:

monquixote said:

I can't believe this is not completely bloody obvious to anyone with GCSE physics.

If you are struggling then  imagine pushing it with your arm

If you are still struggling also consider that the tyres don't have infinite traction.

Tyres do not have infinite traction but then jets do not have infinite thrust.

One way of simplifying it might be to say, could a 747 take off if someone locked the wheels so they couldn't turn? That would create the situation in the question because a runway is just a conveyor belt moving at 0mph.

Yes but they have vastly more thrust than wheels have traction.

CabbageCat

monquixote said:

CabbageCat said:

monquixote said:

I can't believe this is not completely bloody obvious to anyone with GCSE physics.

If you are struggling then  imagine pushing it with your arm

If you are still struggling also consider that the tyres don't have infinite traction.

Tyres do not have infinite traction but then jets do not have infinite thrust.

One way of simplifying it might be to say, could a 747 take off if someone locked the wheels so they couldn't turn? That would create the situation in the question because a runway is just a conveyor belt moving at 0mph.

Yes but they have vastly more thrust than wheels have traction.

OK. So do kids in physics GCSE learn how fast a tyre can be dragged at 100mph under several tonnes before it fails?

Edit: And I guess that the wheel's bearings have less traction than the tyre so if the tyre were being dragged the bearings would move before the tyre does. So the wheel would turn faster. So the conveyor would go faster.

Nitefly

Looks like another win for the wisdom terrorist...

Sporky

ICBM said:

You don't understand the basic physics of powered flight.

That's (a) untrue, and (b) awfully patronising. Stop insulting people and start thinking about the question that was actually asked.

ICBM said:

The wheels, their speed of rotation or its direction are IRRELEVANT. The only thing that matters is the thrust and drag, which determines the airspeed. All that happens if the conveyor moves backwards is that the wheels turn faster.

You're not thinking through the consequences of the wheels' rotation (and therefore the conveyor belt's movement). You can shout that they're irrelevant all you want, but they are not. They are critical to the question.

ICBM said:

No it does not. It's a perfectly valid thought experiment.

It's actually a useful one, because the answer you give determines whether you understand the problem or not.

I didn't say it wasn't valid, and I agree that the answer given determines whether you understand the question. Thus far you're relying on discarding information given in the question in order to form your answer.

I vote we follow this thread up with one on the Monty Hall problem.

dtr said:

When the question says the conveyor belt is designed to match the speed of the plane's wheels, some folks stick to believing in a magical conveyor belt, however impossible such a thing would be in reality

The opposing view is to believe that the question is "wrong", and to base your answer on discarding information given in the question. At that point the answer might as well be "my cat's breath smells of catfood" because you're not actually addressing the puzzle that was set.

Maynehead

ICBM said:

Sambostar said:

You still don't understand the question. No ground speed can be achieved nor any acceleration as the forward thrust of the engines and acceleration is counteracted by the increasing backwards speed of the conveyor, effectively meaning that the plane is static so there is no airspeed.

NO IT ISN'T!!

You don't understand the basic physics of powered flight.

The wheels, their speed of rotation or its direction are IRRELEVANT. The only thing that matters is the thrust and drag, which determines the airspeed. All that happens if the conveyor moves backwards is that the wheels turn faster.

Sporky said:

I do indeed have a scientific and engineering education, but I also love logic puzzles and hypothetical questions, particularly ones that don't quite make sense. Such as this one.

No it does not. It's a perfectly valid thought experiment.

It's actually a useful one, because the answer you give determines whether you understand the problem or not.


The answer to the question I asked earlier is Astrophysics and General Physics, by the way...

Then it is even more shocking that you lack the ability to fully grasp the core of the question, which when distilled down is in fact nothing to do with planes or conveyor belts. It is a logic puzzle that has only one rational solution.

Your posts have come across quite condecending, it is clear that you assume that we have a more shallow understanding of the problem than you do, aided by your degree in physics. In fact, it is us who have the deeper, higher level understanding.

You start off by completely misinterpreting the question, then you say that the motion of the wheels and the conveyor are irrelevant to the question, when they are actually the fundamental basis of the question. You then refer us to YouTube videos demonstrating experiments that in no way conforms to the parameters set out by the question, and cannot possibly be used for empirical evidence. If you submitted a white paper with the arguments and evidence you presented in this thread it would have been thrown straight out.

Why don't you start by fully analysing the question, and offer rational, reasoned counter arguments to the solutions presented that you don't agree with? Surely this should be second nature to someone with a scientific degree, or did your final year dissertation simply say "I'm right, you're wrong"?