Plane

[Whereditgo]

If you walk on an escalator in the opposite direction to it's travel and at the same speed you will remain stationary with respect to the escalator. The thrust is made by your muscles and legs. As long as the speed of the escalator is balanced to the speed of you walking everything is in balance. Things only react differently if that balance is disturbed by your pace or the speed of the escalator.

What simple law of motion is this breaking.

But when you walk you exert a force against the ground with your legs/feet as you say, an aircraft gets it's thrust from a different force that has nothing to do with contact with the ground (other than the rolling resistance of the tyres).

 

[RichA]

For those who live in the real world:

The "conveyor belt" was set to be a constant 25mph to match the plane's takeoff speed.
It's speed did not increase as the plane's wheels sped up.
As the plane took off its wheels were rotating at 50mph and the conveyor belt was still 25mph.
It answered a different question to the one posed in this thread.

 

[RichA]

But we know this is nonsense in terms of a plane moving along the belt. The wheels will always rotate faster and faster.
As long as you're ok with your outcome not existing in the real world then that's fine.

And in the puzzle you posted on page 1, the belt will speed up to match them perfectly.
If the plane moves forward it's wheels are either rotating faster than the belt or they have lost traction.

 

[Golf is fun]

The problem with that demonstration is that the wheel speed was not measured. The wheels had to have been rotating faster than the sheet was being pulled in the opposite direction.

See my previous post, in particualr this bit:

 

[RichA]

This overlooks one key problem - in the real world for the conveyor to counteract the wheels turning it has to move in the direction of the planes travel, for the conveyor to match the speed of the wheels in the opposite direction it has to move in the opposite direction of the planes travel. Therefore to read the question literally the conveyor has to be moving both forwards and backwards at the same time - Schrodinger's conveyor!

Going back to your trundle wheel, stand along side a moving conveyor with a trundle wheel on the conveyor, walk at the exact same speed as the conveyor observe that the wheel is not turning, nor skidding, but you are moving, and the wheel is moving relative to everything in the world except the conveyor and you. This is the only way a conveyor can stop a free wheel turning.

None of the puzzle works in the real world. It's a thought experiment rather than one of mechanics and mathematics.

If the conveyor belt moves in the same direction as the plane is pointing then the wheel will never rotate and the belt will never move. The thrust will eventually force the plane to slide forward on locked wheels until the tyres wear out.
If it's argued that the conveyor belt is reactive and the wheels are allowed to turn a microsecond before the conveyor starts moving the same direction then the plane reaches takeoff speed in half its normal time and distance. But then it isn't counteracting the rotation of the wheels - it's amplifying the effect.

 

[RichA]

Going back to your trundle wheel, stand along side a moving conveyor with a trundle wheel on the conveyor, walk at the exact same speed as the conveyor observe that the wheel is not turning, nor skidding, but you are moving, and the wheel is moving relative to everything in the world except the conveyor and you. This is the only way a conveyor can stop a free wheel turning.

But it's not the only way that it can counteract its rotation.
If the belt is moving and the wheel is not rotating then the belt is not matching the speed of the plane's wheels and this doesn't fit with the puzzle's criterion.

 

[Deleted member 35927]

If you walk on an escalator in the opposite direction to it's travel and at the same speed you will remain stationary with respect to the escalator. The thrust is made by your muscles and legs. As long as the speed of the escalator is balanced to the speed of you walking everything is in balance. Things only react differently if that balance is disturbed by your pace or the speed of the escalator.

What simple law of motion is this breaking.

You're getting in a muddle, there is no correlation between walking on an escalator and a jet plane pushing through the air.

 

[Golf is fun]

The problem with that demonstration is that the wheel speed was not measured. The wheels had to have been rotating faster than the sheet was being pulled in the opposite direction.

See my previous post, in particular this bit:

This overlooks one key problem - in the real world for the conveyor to counteract the wheels turning it has to move in the direction of the planes travel, for the conveyor to match the speed of the wheels in the opposite direction it has to move in the opposite direction of the planes travel. Therefore to read the question literally the conveyor has to be moving both forwards and backwards at the same time - Schrodinger's conveyor!

If a conveyor moves in the opposite direction to a freewheel it will always make that wheel spin faster than it normally would. There is no way for a conveyor moving in the opposite direction to make the wheel not rotate, it can only make it rotate faster.
If the conveyor moves in the same direction as the wheel it can stop it rotating, but it will be moving the wheel and whatever is attached to it forward.

Ultimately there are 3 possibilities here which hopefully at least everyone agrees on:
1) The conveyor moves in the opposite direction to plane, in which case ignoring wheel bearing and tyre failure for simplicities sake the plane takes off. Obviously in the real world there is a point where if you make the wheel spin fast enough the resulting heat will cause the tyres or bearings melt or fail in some way.
2) The conveyor moves in the same direction of the plane - hopefully it's obvious to everyone that this moves the plane forward and it will take off.
3) The conveyor moves in the opposite direction to the plane and in the same direction as the plane - this satisfies the question, but hopefully it is also obvious to everyone a conveyor can't move in 2 directions at the same time. No one can answer what happens in this scenario, because it's just a work of fiction.

 

[Voyager EMH]

Deep breath.
You're on the belt on your roller skates. Your mate is holding you stationary - your wheels are travelling at the same speed as the belt.
Your mate pushes you forward at 3mph. This can only be achieved if your wheels are travelling 3mph faster than the belt.
If the wheels and belt's speeds are identical then your wheels are slipping on the belt. There cannot be perfect traction.

You're on the belt on your roller skates. Your mate is holding you stationary - your wheels are travelling at the same speed as the belt.
The wheels are not travelling they are rotating.

Your mate pushes you forward at 3mph. This can only be achieved if your wheels are travelling 3mph faster than the belt.
First sentence the wheels are not moving/travelling - they have 0mph - but they rotating at the same speed as the belt.
A push is applied - the wheels are moving/travelling at 3mph - they are still rotating at the same speed as the belt, because there is no slipping.
If they were not rotating at the same speed as the belt there would have to be some slipping going on and then there would not be 3mph movement/travelling.

If the wheels and belt's speeds are identical then your wheels are slipping on the belt. There cannot be perfect traction.
If the wheels' rotation speed and the belt's speeds are identical there is no slipping. It will always be like this, because the wheels passively react and match the belt's speed.

From mate-holding-position to mate-pushing-position no slipping occurs and that is why wheels go from 0mph to 3mph - still no slipping - belt speed and wheels rotation matched.

That is why plane takes off as normal with no "extra" forces to be overcome than it takes to get the wheels of a plane moving on a normal runway.

 

[Golf is fun]

But it's not the only way that it can counteract its rotation.
If the belt is moving and the wheel is not rotating then the belt is not matching the speed of the plane's wheels and this doesn't fit with the puzzle's criterion.

Please explain how a conveyor belt can counteract a wheels rotation whilst moving in the opposite direction to a wheel?

If you can do this you're on the way to inventing perpetual motion, because you are getting 2 opposite forces of equal magnitude out of a conveyor when only putting in enough energy for 1 of those forces. You've broken the first law of thermodynamics - energy can only be transferred, not created or destroyed.

 

[RichA]

Please explain how a conveyor belt can counteract a wheels rotation whilst moving in the opposite direction to a wheel?

If you can do this you're on the way to inventing perpetual motion, because you are getting 2 opposite forces of equal magnitude out of a conveyor when only putting in enough energy for 1 of those forces. You've broken the first law of thermodynamics - energy can only be transferred, not created or destroyed.

Language innit.
An antidote counteracts a venom by neutralising its effects.
Wearing Kevlar trousers counteracts a venom by preventing the snakebite in the first place.
You can counteract the cause or the effect of something.
The proposed solutions depend on whether you think the conveyor belt is reactive or proactive.

 

[Swango1980]

I think we've made good progress on this thread to be fair.

In the early stages, many who said the plane would not take off seemed to basically suggest that the belt would counter the force of the engines, and it would basically act like a treadmill. At least we seem to have crossed that hurdle for the majority of people, to understand that can't happen

 

[Voyager EMH]

And in the puzzle you posted on page 1, the belt will speed up to match them perfectly.
If the plane moves forward it's wheels are either rotating faster than the belt or they have lost traction.

If the plane's wheels are rotating faster than the belt, they would have to have some power causing them to do this. The wheels have no power to do this so this does not happen.
If the plane's wheels are rotating faster than the belt, they would be some slipping going on, for this to be occurring.

They wheels can do no more than react passively to the belt passing beneath them. At all times. And the belt and the wheels will be matched at all times with no slipping.

It does not make any difference whether the belt speeds up or not, the wheels rotation will match the belt's speed. The wheels can not do otherwise, because they have no power to do otherwise.

On a normal runway the only thing to be overcome is friction at the start from a stationary position.
Exactly the same on a conveyor.
The post#1 scenario does not include a conveyor that can "think ahead" and "predict" in order to keep the plane stationary.
Once the plane's wheels start to turn the plane will take off as normal.

 

[Golf is fun]

Language innit.
An antidote counteracts a venom by neutralising its effects.
Wearing Kevlar trousers counteracts a venom by preventing the snakebite in the first place.
You can counteract the cause or the effect of something.
The proposed solutions depend on whether you think the conveyor belt is reactive or proactive.

Not really, it's science. A chemist could explain the mechanism through which the antidote works, most people can explain how the kevlar requires more pressure to be punctured than the snakes fang can exert.
Please explain how a conveyor moving in the opposite direction to a wheel can counteract that wheels rotation?

Through what mechanism or scientific principles does the conveyor do this?
Presumably you understand and can explain given the conviction you've argued this.

 

[RichA]

You're on the belt on your roller skates. Your mate is holding you stationary - your wheels are travelling at the same speed as the belt.
The wheels are not travelling they are rotating.

Your mate pushes you forward at 3mph. This can only be achieved if your wheels are travelling 3mph faster than the belt.
First sentence the wheels are not moving/travelling - they have 0mph - but they rotating at the same speed as the belt.
A push is applied - the wheels are moving/travelling at 3mph - they are still rotating at the same speed as the belt, because there is no slipping.
If they were not rotating at the same speed as the belt there would have to be some slipping going on and then there would not be 3mph movement/travelling.

If the wheels and belt's speeds are identical then your wheels are slipping on the belt. There cannot be perfect traction.
If the wheels' rotation speed and the belt's speeds are identical there is no slipping. It will always be like this, because the wheels passively react and match the belt's speed.

From mate-holding-position to mate-pushing-position no slipping occurs and that is why wheels go from 0mph to 3mph - still no slipping - belt speed and wheels rotation matched.

That is why plane takes off as normal with no "extra" forces to be overcome than it takes to get the wheels of a plane moving on a normal runway.

The outside surface of the wheel is travelling in a circle at the same speed as the belt.

During the push the wheels must be rotating and travelling 3mph faster than the belt, otherwise there would be no movement.

Imagine the belt is moving at a constant snails pace while the rider is held in place. The pusher runs at 10mph and pushes the rider forward. The wheels absolutely have to be rotating faster than the belt.
The wheels have to be faster than the belt for forward motion with perfect traction.

 

[Voyager EMH]

I think we've made good progress on this thread to be fair.

In the early stages, many who said the plane would not take off seemed to basically suggest that the belt would counter the force of the engines, and it would basically act like a treadmill. At least we seem to have crossed that hurdle for the majority of people, to understand that can't happen

Plane going at 150mph in one direction and belt going at 150mph in other direction causing a stationary plane - this is very easy to nod your head at for the majority of people, I believe.

The "plane going at" meaning "power of engine at" is what is assumed by the head nodders. The two "cancelling each other out" conclusion is an easy one to jump to.

And a plane "going at" 185mph "beating" a belt going at 200mph in the opposite direction - CAN'T BE DONE - very very easy indeed to nod your head at that.

Perception, perception, perception.

This is why I find the thread - and all the comments made - very interesting.

 

[RichA]

Plane going at 150mph in one direction and belt going at 150mph in other direction causing a stationary plane - this is very easy to nod your head at for the majority of people, I believe.

The "plane going at" meaning "power of engine at" is what is assumed by the head nodders. The two "cancelling each other out" conclusion is an easy one to jump to.

And a plane "going at" 185mph "beating" a belt going at 200mph in the opposite direction - CAN'T BE DONE - very very easy indeed to nod your head at that.

Perception, perception, perception.

This is why I find the thread - and all the comments made - very interesting.

If the belt is travelling backwards at 5mph and the plane is travelling forwards at 185mph, do you perceive that it's wheels are rotating at 5mph to match the belt?

 

[BobbyA]

You obviously don't understand how the plane normally moves forward on a runway from a standing start, this is part of the problem with your comprehension.

The plane doesn't fly through the air from a standstill, the engines push it forward and the tyres react with the opposite force of the solid tarmac to allow the plane to roll.

If there is no solid resistance, the plane can't roll. And guess what, the conveyor moving in the opposite direction as per one interpretation of the scenario is the lack of solid resistance.

You really are not understanding the forces working here.

The plane doesn't fly through the air from a standstill, the engines push it forward and the tyres react with the opposite force of the solid tarmac to allow the plane to roll.
This is incorrect, the only force on the wheels is the downward force of the plane, the forward force moving the plane forward is from the thrust of the engine nothing to do with the wheels, all the wheels are doing are moving in the opposite direction to the ground and hold the plane up. Imagine the plane was held in the air by some sort of massive crane and rope, as soon as the thrust is applied the plane will move forward. Therefore proving the plane moves forward because of thrust from the engine regardless of the wheels / ground. Likewise this thrust is what keeps the plane moving forward in the air with no ground forces. If you took the wheels off and just had the uprights, the plane would still move forward but would require more forward thrust as there is more friction.

If there is no solid resistance, the plane can't roll. And guess what, the conveyor moving in the opposite direction as per one interpretation of the scenario is the lack of solid resistance.
Again incorrect, what resistance does the plane experience in the air, get yourself a piece of paper and string, roll the paper up and tie it the string around it and tie it up somewhere, blow on the paper, does it move forward, if it does this replicates the thrust of the jet engine. there is no need for resistance. Or even easier, put a straw on a table and blow it, it will move forward, no need for wheels or ground forces to provide forward movement.

Actually an even easier example, imagine a heavy box, to move it forward you have to give it a big push (thrust) no wheels, nothing travelling in the opposite direction, it still moves forward.

 

[Swango1980]

Just had the funniest moment in work this week (been a slow week).

I mentioned this thread to a colleague on Monday, saying it was all kicking off. So, he read some of the posts and asked a few questions this afternoon. One of the directors overheard, so we ran the OP by him. He instantly said the plane wouldn't take off. I said it would. He then tried to explain how wings worked. I kept trying to explain how engines work, but he kept interrupting thinking I didn't understand how wings work. He even shouted at one point "I'm a physicist". Telling me about air flow, etc. Five minutes later I got a word in, and reminded him the plane still moved forward, and asked him how the jet engine applies thrust. At that point he went quiet for about 5 seconds, and then went "ahhh, you're right".

My mate was sitting in between us just laughing his head off, as it was like the forum playing out in real life

 

[RichA]

Not really, it's science. A chemist could explain the mechanism through which the antidote works, most people can explain how the kevlar requires more pressure to be punctured than the snakes fang can exert.
Please explain how a conveyor moving in the opposite direction to a wheel can counteract that wheels rotation?

Through what mechanism or scientific principles does the conveyor do this?
Presumably you understand and can explain given the conviction you've argued this.

I'm mostly confused that anybody can think that "opposite direction" as used in the wording of the puzzle can be interpreted to mean "same direction".