Skill's weekend teaser

[MrGecko]

Bramble, when you are flying / during take off, do you ever look at a little dial that is telling you how many revolutions per minute the wheels are doing?

 

[Skill Leverage]

mate, you are beyond help. Gecko, Fifty2, new_trader and I have all answered this question, and you are just too plain dumb to see it. Seriously, read over the posts again, and see if the penny can drop. It is literally like talking to a brick wall, you cannot see the wood for the trees at the moment.

 

[Skill Leverage]

You just cannot understand that it doesn't matter if the conveyor is matched, unmatched, still or spinning at a million miles an hour. Maybe this will do it:

the groundspeed of the plane is not affected in any way by the speed of the conveyor belt. It is only affected by the thrust of the engines.

 

[TheBramble]

While the engines are off, the plane has a relative groundspeed of 0. When the engines are turned on, they pull the plane through the air, giving it a +ve velocity relative to both the air and the ground.

So for sake of example, if it were possible to have a 747 with engines off at 30,000 feet at 0 mph and then switch the engines on, you reckon it would simply fly off under it's own steam do you?

My best guess is that it, and everyone on board, would feel a bit of a dropping sensation as is plummeted earthward until it had genetaed the minimum airflow to generate sufficent lift to fly.

 

[Skill Leverage]

the only thing the conveyor belt affects is the rotational speed of the plane's wheels. The wheels are not in any way responsible for forward motion of the plane.

 

[searchlight]

(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 the opposite direction). Can the plane take off?" (The Straight Dope: 060203.)

The implicit assumption is that if the conveyor belt's speed backward exactly counteracts the airplane's "speed" (whatever that means) forward, the plane remains stationary relative to the earth and, more importantly, to the air. (We assume the winds are calm.) With no wind moving past its wings, the plane generates no lift and can't take off.

But the assumption is false. While the conveyor does exert some modest backward force on the plane, that force is easily overcome by the thrust of the engines pulling the plane ahead. The plane moves forward at roughly its usual speed relative to the ground and air, generates lift, and takes off. Many people have a hard time grasping this (although it can be easily demonstrated in the lab), but eventually they do, smack their foreheads, and move on. We'll call this Basic Realization #1.

Message-board discussions of this question tend to feature a lot of posters who haven't yet arrived at BR #1 talking right past those who have, insisting more and more loudly that the plane won't take off. Then there's a whole other breed of disputants who, whether or not they've cracked the riddle as originally posed, prefer to reframe it by proposing progressively more esoteric assumptions, refinements, analogies, etc. Often they arrive at a separate question entirely: Is there a way to set up the conveyor so that it overcomes the thrust of the engines and the plane remains stationary and doesn't take off?

The answer is yes. Understanding why is Basic Realization #2.

The conveyor doesn't exert much backward force on the plane, but it does exert some. Everyone intuitively understands this. To return to the analogy in my original column, if you're standing on a treadmill wearing rollerblades while holding a rope attached to the wall in front of you, and the treadmill is switched on, your feet will initially be tugged backwards. Partly this is due to friction in the rollerblade wheel bearings, but partly--this is key--it's because the treadmill is accelerating the rollerblade wheels and in the process imparting some angular (rotary) but some linear (backward) momentum to them. You experience the latter as backward force. Eventually the treadmill reaches a constant speed and the rollerblade wheels cease to accelerate. At this point you can easily haul in the rope and pull yourself forward.

But what if the treadmill continues to accelerate? Different story. In principle it's possible to accelerate the treadmill at a rate that will exactly counteract any forward force you care to apply. (This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force.) The only mathematics needed to demonstrate this is the well-known physics axiom F = ma--that is, force equals mass times acceleration. Given that the conveyor exerts some backward force F on the plane, we simply crank up the acceleration as much as necessary to equal any forward force F generated by its engines. Result: The plane stands still and doesn't take off. Welcome to BR #2.

You may say it's impossible to build a constantly accelerating treadmill, that eventually we run into the limitation imposed by the speed of light, etc. True but irrelevant--BR #2 has an intrinsic elegance that transcends such practical concerns. Why didn't I bring it up in the first place then? You've got to be kidding. It took an entire column to get BR #1 across, and a second one to convey (I hope) BR #2. One fricking thing at a time.

— Cecil Adams

I found above explanation is bit better. May be this will help.(google searched it.)

 

[Skill Leverage]

So for sake of example, if it were possible to have a 747 with engines off at 30,000 feet at 0 mph and then switch the engines on, you reckon it would simply fly off under it's own steam do you?

My best guess is that it, and everyone on board, would feel a bit of a dropping sensation as is plummeted earthward until it had genetaed the minimum airflow to generate sufficent lift to fly.

correct. This is, again, completely and utterly irrelevant to the problem at hand. You have introduced yet another force on the plane at standstill, that of gravity. Stop changing the parameters and listen to what is being said to you man, for God's sake it's pathetic.

 

[Skill Leverage]

(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 the opposite direction). Can the plane take off?" (The Straight Dope: 060203.)

The implicit assumption is that if the conveyor belt's speed backward exactly counteracts the airplane's "speed" (whatever that means) forward, the plane remains stationary relative to the earth and, more importantly, to the air. (We assume the winds are calm.) With no wind moving past its wings, the plane generates no lift and can't take off.

But the assumption is false. While the conveyor does exert some modest backward force on the plane, that force is easily overcome by the thrust of the engines pulling the plane ahead. The plane moves forward at roughly its usual speed relative to the ground and air, generates lift, and takes off. Many people have a hard time grasping this (although it can be easily demonstrated in the lab), but eventually they do, smack their foreheads, and move on. We'll call this Basic Realization #1.

Message-board discussions of this question tend to feature a lot of posters who haven't yet arrived at BR #1 talking right past those who have, insisting more and more loudly that the plane won't take off. Then there's a whole other breed of disputants who, whether or not they've cracked the riddle as originally posed, prefer to reframe it by proposing progressively more esoteric assumptions, refinements, analogies, etc. Often they arrive at a separate question entirely: Is there a way to set up the conveyor so that it overcomes the thrust of the engines and the plane remains stationary and doesn't take off?

The answer is yes. Understanding why is Basic Realization #2.

The conveyor doesn't exert much backward force on the plane, but it does exert some. Everyone intuitively understands this. To return to the analogy in my original column, if you're standing on a treadmill wearing rollerblades while holding a rope attached to the wall in front of you, and the treadmill is switched on, your feet will initially be tugged backwards. Partly this is due to friction in the rollerblade wheel bearings, but partly--this is key--it's because the treadmill is accelerating the rollerblade wheels and in the process imparting some angular (rotary) but some linear (backward) momentum to them. You experience the latter as backward force. Eventually the treadmill reaches a constant speed and the rollerblade wheels cease to accelerate. At this point you can easily haul in the rope and pull yourself forward.

But what if the treadmill continues to accelerate? Different story. In principle it's possible to accelerate the treadmill at a rate that will exactly counteract any forward force you care to apply. (This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force.) The only mathematics needed to demonstrate this is the well-known physics axiom F = ma--that is, force equals mass times acceleration. Given that the conveyor exerts some backward force F on the plane, we simply crank up the acceleration as much as necessary to equal any forward force F generated by its engines. Result: The plane stands still and doesn't take off. Welcome to BR #2.

You may say it's impossible to build a constantly accelerating treadmill, that eventually we run into the limitation imposed by the speed of light, etc. True but irrelevant--BR #2 has an intrinsic elegance that transcends such practical concerns. Why didn't I bring it up in the first place then? You've got to be kidding. It took an entire column to get BR #1 across, and a second one to convey (I hope) BR #2. One fricking thing at a time.

— Cecil Adams

I found above explanation a bit better. May be this will help.(google searched it.)

Good post, but the second scenario in this is different to our one, in that they are relating the speed of the belt to the thrust of the engines, something which we are not doing.

 

[TheBramble]

Bramble, when you are flying / during take off, do you ever look at a little dial that is telling you how many revolutions per minute the wheels are doing?

There have never been any such dial in any plane I have flown. Largely I imagine because it would be completely irrelevant.

One of the dials I do keep an eye on though indicates airspeed which, believe it or not, correlates quite closely with groundspeed (plus or minus wind vector).

 

[Skill Leverage]

Bramble, read the above post from searchlight, SMACK YOUR FOREHEAD, and move on.

 

[Skill Leverage]

There have never been any such dial in any plane I have flown. Largely I imagine because it would be completely irrelevant.

One of the dials I do keep an eye on though indicates airspeed which, believe it or not, correlates quite closely with groundspeed (plus or minus wind vector).

Groundspeed in that sense is completely different to the speed of a moving belt; that is not the plane's groundspeed, because the belt DOES NOT MOVE THE PLANE.

For pity's sake read searchlight's post, and then tell me how much of an idiot you are. In a woman's voice.

 

[TheBramble]

But the assumption is false. While the conveyor does exert some modest backward force on the plane, that force is easily overcome by the thrust of the engines pulling the plane ahead...

Ahead of what? The conveyor on which it is situated and which we are told will always match the speed of the plane? It can never move ahead or behind it's initial starting point on the conveyor relative to any reference point in-line but external to the moving conveyor.

 

[MrGecko]

There have never been any such dial in any plane I have flown. Largely I imagine because it would be completely irrelevant.

One of the dials I do keep an eye on though indicates airspeed which, believe it or not, correlates quite closely with groundspeed (plus or minus wind vector).

Precisely.

In this case, the speed at which the wheels are revolving is completely irrelevant to the airspeed or groundspeed.

 

[Skill Leverage]

It Will Match The Speed Of The Wheels!!!!! Oh My God!!!!!!!

 

[Skill Leverage]

Ahead of what? The conveyor on which it is situated and which we are told will always match the speed of the plane? It can never move ahead or behind it's initial starting point on the conveyor relative to any reference point in-line but external to the moving conveyor.

READ WHAT IS IN FRONT OF YOU FOR GOD'S SAKE. The conveyor belt's speed is irrelevant. It is this that you cannot get into your pea head.

You have lost. End of story. You are the weakest link, etc.

 

[MrGecko]

Ahead of what?

Ahead of the air.

It can never move ahead or behind it's initial starting point on the conveyor relative to any reference point in-line but external to the moving conveyor.

No. It can move relative to the air (and to the ground).

 

[Skill Leverage]

Please God, let him smack his forehead the gym shuts in a few hours...

 

[Aspire]

You need to understand that it doesn't matter how fast the conveyor belt is going it does not have any effect on the speed or position of the plane.

Imagine the conveyor is made from something really really slippery. A hundred times more slippery than ice. Then when you turn on the conveyor it just slips under the plane and the plane stays exactly where it is. It doesn't matter how fast the conveyor goes or in what direction it is just so slippy that the plane stays still.

This is the situation we have here except the conveyor doesn't slip against the wheels it is the wheels slipping against the plane because they are not attatched to anything that drives the plane, just a slippery axle that the wheels spin around.

So now, when the pilot turns on teh engines the plane moves forward with the same groundspeed (compared to the ground under the conveyor belt) that it would even if there was no conveyor belt there because the conveyor makes no difference.

 

[Skill Leverage]

See, Aspire got it wrong at first, and after ONE POST of explanation, he got it, and now he has given a really great analogy of the situation.

Therefore, it's not our logic, or our explanation of it that's at fault; it's you Bramble.

 

[barjon]

Ah!! Aren't Sunday's fun again with dear old Tony (Bramble) back stirring the porridge

Thank you all for keeping it civilised :clap:

cheers

jon