Skill's weekend teaser

[MrGecko]

Put it this way. You have a box on wheels on a conveyor belt. You push the box with your finger and at the same time someone winds the belt in the opposite direction. What happens? Ah, you ask where does the thrust from my finger go?
And for the idiot who called me a pleb, we can put some wings on the box if you like! LOL

No, this is wrong for the "its not a car" reason.

 

[TheBramble]

All the time the wheels are on the belt and going at the same speed, is it not irrelevant about where the power is coming from?

It could be a clown of a sodding unicycle...correct.

 

[TheBramble]

err... hang on....

Bramble, the problem as described by Skill is on the Right, and we are all on the left?

Is that your position?

EDIT:

Err... well, yes, I am a gnats c0ck away from changing my mind and saying the plane DOESN'T take off. If you did it in real life, the plane would take off - but Skill has described a situation that could never exist in real life - Using the rollers in the diagram attached, we are (were) all thinking that the conveyor belt works like the one on the left (as it would in real life) - yet, look at the translational displacement of the dot, and it's "mirror" (the square) on the lower roller... THE DISPLACEMENT IS THE SAME

The problem initially stated that the displacement would be OPPOSITE, which is what we have on the right... -2r + 2r = 0.

Bramble, can you confirm?

Yes Gecko, I can confirm. You are a gnat's **** away from changing your mind. -2r + 2r = +x-x = 0

 

[raysor]

MrGecko,
Have a read of Ockam's Razor!

Great fun reading your posts!! Brilliant.

 

[megamuel]

raysor, you are an IDIOT and a PLEB. Go and read the whole thread again you bloody muppet. What are you talking about the breaks for? Of course if the breaks were on the plane wouldn't go anywhere... But THE BRAKES ARE NOT ON, the wheels are free rolling. The jet engines push the plane forward, when the plane is going forward, guess what AIR FLOWS AROUND THE WINGS! Then, the plane takes off. Where is your magical force coming from that stops the plane from flying? Skill, come back, this is boring!

 

[megamuel]

Put it this way. You have a box on wheels on a conveyor belt. You push the box with your finger and at the same time someone winds the belt in the opposite direction. What happens? Ah, you ask where does the thrust from my finger go?
And for the idiot who called me a pleb, we can put some wings on the box if you like! LOL

Thats the most stupid thing anyone has said on this thread...

 

[raysor]

raysor, you are an IDIOT and a PLEB. Go and read the whole thread again you bloody muppet. What are you talking about the breaks for? Of course if the breaks were on the plane wouldn't go anywhere... But THE BRAKES ARE NOT ON, the wheels are free rolling. The jet engines push the plane forward, when the plane is going forward, guess what AIR FLOWS AROUND THE WINGS! Then, the plane takes off. Where is your magical force coming from that stops the plane from flying? Skill, come back, this is boring!

Brilliant, you must be clever being able to describe how an aeroplane takes off!!
Haven't you forgotten something apart from your manners.
By the way a jet engine doesn't push the plane forwards. ******!

 

[Skill Leverage]

Oh dear.

I can't be bothered to do anything more than skim these posts but it would seem Brambles now has us combining the theoretical world with the real world as suits his purpose. I've already acknowledged the fact that, as worded, the problem creates something of a paradox and have provided the correction for this. Stop clutching at straws and for God's sake move on.

 

[Skill Leverage]

If we can all accept that I should have worded the question 'The belt moves at the same speed as the plane in the opposite direction', the plane takes off. Trying to convince Brambles of this however will be harder than getting Patrick Swayze to give up the Marlboro Reds, so I suggest anyone who wants to keep their sanity steers well clear of the thread.

 

[MrGecko]

Ok, I have just got back from a swim at the gym. I did 38 lengths and didn't take off (references to my buoyancy are permitted).

Right, I have had a bit of a think about this, and my conclusions are:

1) The Plane doesn't take off. Whether it was what the OP meant or not is irrelevent. There is only one correct solution and that is that the plane doesn't take off. I got it wrong. Again.

2) We all really knew what Skill was trying to get at, and in this case, the plane does take off.

With regard to 1), then I should apologise to Brambles for giving him a bit of agro throughout the discussion; I have accused him if things which infact I am guilty of (in particular, failing to consider things that didn't make sense in the real world, but were in fact true), so, sorry Brambles.

As for 2), in the real world, the plane takes off. Don't talk to me about carts and wheels and whatever - the plane takes off. Somewhere, amongst my pretty (but but so relevant) diagrams, lies a proof for it that some smart chappy can go and figure out - a proof for a different problem than the one here. Up until about two posts ago, everything I said was with regard the that problem, not this problem.

Thats about it, except:

* If anyone thinks the plane doesn't take off because of the conveyor "eating up" the thrust of the engine, you've got it wrong.

* If anyone things the plane does take off because the engines move relative to the air, the wheels dont matter one bit, or you could do it on ice, you've got it wrong. Look at my last diagram.

Much Love

G

P.S. This is what I think until someone changes my mind.

 

[Skill Leverage]

1) I see your point Gecko, but I acknowledged the poor wording of the original post a long, long time ago.

2) I do not agree with, at all. With zero friction between the wheels and their axels, the net lateral force on the plane due to the wheels is zero regardless of their speed. It doesn't matter whether the rotational displacement is equal or opposite, because they are not part of the system of forces that dictates whether or not the plane moves; real life or not, they might as well not even be there as far as the engines are concerned.

EDIT : I will admit that the plane's wheels matter to the problem for the reasons that ezreddy pointed out - Putting stipulations on their movement as being opposite to that of the belt, or other scenarios will create situations that are impossible to initiate or resolve; however, as you said we all know what I meant, and if I had known that we would still be here 60 pages later a) I would have taken a litle more time to check the wording of my notes b) I would have burned them sooner than post anything on here.

 

[Skill Leverage]

Your diagram assumes that, should the belt start moving before the plane's engines are switched on, the plane would move laterally. This is incorrect.

At least that's what I'm seeing, as stated I did little more than skim the last 10 pages as I really can't be bothered with this any more.

 

[MrGecko]

1) I see your point Gecko, but I acknowledged the poor wording of the original post a long, long time ago.

2) I do not agree with, at all. With zero friction between the wheels and their axels, the net lateral force on the plane due to the wheels is zero regardless of their speed. It doesn't matter whether the rotational displacement is equal or opposite, because they are not part of the system of forces that dictates whether or not the plane moves; real life or not, they might as well not even be there as far as the engines are concerned.

1) I ignored you

2) err... 2) says that the plane takes off...

 

[MrGecko]

Your diagram assumes that, should the belt start moving before the plane's engines are switched on, the plane would move laterally. This is incorrect.

At least that's what I'm seeing, as stated I did little more than skim the last 10 pages as I really can't be bothered with this any more.

No, thats not what I'm saying, and thats why the plane takes off

 

[MrGecko]

The conveyor belt moving of its own accord does not move the plane, it just makes the wheels go around. Once the plane starts to produce thrust, it will have a force causing it to move in the fixed reference frame - in order for it to remain stationary, there must be another force acting directly on the plane;

Using the translational / angular terminilogy, a translational displacement of plane the cannot be opposed by the conveyor; the conveyor adds angluar velocity to the wheels, not translational velocity to the plane.

Imagine the plane and the conveyor are stood still, no engines, no friction... if the conveyor starts to move, the plane stays exactly where it is, its the wheels that spin around.

bump

 

[megamuel]

"Thank you for your email to Professor Hawking.

As you can imagine, Prof. Hawking receives many such every day. He very much regrets that due to the severe limitations he works under, and the enormous number of requests he receives, he is unable to compose a reply to every message, and we do not have the resources to deal with many of the specific scientific enquiries and theories we receive.

Please see the website Professor Stephen W. Hawking for more information about Professor Hawking, his life and his work.

Yours faithfully

Sam Blackburn
Technical Assistant to
Professor S W Hawking CH CBE FRS
Department of Applied Mathematics and Theoretical Physics,
University of Cambridge,
Cambridge,
CB3 0WA.
United Kingdom.
http://www.hawking.org.uk"

:smart:

 

[Skill Leverage]

Sorry G, I think I meant with regard to your explanation of 1)... basically I'm saying that Brambles wasn't arguing the fact that the plane doesn't take off due to the paradoxical nature of the OP, rather because he didn't understand the nature of the wheels' movement having no effect on the lateral force.

 

[Skill Leverage]

I am confusing even myself now because I haven't properly read what everyone is saying, and 100% cannot be bothered to do so. It's probably best therefore that I leave my contribution to the conversation as it stands, as I can't be bothered to keep up to date with it.

SL

 

[Skill Leverage]

Basically Gecko I read your post quickly and thought that you had changed your conclusion to something where the plane takes off, but not due to horizontal movement rather due to being in a situation similar to being tethered in a wind tunnel. Apologies.

 

[MrGecko]

OK...

I am inclined to disagree here... I think Brambles knew exactly what the "gist" of the problem was, but answered a different problem; the whole argument has been about why the two solutions to the two different problems are... well, different.