# Which ones (if any) of these sentences are true?



## RogerS (4 Aug 2010)

a) Absolute motion is a redundant concept

b) There is no value in the concept of absolute space

c) No matter how fast we run, a light beam will still recede away from us at the speed of light

d) Imagine a car 4m long trying to fit into a garage 3.9m long. If the car travels faster than 22% of the speed of light then it will shrink enough to fit

e) Friction between the seas and the earths' surface causes the moon to drift slightly farther away from earth


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## StevieB (4 Aug 2010)

RogerS":10c0vbcw said:


> a) Absolute motion is a redundant concept
> 
> b) There is no value in the concept of absolute space
> 
> ...



a) Not redundant if the fixed point you are referencing to is effectively motionless (think celestial body) but in absolute terms, since the universe is still expanding then nothing is motionless, so in a pedantic kind of way absolute motion is impossible to compute.

b) see a) hypothetical but useful

c) tricky, my gut reaction would be light speed minus speed you run, but as this is the minutest % then to all intents and purposes this is correct. Caesium clocks and time dilation shows how speed effects the passage of time, but you run alot slower than a plane flies.

d) Impossible to say - no car will ever travel that fast (even with the Stig driving) and stopping it in the garage will be impossible as once it stops it will expand again and not fit.

e) No idea - the moons gravitational pull causes the tides, so effectively if friction causes us to pull further away gravitational effects become less, which reduce friction, which stops the drift.... will it not reach equilibrium?

how did I do?! 2/5 I suspect 

Steve


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## Anonymous (4 Aug 2010)

i am guesing these are theories which can never be proven, regardless of how good the hadron collider is those particle collisions are not at the speed of light.

with regards to D, the car is a fixed size object it wouldn't expand (lets forget heat causing the materials of the car to expand when travelling at that kind of speed) nor would it shrink.

E) i do believe the moon is creaping further away from us, but not sure why?


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## PeterBassett (4 Aug 2010)

a) Don't know
b) Don't know
c) True
d) I make that the car will be 2mm too long to fit at 22%C. If you round down it will fit, but only for a very short amount of time. ;-)
e) True


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## Lons (4 Aug 2010)

D: Sounds like my missus trying to get into a garage - would demolish it so pointless argument :lol: :lol:


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## RogerS (5 Aug 2010)

In fact they are all correct according to Professor Brian Cox and Albert Einstein.

I can highly recommend 'Why does e=mc2' by Brian Cox and Jeff Forshaw. 

The one statement that I can't really get my head round is (c). Common sense suggests that if you had a mate holding a flashlight and you then ran fast enough then you could get to the front of the lightbeam and run alongside it. But if we are to follow Maxwell's equations to the letter, then no matter how fast we run, the beam still recedes away from us at the speed of light. If it were not so then it would mean that the speed of light is different for your mate with the flashlight than it is for you. But since the speed of light is a constant, this can't be so.

Must go and lie down now as my brain is starting to hurt again.


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## woodbloke (5 Aug 2010)

RogerS":3c5puldl said:


> Must go and lie down now as my brain is starting to hurt again.


Mine started to hurt after I read the first question :lol: - Rob


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## PeterBassett (5 Aug 2010)

e is because the moon pulls the seas via gravity. This causes the seas to bulge towards the moon and away from the moon, on the front and back of the earth. Front and back here with respect to the moon.

Now, if the earth were tidally locked to the moon, this bulge alone would actually cause the moon to eventually crash into the earth.

However, since the earth rotates once in 24 hours and the moon orbits once in a month or so the face that the earth presents to the moon is always moving forward. It rotates in the same direction as the moons orbit.

Due to friction with the sea this movment drags the bulge of tidal water slightly ahead of where it would be if the earth were tidally locked.

Since the bulge is now ahead of the moon in its orbit the extra mass there pulls at the moon. Pulls it forward along it's orbit, causing its orbital velocity to increase.

Since the velocity has increased it must orbit higher.

Pete


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## Benchwayze (6 Aug 2010)

I do know that the Earth's gravitational pull causes 'Moon-quakes' that are more severe and longer lasting than quakes on Earth.

Now ... Brain burning out I'm afraid. Absolutely...

John


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## kasandrich (6 Aug 2010)

Heres another puzzler for you, 

If I stand facing an oncoming train, and when it gets into range I fire a pea shooter at it, will the train stop?

My case is that the pea will bounce off, doing a complete change of direction 180degrees, as it changes direction, there must be a point in time where it is stationary, and not going in either direction, you must therefore deduce that the pea stops, when the pea is stationary it will be in contact with the front of the train, therefore it follows that if the pea is stationary and in contact with the train, the train must also be stationary. :roll:


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## bugbear (6 Aug 2010)

kasandrich":1beo11fo said:


> Heres another puzzler for you,
> 
> If I stand facing an oncoming train, and when it gets into range I fire a pea shooter at it, will the train stop?
> 
> My case is that the pea will bounce off, doing a complete change of direction 180degrees, as it changes direction, there must be a point in time where it is stationary, and not going in either direction, you must therefore deduce that the pea stops, when the pea is stationary it will be in contact with the front of the train, therefore it follows that if the pea is stationary and in contact with the train, the train must also be stationary. :roll:



This reasoning assumes that both peas and trains are perfectly inelastic, which is (obviously) not the case.

BugBear


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## RogerS (6 Aug 2010)

I like that one, Richard.

Here's another. Imagine a treadmill large enough to stick a plane on. Now fire up the planes engines to take off speed and set the treadmill going until it also reaches take-off speed.

Question - will the plane take-off?


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## kasandrich (6 Aug 2010)

RogerS":35gdv5qi said:


> I like that one, Richard.
> 
> Here's another. Imagine a treadmill large enough to stick a plane on. Now fire up the planes engines to take off speed and set the treadmill going until it also reaches take-off speed.
> 
> Question - will the plane take-off?



Yes because a planes engines work on the air not the ground.


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## dannykaye (6 Aug 2010)

a) True

b) True

c) two light beams travelling towards each other an observer travelling with one can measure the passing speed of the other and it will be c not 2c 

d) difficult because it is shorter dur to dilation, you could never stop it tho 

e) True

here's one that I can never satisfy myself about, an astronaut is floating in space at the centre of a perfect spherical mirror. he turns on a flashlight, what does he see


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## Jacob (6 Aug 2010)

bugbear":33nwm1lb said:


> kasandrich":33nwm1lb said:
> 
> 
> > Heres another puzzler for you,
> ...


Makes no (difference). Except that if elastic, not all particles of the train and/or pea are traveling at the same time with the same speed and direction.


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## barkwindjammer (6 Aug 2010)

Eat your peas up and get back to work :?


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## RogerS (6 Aug 2010)

kasandrich":2dn0wsgx said:


> RogerS":2dn0wsgx said:
> 
> 
> > I like that one, Richard.
> ...



I don't think so. The plane needs airflow to generate lift. It's wheels might be going round and the engines at full-blast but there's no airflow.


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## RogerS (6 Aug 2010)

dannykaye":2384ne0o said:


> a) True
> 
> b) True
> 
> ...



I'm not sure that he will see anything if it is a perfect mirror. By perfect, any light will be reflected back down it's own path and to the torch. By perfect there will be no internal light scatter inside the mirror so there would be no stray photons coming out at other angles to then bounce off other parts of the mirror. Am I right?


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## barkwindjammer (6 Aug 2010)

a jet or prop would be thrusting the aircraft forward, its going to move forward and colide with the giant treadmill, the treadmill is in a static position


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## RogerS (6 Aug 2010)

Mr G Rimsdale":3kuglxnw said:


> bugbear":3kuglxnw said:
> 
> 
> > kasandrich":3kuglxnw said:
> ...



That is so wrong. If the objects are elastic (which all objects on this planet are to some extent) then the energy from the collision gets absorbed by each object by deforming and then into heat, I believe.

I'm still trying to work out, theoretically, what would happen if they were both inelastic....common sense says that the initial premise should hold and yet if both are stationary then there would have to be a time - (a noticeable time due to the inertia in the train) while the train got back up to speed.


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## RogerS (6 Aug 2010)

barkwindjammer":19g9pn3j said:


> a jet or prop would be thrusting the aircraft forward, its going to move forward and colide with the giant treadmill, the treadmill is in a static position



The jet might be thrusting the plane forward but the tread of the treadmill is going backwards underneath so the plane will remain stationary.


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## dannykaye (6 Aug 2010)

RogerS":oweu97ja said:


> I'm not sure that he will see anything if it is a perfect mirror. By perfect, any light will be reflected back down it's own path and to the torch. By perfect there will be no internal light scatter inside the mirror so there would be no stray photons coming out at other angles to then bounce off other parts of the mirror. Am I right?



except it is not a flat mirror and the torch might not be exactly in the centre


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## barkwindjammer (6 Aug 2010)

RogerS":1qxbj7ao said:


> barkwindjammer":1qxbj7ao said:
> 
> 
> > a jet or prop would be thrusting the aircraft forward, its going to move forward and colide with the giant treadmill, the treadmill is in a static position
> ...



This would equate if it were the wheels of the plane creating forward motion


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## Jacob (6 Aug 2010)

barkwindjammer":149lro4a said:


> Eat your peas up and get back to work :?


Impossible - elastic peas bounce all over the place.

Re plane - if the treadmill is going at take-off speed then the plane (if fixed, PS to the treadmill) is being forced through the air at take-off speed and hence will take off, at a tangent, the instant it is released (horizontally), even without the engines fired up, but without which it would come down again quickly.

PS assuming the treadmill is sending the plane forwards! Or can they fly backwards?


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## Racers (6 Aug 2010)

Hi,

Mythbusters did the conveyor belt /plane with a real plane and it took off, the propeller pushes against the air not the ground/treadmill.

http://www.youtube.com/watch?v=OyUpDQ-fpTc

Pete


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## barkwindjammer (6 Aug 2010)

If a plane is 'fixed' then it wont take off whether or not a giant treadmill is used to 'spin the wheels', aircraft are propelled forward by thrusting air backwards and not by propelling them forward by the wheels, it is true that the wings are what create the lift, so to sum up

A: a fixed aircraft wont move forward let alone up

B: the wheels are 'free wheels' and provide no forward movement

C: air thrusted backwards provides the forward force

D: rubber peas are tricky to eat without the aid of adhesive cutlery

so, the question was 'will the plane take off ?'
answer no

'What will the plane do'?
answer colide with the rotating treadmill, as its forced forward and the treadmill isn't 

Q: does anyone know of a foodsafe adhesive? :wink:


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## Jacob (6 Aug 2010)

RogerS":11ujvzdn said:


> Mr G Rimsdale":11ujvzdn said:
> 
> 
> > bugbear":11ujvzdn said:
> ...


No it's right. The difference is that not the whole pea (or the whole train) is acted upon (or reacts) simultaneously. There'd be a pressure wave.


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## RogerS (6 Aug 2010)

But if the plane is thrust forward by virtue of the propeller then that will make the wheels roll but if the treadmill moves backwards at the same rate then the plane will remain stationary. If it remains stationary than it will have no airflow and hence no lift.


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## RogerS (6 Aug 2010)

Mr G Rimsdale":nm8tm8o1 said:


> RogerS":nm8tm8o1 said:
> 
> 
> > Mr G Rimsdale":nm8tm8o1 said:
> ...



You can only have a pressure wave if the object is elastic.


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## dannykaye (6 Aug 2010)

RogerS":38mrs9n1 said:


> But if the plane is thrust forward by virtue of the propeller then that will make the wheels roll but if the treadmill moves backwards at the same rate then the plane will remain stationary. If it remains stationary than it will have no airflow and hence no lift.



the propeller will move the plane forward, if the treadmill is still the wheels will revolve at, say, x meters per, if the treadmill is moving at x meters per, in one direction the wheels will not revolve in the other they will tevolve at 2x. the motion of the plane and that of the wheels coupled to the treadmill are not connected as they have what we might call a loose joint in the system, ie the axle of the wheels


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## Jacob (6 Aug 2010)

RogerS":3ra8nllt said:


> Mr G Rimsdale":3ra8nllt said:
> 
> 
> > RogerS":3ra8nllt said:
> ...


Deliberate error - I meant elastic when I said inelastic.


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## RogerS (6 Aug 2010)

Thanks, Danny, I get it now.


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## llangatwgnedd (6 Aug 2010)

Get a life you daft bats.


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## Jake (6 Aug 2010)

Good to see a topic wot sorts the wheat from the chaff.


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## barkwindjammer (6 Aug 2010)

Jake":1lit6mo2 said:


> Good to see a topic wot sorts the wheat from the chaff.


as peeps that used to use CB radio would say
roger !


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## Jacob (6 Aug 2010)

It's like being back at school (3rd form). We had a term long argument about who was the greatest guitarist in the world - Julian Bream, Hank Marvin or Duane Eddy.


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## Sportique (7 Aug 2010)

It seems to me that the pea will bounce on the out-going tide, this will cause the train to go into reverse colliding with the treadmill and forcing the plane to vanish up its own whatsit.

I think the mirror is a red herring and astronauts cannot eat herrings.

Anyone read Isaac Asimov writings about the moon (The Tragedy of ...) ?

Dave


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## Benchwayze (7 Aug 2010)

kasandrich":4fjs02jc said:


> Heres another puzzler for you,
> 
> If I stand facing an oncoming train, and when it gets into range I fire a pea shooter at it, will the train stop?
> 
> My case is that the pea will bounce off, doing a complete change of direction 180degrees, as it changes direction, there must be a point in time where it is stationary, and not going in either direction, you must therefore deduce that the pea stops, when the pea is stationary it will be in contact with the front of the train, therefore it follows that if the pea is stationary and in contact with the train, the train must also be stationary. :roll:



As any camera can prove with fast enough film! :lol: :lol: :lol:


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## Benchwayze (7 Aug 2010)

RogerS":1su7tucv said:


> dannykaye":1su7tucv said:
> 
> 
> > a) True
> ...



He would see the effects of spherical abberation. A mirror whose surface is part of a perfect sphere gives a distorted image at its focus. Which is why a telescope mirror is figured to a parabolic curve to bring the reflected rays to a predetermined focus. 

I think! :roll:


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## dannykaye (7 Aug 2010)

Mr G Rimsdale":3nvqjn1q said:


> It's like being back at school (3rd form). We had a term long argument about who was the greatest guitarist in the world - Julian Bream, Hank Marvin or Duane Eddy.



Have twangy guitar almost has it but my vote goes to ragtime cowboy jew


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## big soft moose (7 Aug 2010)

dannykaye":rp8fma3y said:


> Mr G Rimsdale":rp8fma3y said:
> 
> 
> > It's like being back at school (3rd form). We had a term long argument about who was the greatest guitarist in the world - Julian Bream, Hank Marvin or Duane Eddy.
> ...



be serious, everyone knows hendrix was the greatest guitarist in the world


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## Jenx (7 Aug 2010)

Mr G Rimsdale":2hho79gb said:


> It's like being back at school (3rd form). We had a term long argument about who was the greatest guitarist in the world - Julian Bream, Hank Marvin or Duane Eddy.



ANSWER : 
None, .. because Rory Gallagher was :wink: :lol:



( and for BSM's benefit there ... when asked 'whats it like to be the greatest guitarist in the world ? Jimi Hendrix replied.. "Ask Rory Gallagher" ) :wink:


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## dannykaye (7 Aug 2010)

Rory was well cool as was hendrix, but I like Stephan Grossman, mind you Stephan reconed the Rev Gary Davis and he was pretty amazing.


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## RogerS (7 Aug 2010)

Jenx":2tv0y6yb said:


> Mr G Rimsdale":2tv0y6yb said:
> 
> 
> > It's like being back at school (3rd form). We had a term long argument about who was the greatest guitarist in the world - Julian Bream, Hank Marvin or Duane Eddy.
> ...



Never 'eard of 'im. Was he in Oasis?


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## Benchwayze (8 Aug 2010)

big soft moose":3t20ph5z said:


> dannykaye":3t20ph5z said:
> 
> 
> > Mr G Rimsdale":3t20ph5z said:
> ...



Everyone except the devotees of Django Reinhardt.

Ultimately, Pete, I suppose it depends on your area of musical taste. And anyone who would set fire to their guitar, must have some issues going on!  

John


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## PeterBassett (8 Aug 2010)

RogerS":2rhgqg3y said:


> I like that one, Richard.
> 
> Here's another. Imagine a treadmill large enough to stick a plane on. Now fire up the planes engines to take off speed and set the treadmill going until it also reaches take-off speed.
> 
> Question - will the plane take-off?



NO You are not allowed to ask this question. This question is the forum killer.

I used to frequent a motorbike forum and this question caused more arguments than anything else I've ever seen.

Of course the answer is that the plane will take off as normal. But you try telling that to some people!


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## PeterBassett (8 Aug 2010)

RogerS":3b0ip0m8 said:


> barkwindjammer":3b0ip0m8 said:
> 
> 
> > a jet or prop would be thrusting the aircraft forward, its going to move forward and colide with the giant treadmill, the treadmill is in a static position
> ...



GAH! NO! STOP THR PAIN!!1111111 :lol:


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## PeterBassett (8 Aug 2010)

dannykaye":3nk3qe86 said:


> here's one that I can never satisfy myself about, an astronaut is floating in space at the centre of a perfect spherical mirror. he turns on a flashlight, what does he see



Unless the flashlight head itself is very carefully positioned at the center of the sphere then eventually all the light will fall back onto the astronaught.

What will he see? It depends on the position of his eyes within the sphere. Basically, everywhere he looks he will see a distorted very of a part of himself.

One example : If one of his eyes is bam in the center of the sphere and he looks directly forward with that one eye only he will see his own black pupil over the whole of the sphere and whatever scattered images are made from the flashlight, which will cause various distorted images of his other body parts to appear.

Pete


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## Anonymous (8 Aug 2010)

PeterBassett":y7zjnzyf said:


> RogerS":y7zjnzyf said:
> 
> 
> > I like that one, Richard.
> ...



lol

The velocity of the plane going forward is equal to the velocity of the treadmill going backwards so therefore the plane remains stationary

Simples

in order for the plane to take off it needs to be travelling forward at speed which the counter action of the treadmill will not allow.


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## Anonymous (8 Aug 2010)

in fact i think this is called an equilibrium


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## Jake (8 Aug 2010)

mark270981":1sp73azq said:


> lol
> 
> The velocity of the plane going forward is equal to the velocity of the treadmill going backwards so therefore the plane remains stationary
> 
> ...



Wheat nevers says "simples". It's a chaff phrase.


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## Anonymous (8 Aug 2010)

Jake":14a1zhmt said:


> mark270981":14a1zhmt said:
> 
> 
> > lol
> ...



well it was for your benefit


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## Jake (8 Aug 2010)

Well that's me told.

You're so wrong, the simples will very quickly hit the ton.


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## Anonymous (8 Aug 2010)

I was very nearly pedantic and corrected your grammar, but you beat me to it.


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## Jake (8 Aug 2010)

It was stylistic, but I thought actually it didn't work.


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## Anonymous (8 Aug 2010)

Why didn't you think it would work?


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## Jake (8 Aug 2010)

It was intended as mild sarcasm, but it didn't play out and I thought it would just prompt a great big feeling of superiority. Whereas the point is the plane doesn't give a rubbish about its wheels.


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## Anonymous (8 Aug 2010)

nah mate, i'd never feel superior to anyone regardless of wealth, stature or education.

Its a tough argument, think mouse in a wheel, the mouse can never outrun the wheel.


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## Jake (8 Aug 2010)

The mouse is trying to put power through its feet through the wheel.

Tie a firework to a treadmill. Result?


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## Anonymous (8 Aug 2010)

put the rocket on wheels, the wheels make the motion, the treadmill counteracts the motion of said wheels.

if the plane had no wheels then i would agree with you.

we could go all night on this.

so probably best to agree to disagree, don't you think?


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## Tom K (8 Aug 2010)

kasandrich":wclj8eye said:


> Heres another puzzler for you,
> 
> If I stand facing an oncoming train, and when it gets into range I fire a pea shooter at it, will the train stop?
> 
> My case is that the pea will bounce off, doing a complete change of direction 180degrees, as it changes direction, there must be a point in time where it is stationary, and not going in either direction, you must therefore deduce that the pea stops, when the pea is stationary it will be in contact with the front of the train, therefore it follows that if the pea is stationary and in contact with the train, the train must also be stationary. :roll:



It doesn't matter coz the train will kill you :lol:


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## Jake (8 Aug 2010)

mark270981":7g9f01ic said:


> put the rocket on wheels, the wheels make the motion, the treadmill counteracts the motion of said wheels.
> 
> if the plane had no wheels then i would agree with you.
> 
> ...



Yes, but only because you are missing the point. The treadmill can't exert any force through the wheels of the plane to the plane because there is a bearing in the wheel. For the same reason, the place can easily accelerate when the runway is static.

It is a very good reason for not powering aircraft through their wheels cos otherwise they would rapidly fall out of the sky on takeoff


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## PeterBassett (8 Aug 2010)

mark270981":znroqi6s said:


> The velocity of the plane going forward is equal to the velocity of the treadmill going backwards so therefore the plane remains stationary
> 
> Simples



Why? WHY WhY?! The pain it won't StoP?!

PLease StOp TaLllking Abbooott Wheeleee.ss. 

Teyyyyy don.tt natter!

Ohhh Ggggodddd. Ears Beleeeeding.

Wheeeeeelsss.,,,. NO...........

If this is a wind up, it is excellent and you are to be congratulated. If not, you have hereby lost your right to teach physics to kids. :lol:

I told you it was a forum killer. 8)


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## Anonymous (8 Aug 2010)

i just googled the question and well there is no specific answer, just lots and lots of arguments

Jake we need to agree to disagree, you have your opinion i have mine.


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## PeterBassett (9 Aug 2010)

No, there really is an answer.

There really is.


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## Sportique (9 Aug 2010)

If the treadmill is as long as the required runway, then, regardless of what the treadmill is doing (forwards, backwards or stationary) the plane will move forward through its engine thrust, reach take off AIR speed and leave the treadmill.

The speed of the wheels is irrelevant - if the treadmill is moving against the motion of the plane then the wheels will simply turn faster. If the treadmill is moving in the same direction as the plane then the weels will be stationary - but the plane will still reach take-off AIR speed.

As the man said, the wheels have nothing to do with air speed.

Dave


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## Tom K (9 Aug 2010)

The planes wheels aren't powered they just stop those nasty scratches on the undercarriage. Think Harrier jump jet.


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## Benchwayze (9 Aug 2010)

Tom K":1ktgtddo said:


> The planes wheels aren't powered they just stop those nasty scratches on the undercarriage. Think Harrier jump jet.



True. 
But, without friction - nothing moves. With too much friction - nothing moves. Friction is a contrary Bu**er! 

Thus, without wheels a normal aircraft has nothing with which to overcome friction and it wouldn't move without freakin' enormous amounts of power. ('No wheels' works okay on water though, although there's still friction. And on ice or snow, either wheels or skids work.. (Don't they?) 

Once the speed of the craft overcomes friction, airlift pulls the plane into flight and the wheels/skids become redundant, until the plane needs to land again. 

I'd say the main reasons Harriers have wheels are the same reasons most helicopters have them; for a soft landing and to move the aircraft around on the deck. (Although Harriers don't always take of vertcally.) 

But I don't know much about aircraft. 

John


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## Jacob (9 Aug 2010)

Sportique":ycl8b2p1 said:


> If the treadmill is as long as the required runway, then, regardless of what the treadmill is doing (forwards, backwards or stationary) the plane will move forward through its engine thrust, reach take off AIR speed and leave the treadmill.
> 
> The speed of the wheels is irrelevant - if the treadmill is moving against the motion of the plane then the wheels will simply turn faster. If the treadmill is moving in the same direction as the plane then the weels will be stationary - but the plane will still reach take-off AIR speed.
> 
> ...


Spot on. But if the plane wheels were chocked on a forwards moving treadmill it would lift off even without engines, and then come down again. 
I've suddenly realised my previous answers to this stupid question are gibberish - I envisaged "treadmill" as a round drum (a traditional treadmill) with the plane attached to the outside (well why not?) whereas what was meant was a modern gym type conveyor belt treadmill :roll:


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## Jacob (9 Aug 2010)

RogerS":1c13cvdn said:


> a) Absolute motion is a redundant concept
> 
> b) There is no value in the concept of absolute space
> ...


Getting back to the original question - a trip down memory lane for me as the best mark I ever got for an essay, as a student many years ago, was on "Compare and contrast Newton's and St Augustine's views of space and time"
Newton hypothesised "_absolute_ space and time" but St Augustine didn't make this mistake and hence could be seen as more scientifically correct, albeit less scientifically productive.


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## Tom K (9 Aug 2010)

Benchwayze":1q8o0ybs said:


> Tom K":1q8o0ybs said:
> 
> 
> > The planes wheels aren't powered they just stop those nasty scratches on the undercarriage. Think Harrier jump jet.
> ...



I think its more about lift actually and wether you can get air under the wings. :lol:


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## Racers (9 Aug 2010)

Hi, chaps 

Way back on page 2 I posted a link to a video of a plane taking off on a conveyer belt, I can't believe you are still arguing about it.



http://www.youtube.com/watch?v=OyUpDQ-fpTc 


Pete


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## bugbear (9 Aug 2010)

RogerS":16j9ve7i said:


> You can only have a pressure wave if the object is elastic.



I don't suppose they'd be much of a pressure wave in "pea vs train".

To see why the train doesn't stop, it's easiest to exaggerate the elasticity.

If you visualise a nice big (strong) trampoline mounted on the front of the train, and a cannon ball going into the trampoline, it's quite intuitively obvious that the trampoline will give a bit, and then bounce the cannon ball back.

It also fairly obvious that the train doesn't stop, although it will be fractionally slowed down.

With different (but non zero) levels of elasticity and mass in the pea vs train example, this is what's happening.

It is obvious that a fly cannot stop a train; the game here is to spot the flaw in the reasoning, not the conclusion.

Next stop - Xeno's paradox.

BugBear


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## Vormulac (9 Aug 2010)

I suspect given the nature of the question that we are not talking about involving practical issues such as friction and bearing efficiency in our little aeroplane connundrum. Given that the 'treadmill' is there to ensure that the aircraft remains stationary regardless of the thrust of the engines the only factor to take into account is airflow over the lift producing surface. If it is a jet aircraft where thrust is directed straight through the engine (wherever that is mounted) then I very much doubt there would be any airflow over the wing and so it would not take off; however, if it is a prop-driven aircraft or a channel wing design then the engine is directing high speed air over the wing, creating lift, and it would.


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## SBJ (9 Aug 2010)

c'mon guys - don't you watch Mythbusters???

http://mythbustersresults.com/episode97


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## RogerS (9 Aug 2010)

Racers":njk65nag said:


> Hi, chaps
> 
> Way back on page 2 I posted a link to a video of a plane taking off on a conveyer belt, I can't believe you are still arguing about it.
> 
> ...



I had a quick look and then read all the comments ..which confused things!

Suffice to say I now am firmly in the 'take-off' camp


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## RogerS (9 Aug 2010)

Vormulac":1kvenb6d said:


> I suspect given the nature of the question that we are not talking about involving practical issues such as friction and bearing efficiency in our little aeroplane connundrum. Given that the 'treadmill' is there to ensure that the aircraft remains stationary regardless of the thrust of the engines the only factor to take into account is airflow over the lift producing surface. If it is a jet aircraft where thrust is directed straight through the engine (wherever that is mounted) then I very much doubt there would be any airflow over the wing and so it would not take off; however, if it is a prop-driven aircraft or a channel wing design then the engine is directing high speed air over the wing, creating lift, and it would.



No! True up to a point but the aircraft is thrust forward by the propellers or the jets. The wheels are a red-herring. So is the treadmill. Given a long enough treadmill then the aircraft will accelerate under the thrust until it gets sufficient groundspeed aka air over the wings to create lift.


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## PeterBassett (9 Aug 2010)

The point is that ground speed is completly irrelevant.

Ground speed does not cause an aeroplane to fly. AIRspeed does.

All you need to figure out that the treadmill has no effect is to imagine a plane taking off under these two circumstances.

1) Into a 50mph head wind
2) With a 50mph tail wind.

The difference in take off AIRspeed in these two examples is 0mph. If the plane requires 200mph airspeed to take off the plane will take off at 200mph airspeed.

The difference in take off ground speed between these two examples is 100mph. With a head wind the plane will achieve take off with a 150mph ground speed. With a tail wind it will have a 250mph ground speed before it takes off.

The treadmill example is *exactly* the same except the system is transformed so that is is the "ground" that is moving relative to the plane and not the air, in the case of wind.

Everything else is the same. It doesn't matter what velocity the plane has relative to the ground/treadmill. It is the air speed that controls the take of point. The treadmill could be going 500mph backwards and the only thing would happen is the planes wheel bearings would scream. The plane would still take off when it had achieved 200mph air speed and, in this case, 700mph "ground" speed (or treadmill speed).

The plane cannot be slowed by the treadmill no matter how fast it goes (within the tolerance of the wheel bearings). The effect is so small as to be none existant.

Pete


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## bugbear (9 Aug 2010)

I'm just glad no-one's mentioned the Monty Hall puzzle.

Or the lorry load of budgies.

Oops. ;-)

BugBear


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## PeterBassett (9 Aug 2010)

The monty hall one is great. I had to write a simulation of it to prove the outcome to a co worker.

They did the budgies on mythbusters I recall.


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## RogerS (9 Aug 2010)

And then there's the Two Door problem. Two doors - one is death, the other means life and two people - one outside each door. One person always tells the truth and the other always lies. You are allowed one question to only one of the two.


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## Kalimna (9 Aug 2010)

For the idots out there (me!) what is the Monty Hall puzzle? And what is Xenos' paradox? 

Oh, and from previous page, regarding friction. You don't need friction to move, just ask anyone who observes motion through a vacuum (or as damned close to a vacuum as to render friction zero). What you need is a force that overcomes another force, be it friction, gravity, momentum etc (and I realise Im not using the term 'force' in it's strictest scinetific definition).

Adam S


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## PeterBassett (9 Aug 2010)

The Monty Hall Puzzle. No peeking, it's all over the internet.

You're on a game show, and you're given the choice of three doors: 

Behind one door is a car; behind the others, goats. 

You pick a door, say No. 1, and the host, who knows what's behind the doors, opens one of the doors with a goat, say No. 3. He will always open a loosing door after you have initially chosen.

He then says to you, "Do you want to pick door No. 2?"

Is it to your advantage to switch your choice? 

Do you get better, worse or the same odds of winning if you change the door you picked to No. 2?


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## Kalimna (9 Aug 2010)

Ahh, I think I know this one - it was (or at least a version) in part of a maths/sciece documentary series (Marcus de Sautoy I think was presenting, and for my money much better at it than Brian Cox who has far too many NigellaLawsonesque floaty camera moves..... but i digress).

I can't remember the logic, but I think it is to your advantage to change your mind.

Adam


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## PeterBassett (9 Aug 2010)

I think Marcus de Sautoy did a program with Alan Davies and they covered it.


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## MikeH (9 Aug 2010)

Ah Roger, I sort of remember the life and death doors one. You ask either person which door the other person would say if you asked them which door leads to death and then take that door.
If you had asked the truth giver they would have said that the lier would say the life door.
If you had asked the lier they would tell you that the truth person would tell you the other door.
Or it's something like that anyway!


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## Jacob (9 Aug 2010)

Just back from the boozer so may get this right!
Your first choice is 1:3 against winning the car. After one goat is eliminated the chances of the remaining choice being a car is 1:2. So you change your choice, 1:2 is better than 1:3.


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## big soft moose (9 Aug 2010)

Mr G Rimsdale":2bhe3jqd said:


> Just back from the boozer so may get this right!
> Your first choice is 1:3 against winning the car. After one goat is eliminated the chances of the remaining choice being a car is 1:2. So you change your choice, 1:2 is better than 1:3.




initial reasoning is right but answer is wrong - once one goat is eliminated you have a !:2 chance of winning the car regardless of whether you change your choice or not (ie there are two doors left and its 50/50 which has the car)

so there is no advantage to changing your choice, but equally no disadvantage either.


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## SBJ (9 Aug 2010)

Sorry BSM you are wrong.

you have a 1 in 3 chance of picking the car initially and a 2 in 3 chance of not picking it. 
If you get it right initially and change you lose 1 in 3 times. 

If you get it wrong initially ( which you will do 2 in 3 attempts, statistically) and then change you will win because the presenter has to remove the second goat.

So in theory, you are better off by selecting the wrong option first time - not that you'd know.


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## big soft moose (9 Aug 2010)

ahhh pineapples - you are right , and i was wrong - the logic still seems counter inituitive as at the point of choosing its 1/2 as the 3rd option has been removed - but all the sites i googled agree with your interpretation so fairplay


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## Kalimna (10 Aug 2010)

BSM - Unfortunately for your case I think SBJ is right, and Wikipedia (the source of all fact in the universe!) backs this up :

"An even simpler solution is to reason that switching loses if and only if the player initially picks the car, which happens with probability 1/3, so switching must win with probability 2/3"

There is a lot more explanation in the whole article, but the above line sums it up neatly.

The above can be shown to be the case by experiment, and the initially intuitive answer is, indeed, false. I'm not very good at statistics, and had trouble believing the answer, but the experimental evidence is impossible to refute.

Adam


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## SBJ (10 Aug 2010)

big soft moose":23vo0e8d said:


> ahhh pineapples - you are right , and i was wrong - the logic still seems counter inituitive as at the point of choosing its 1/2 as the 3rd option has been removed - but all the sites i googled agree with your interpretation so fairplay



Talk about timing, every time I went to quote you your post changed!!!

What you have to consider is that the presenter introduces an element of knowledge AFTER the intiial choice which you can use to influence your decision.


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## big soft moose (10 Aug 2010)

SBJ":2q985myn said:


> big soft moose":2q985myn said:
> 
> 
> > ahhh pineapples - you are right , and i was wrong - the logic still seems counter inituitive as at the point of choosing its 1/2 as the 3rd option has been removed - but all the sites i googled agree with your interpretation so fairplay
> ...



the real question tho is what sort of goat and what sort of car - if its a smart car you might be better of with the goat :lol:


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## bugbear (10 Aug 2010)

Kalimna":1m54rx5x said:


> The above can be shown to be the case by experiment, and the initially intuitive answer is, indeed, false. I'm not very good at statistics, and had trouble believing the answer, but the experimental evidence is impossible to refute.
> 
> Adam



So the "take home" for today: "Common sense answers can be wrong".

Indeed, most good logic or maths puzzles derive much of their appeal from this.

BugBear


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## Anonymous (10 Aug 2010)

Their experiment wasn't exactly scientific was it


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## Kalimna (10 Aug 2010)

Mark - yes it was scientific, though not quite as rigorous, perhaps, as the mathematical proof of Fermats Last Theorem. The sample size wasn't huge (50, I think), but the trend was clear and for a tv documentary, I can't think of any way it may have been improved given the time and audience constraints (de Sautoys' docu). Plus, that wasn't the only experimental evidence.

Bugbear - yep, common sense isn't always correct, especially where evidence is counter to what we already 'know'. Just look at any studies involving memory and recall.


Adam


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## Jacob (10 Aug 2010)

Er - surely common sense _does_ provide the answer? 
Clearly the first choice has odds against being the car; is more likely to be goat. The second choice (after removing a goat) has even odds, equal likelihood and so is the better bet.
S'obvious!


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## PeterBassett (10 Aug 2010)

Its better than 1:2. The odds if your change your mind are 2:3. The odds if you stick with your door are 1:3.


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## Anonymous (10 Aug 2010)

Well I have just emailed Stephen Hawking the question through the official channels regarding this plane thing.

I very much doubt he will reply as its probably just trivial to him, but thought it was worth a try.

Mstance hasn't changed on it, the plane remains firmly on the conveyor belt. Untill someone who is qualified to challenge my opinion it remains at that.

The Mythbusters experiment wasn't conducted to the true letter of the question so is therefore inconclusive.


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## Jacob (10 Aug 2010)

PeterBassett":1vzlx3h0 said:


> Its better than 1:2. The odds if your change your mind are 2:3. The odds if you stick with your door are 1:3.


No. The odds are evens on the change of mind. Still better than 1:3 though.


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## Vormulac (10 Aug 2010)

PeterBassett":1chb08s5 said:


> 1) Into a 50mph head wind
> 2) With a 50mph tail wind.
> 
> The difference in take off AIRspeed in these two examples is 0mph.
> [Pete



This isn't true; assuming your take-off speed is 200mph (yes, airspeed - or the speed of the air flowing over the wings creating lift), the wings don't care if the airflow creating the lift is being delivered by a head wind or by the forward motion of the aircraft, a 50mph headwind means the aircraft will only need to generate 150mph of forward speed in order to achieve the 200mph take off speed required. A tailwind on the other hand makes take off significantly more difficult to achieve as it has the opposite effect, additional forward speed is needed by the aircraft to overcome it.

http://www.auf.asn.au/groundschool/umodule11.html

"Take-off into wind!

...
The ground (rolling) speed for take-off is lower. The airspeed during the ground roll equals the ground speed plus/minus the headwind/tailwind component. Thus, if the aircraft is rolling at 30 knots into a 10 knot headwind, the airspeed = 30+10 = 40 knots. If rolling at 30 knots with a 10 knot following wind, the airspeed = 30 –10 = 20 knots."

Anyway, back to our teaser; assuming a perfect frictionless treadmill, as the engines push the aircraft forward, the treadmill runs freely beneath it denying the aircraft any forward motion, so unless there is airflow over the wings being delivered by another source other than the forward motion of the aircraft itself, no lift is generated.

Or so it seems to me


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## RogerS (10 Aug 2010)

Mark

Are you winding us up?

Step One....

Imagine a long length of trackwork high up...bit like a gantry crane. With rails on. There is a trolley on these rails and this trolley can roll freely forward or backwards. Now dangle a plane beneath this trolley on a cable so that the plane is hanging in the air.

Now fire up the jet engines. What's going to happen? The plane is going to move forward under the thrust of its' engines. If you had a long enough set of rails then eventually the speed of the plane will generate enough lift from the airflow going over its' wings and it will fly.

Do you agree?


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## Sportique (10 Aug 2010)

Vormulac
Anyway said:


> Wrong - your "forward motion" above is *ground speed*, you are overlooking the fact that aircraft fly by virtue of their *AIR* speed not gound speed. It is accepted that, in this case, because the "ground" is moving with the aircraft then there will not be any ground speed, but the thrust of the engines will always generate AIR speed (regardless of how quickly or slowly the wheels are/not turning!) - unless the aircraft is tethered and this one is not tethered.
> 
> Dave


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## Racers (10 Aug 2010)

Hi, 

Come on chaps the plane propeller/jet pushes aganst the AIR not the ground. 

Pete


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## PeterBassett (10 Aug 2010)

Mr G Rimsdale":t6tdobz9 said:


> PeterBassett":t6tdobz9 said:
> 
> 
> > Its better than 1:2. The odds if your change your mind are 2:3. The odds if you stick with your door are 1:3.
> ...



These are the odds of winning the car if you Always choose to stick with your original door 33.3% (1:3) No disagreement here i think.






However these are the odds of winning the car if you Always choose to switch doors : 66.6% (2:3)





Note that these odds are of winning the car, not of anything else. 

It's not intuitive at all and that is what makes it so good. Fields medal winners have been caught out by this. I was too until I wrote the above simulation.


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## PeterBassett (10 Aug 2010)

Vormulac":5snggh1t said:


> This isn't true; assuming your take-off speed is 200mph (yes, airspeed - or the speed of the air flowing over the wings creating lift), the wings don't care if the airflow creating the lift is being delivered by a head wind or by the forward motion of the aircraft, a 50mph headwind means the aircraft will only need to generate 150mph of forward speed in order to achieve the 200mph take off speed required. A tailwind on the other hand makes take off significantly more difficult to achieve as it has the opposite effect, additional forward speed is needed by the aircraft to overcome it.



Correct. Did you actually read my post? This is exactly what it says. I even capitalised *AIRspeed*


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## bugbear (10 Aug 2010)

Mr G Rimsdale":1huyxfa6 said:


> Er - surely common sense _does_ provide the answer?
> Clearly the first choice has odds against being the car; is more likely to be goat. The second choice (after removing a goat) has even odds, equal likelihood and so is the better bet.
> S'obvious!



http://en.wikipedia.org/wiki/Monty_Hall ... _not_1.2F2


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## Vormulac (10 Aug 2010)

Sportique":3lf52md6 said:


> Wrong - your "forward motion" above is *ground speed*, you are overlooking the fact that aircraft fly by virtue of their *AIR* speed not gound speed. It is accepted that, in this case, because the "ground" is moving with the aircraft then there will not be any ground speed, but the thrust of the engines will always generate AIR speed (regardless of how quickly or slowly the wheels are/not turning!) - unless the aircraft is tethered and this one is not tethered.
> 
> Dave



Forgive my rather light-hearted example here, but surely then F-14s launching from aircraft carriers would spontaneously leap straight up in the air the moment their engines are pushed to maximum because the engines are 'generating airspeed'... hmm... I think they are actually given a huge dose of forward movement by a steam catapult. Engines produce thrust which drives the aircraft forward, this forward motion creates airspeed and lift, the engines do not of themselves produce lift.


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## Vormulac (10 Aug 2010)

PeterBassett":3fmi502x said:


> Vormulac":3fmi502x said:
> 
> 
> > This isn't true; assuming your take-off speed is 200mph (yes, airspeed - or the speed of the air flowing over the wings creating lift), the wings don't care if the airflow creating the lift is being delivered by a head wind or by the forward motion of the aircraft, a 50mph headwind means the aircraft will only need to generate 150mph of forward speed in order to achieve the 200mph take off speed required. A tailwind on the other hand makes take off significantly more difficult to achieve as it has the opposite effect, additional forward speed is needed by the aircraft to overcome it.
> ...



Sorry, you're absolutely right, you were talking about the speed however it is achieved, I read it rather quickly and got the wrong end of the stick. Carry on!


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## Jacob (10 Aug 2010)

bugbear":1q54k888 said:


> Mr G Rimsdale":1q54k888 said:
> 
> 
> > Er - surely common sense _does_ provide the answer?
> ...


Right. I was beginning to wonder how the odds could be lower for one (1st choice) but evens for the other (2nd choice) as common sense also says they should be higher, if the other is lower!


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## bugbear (10 Aug 2010)

Mr G Rimsdale":3oasq2fm said:


> bugbear":3oasq2fm said:
> 
> 
> > Mr G Rimsdale":3oasq2fm said:
> ...



There are gambling games where the house relies for its profit on the actual odds being a good deal worse for the punter than the "common sense" odds.

These games are, or were, normally offered on street corners with lookouts...

Another probability example, where the maths is fairly easy to follow, is the Birthday Paradox

http://en.wikipedia.org/wiki/Birthday_problem

It's not really a paradox, BTW, but it is surprising.

BugBear


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## big soft moose (10 Aug 2010)

bugbear":3vx4tv7m said:


> ...where the maths is fairly easy to follow,








hmmm so thats fairly easy to follow ...........................so long as you are a mathematical genius


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## Anonymous (10 Aug 2010)

my answer back from a physics professor, unfortunately not professor hawking.

my question:

if a plane is sitting on a rather large conveyor belt that is ready to take off, but as thrust is applied the conveyor counteracts the motion of the planes wheels, regardless of thrust imposed, will the plane take off.

Answer

if the conveyor ensues that the plane essentially stays still (which it does) there will be no airflow over the wings, its the flow of the air over the wings that gives it lift. No flow, No lift, No take off.

I am no longer going to argue this point regardless of the experiment that was conducted as it was inconclusive due to the (un)scientific conditions imposed.


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## bugbear (10 Aug 2010)

big soft moose":2nyr8whu said:


> bugbear":2nyr8whu said:
> 
> 
> > ...where the maths is fairly easy to follow,
> ...



You're too kind ;-)

I googled around, and this was the best expressed explanation I found:



web":2nyr8whu said:


> It turns out to be easier to compute the probability that no two people at the party have the same birthday, and then subtract the answer from 1 to obtain the probability that two people will share a birthday. For simplicity, let's ignore leap years. Thus, there are 365 possible birthdays to consider.
> 
> Imagine the people entering the room one-by-one. When the second person enters the room, there are 364 possible days for her to have a birthday that differs from the first person. So the probability that she will have a different birthday from the first person is 364/365. When the third person enters, there are 363 possibilities of him having a birthday different from both of the first two, so the probability that all three will have different birthdays is 364/365 x 363/365. When the fourth person enters, the probability of all four having different birthdays is 364/365 x 363/365 x 362/365. Continuing in this way, when 23 people are in the room, the probability of all of them having different birthdays is
> 
> ...



BugBear


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## Anonymous (10 Aug 2010)

bugbear":1dtgiq6h said:


> big soft moose":1dtgiq6h said:
> 
> 
> > bugbear":1dtgiq6h said:
> ...



The above maths is very easy to follow if you know what values p and n are.


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## PeterBassett (10 Aug 2010)

mark270981":3ghm3ekk said:


> if the conveyor ensues that the plane essentially stays still (which it does).



How? The conveyor *can't* stop the plane from moving.


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## RogerS (10 Aug 2010)

RogerS":1yw55m4p said:


> Mark
> 
> Are you winding us up?
> 
> ...



Mark...do you agree with the above?


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## Racers (10 Aug 2010)

Hi, Mark

Plane wheels aren't driven so they are free to turn, the plane pushes AIR backwards and since every action has an equal and opposite reaction the plane moves forward, and takes off. 


Pete


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## Dibs-h (10 Aug 2010)

Wow - this plane thing isn't half generating some typage! :lol:

IIRC - the shape of an aerofoil is such that with airflow over it, a pressure differential is created between the upper surface and the lower surface. The lower pressure area above and the higher pressure below - resulting in a net upwards force. At an airspeed above X the lift exceeds the craft's weight and obviously drag and the craft is airborne. No air over the wing - no lift.

Now looking at the treadmill - if it is perfect, i.e. zero friction, any thrust provided by a jet engine, would entirely counteracted by the belt like treadmill - so where is the forward motion and air over the wings?

For a prop type plane - there would be some airflow over the wings, generating some lift, but it would be hard to say whether this would have any significant affect, i.e. whether you need an infinite long treadmill for the air over the wing & lift to get the plane airborne. But zero friction is one thing, infinite treadmill & infinite time to launch are probably equatable to never.

Don't mean to teach anyone to suck eggs - just coming at it from a basic principles thing.

The plane on a gantry\wires things isn't analagous.

Dibs

edit - found this very interesting, http://www.airplaneonatreadmill.com/


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## Anonymous (10 Aug 2010)

nooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.

i can't take it anymore!!!!

i don't know what it is you don't get?

I have had my answer clarified by a physics major, a professor someone who does this kind of thing for a living.

and you are still arguing the point.

the conveyor counteracts thrust through motion in the wheels of the plane. the point you are missing here is the wheels are the major effect.

if the wheels were braked the plane would take off as the plane would skid across the conveyor in order to take off.

But the wheels are free.

If you are running on a treadmill you don't go anywhere do you if the speed is set correctly. 

the propeller or engine of the plane transfers its energy into the wheels (whilst it is in contact with the ground) to get it moving forward, but if the conveyor matches this energy newton for newton the wheels of the plane just spin in the opposite direction equally as fast so the plane cannot go anywhere!!!

I don't understand what is so hard to understand about this?

I am not arguing for the sake of it. 

How much more clarification do we need? I have response from a physics major, I know i'll get in touch with NASA maybe they will shed light on it?


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## PeterBassett (10 Aug 2010)

the propeller or engine of the plane transfers its energy into the wheels (whilst it is in contact with the ground) to get it moving forward.

*NO IT DOESN'T!*

It isn't a car. It doesn't power its wheels. It is a plane. It pushes on the air.


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## bugbear (10 Aug 2010)

mark270981":3dv90ct1 said:


> the conveyor counteracts thrust through motion in the wheels of the plane. the point you are missing here is the wheels are the major effect.



But assuming the bearings in the wheels are reasonable, you can do what you like (via a conveyor, if you wish) to the rims, and no backwards/forwards force will be exerted on the axle.

You'll just spin the wheels.

Now, if we were talking about a (normal) car, which moves forwards by pushing against the ground with its wheels, yes, the conveyor would do all sorts of things.

But all the ground does to an aircraft is hold it up.

In the language of vectors; due to the bearings in the wheels, any force the conveyor applies via the wheels must have a 0 horizontal component.

BugBear


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## PeterBassett (10 Aug 2010)

bugbear":2g80uud9 said:


> In the language of vectors; due to the bearings in the wheels, any force the conveyor applies via the wheels must have a 0 horizontal component.



Bing!


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## Dibs-h (10 Aug 2010)

PeterBassett":cj5bkj4a said:


> bugbear":cj5bkj4a said:
> 
> 
> > In the language of vectors; due to the bearings in the wheels, any force the conveyor applies via the wheels must have a 0 horizontal component.
> ...



Yes I think the interaction between the planes wheel and belt is key - the more I read, and as Bugbear has said - no matter what speed you set the belt to, all it would do is spin the planes wheel faster and faster, i.e. zero horizontal force component on the "leg".

As stated in the article I linked to - if the plane doesn't move, i.e. no air over the wings, it won't lift off. But the bit above has got me wondering, in that if all the conveyor does is spin the wheel of the plane - effectively keeping it stationary - the application of thrust to the plane should push the plane forward, and effectively airborne.

It's this interaction between wheel & belt that's the awkward part.

Just my 2c worth.

Dibs


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## Racers (10 Aug 2010)

Hi. Mark

Planes that land on snow and ice have skids, so how thay take off, or sea planes they don't have wheels when they use water.

Pete


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## RogerS (10 Aug 2010)

Mark...your physics major is wrong. Sorry to tell it how it is !

You still haven't said if you agree with my dangling-plane analogy.


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## PeterBassett (10 Aug 2010)

What about a sea plane taking off upstream on a river.

It can't possibly fly. Physics say so! :lol:


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## Racers (10 Aug 2010)

Hi,

Dont forget the earth is spinning at 1000mph at the equator, so you will have to go faster than that to take off :wink: :wink: 

Pete


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## PeterBassett (10 Aug 2010)

I hadn't thought of that. Ah well, back to the drawing board... :lol: 

Anyway, everyone knows that planes fly from the combined will of the people on board. All the physics stuff has been made up by the military-industrial complex.


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## RogerS (10 Aug 2010)

PeterBassett":ozsppqhe said:


> I hadn't thought of that. Ah well, back to the drawing board... :lol:
> 
> Anyway, everyone knows that planes fly from the combined will of the people on board. All the physics stuff has been made up by the military-industrial complex.



You are absolutely right, Peter. Here's the proof.

Picture a 4 seater Cessna. Picture the size of the wings. Cessna flies at, what, 120 knots? OK. Now picture a jumbo. Flying speed - say 480 knots. OK...so that's 4x the number of passengers. 16 now. Now a jumbo can carry 416 passengers. Which means that the wings need to be 416/16 times larger than a cessna's. ie 26x the area of a Cessna's wings. Are they? I don't think so. Maybe 10x. And that's before we factor in all the cargo and luggage!

Ergo....as PB has said..it's only the will of the passengers that keeps them flying.


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## PeterBassett (10 Aug 2010)

I've changed my mind.

Rabid telepathic spider monkeys.

Aeroplanes are powered by the forced telepathic intervention of rabid spider monkeys.

The plane doesn't fly at all. That would be ridiculous.

The monkeys are whipped until they achieve a zen like state that enables them to make the passengers think they are in the air.

Then the passengers and their cargo are simply beamed to their destination airport via a star trek like transporter.

Thats why airports are so large. Giant underground spider monkey farms and vast whipping arenas.

Why haven't I realised this before?! It's so simple!


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## Anonymous (10 Aug 2010)

RogerS":1bj4ep4r said:


> Mark
> 
> Are you winding us up?
> 
> ...



yes i do agree, but that isn't the case here at all!!



PeterBassett":1bj4ep4r said:


> mark270981":1bj4ep4r said:
> 
> 
> > if the conveyor ensues that the plane essentially stays still (which it does).
> ...



but the wheels do, thats the whole point.

lololol this is so comical, whilst the wheels which are attached to the plane in which have a downward force onto the conveyor have the major effect on this puzzler.

You have to add the wheels into the equation as they are there and in contact with the conveyor.

i know you are all trolling with me - come you can pack it in now!!


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## Racers (10 Aug 2010)

Hi, Mark

Hold a wheel by the axel on a running belt sander and tell my how much force you can feel.

Pete


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## PeterBassett (10 Aug 2010)

mark270981":3rho15z9 said:


> i know you are all trolling with me - come you can pack it in now!!



Ah, it's only in jest. :wink: 

Seriously, Mark, can you explain, in your own words how the conveyor belt would stop the plane from accelerating forwards from the force applied by its prop/jet?


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## Anonymous (10 Aug 2010)

i understand your concept and i very nearly agree with it pete.

ok i'll try this one.

your in a sail boat and the wind is blowing you forward and the current (water) flowing in the exact opposite at the same force the wind is excting on your sails.

Do you move forward?


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## PeterBassett (10 Aug 2010)

Unfortunatley that is not the same system. The drag on the boat from the flow of water is large and well coupled.

The drag on the body of the aircraft from the moving conveyor is decoupled by the wheels and their bearings. 

That is the purpose of the wheels. Do you see the difference there?


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## Anonymous (10 Aug 2010)

but the wheels have the weight of the plane pushing them down.


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## Racers (10 Aug 2010)

Hi, Mark

If the forces are cancling each other out then you will remain stationary.

But the friction in the wheel bearings of the plane are very small.

Pete


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## PeterBassett (10 Aug 2010)

mark270981":34kkx79b said:


> but the wheels have the weight of the plane pushing them down.



I don't follow. This is the case when a plane takes off as normal from an airfield.

The wheels are free to rotate, that is their purpose.


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## Anonymous (10 Aug 2010)

i totally see where you are all coming from.

but you are forgetting the weight of the plane on the wheels its a downward pressure.


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## PeterBassett (10 Aug 2010)

mark270981":27otn3zo said:


> i totally see where you are all coming from.
> 
> but you are forgetting the weight of the plane on the wheels its a downward pressure.



You are going to have to explain how the weight of the aircraft stops it from rolling around on its wheels.


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## Anonymous (10 Aug 2010)

thats the whole point, the plane hasn't taken off, its very in conact with rolling ground.

If the plane was weightless then i would totally agree it would take off, but it isn't regardless of how free the wheel bearings are.


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## Racers (10 Aug 2010)

Hi, Mark

To cause drag on the plane there would need to be friction in the bearings causing heat. 



Pete


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## bugbear (10 Aug 2010)

mark270981":1ww3dvfl said:


> i totally see where you are all coming from.
> 
> but you are forgetting the weight of the plane on the wheels its a downward pressure.



You can press a roller skate down onto a belt sander as HARD as you like, and all that happens is the wheels go round.

BugBear


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## PeterBassett (10 Aug 2010)

Ok, now you agree that the wheels allow the the aircraft to roll freely on the conveyor, to move along it, just turn the engine on and accellerate to take off airspeed.

The conveyor is as long as a normal runway. Perhaps that has been missing in your thoughts?


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## Anonymous (10 Aug 2010)

tell ye what.

tomorrow before i get the workshop i will buy a matchbox car with free wheeling wheels and push it over a beltsander, i am not sure how i will do it in the interests of science ie weight the car down with a brick or something.

and i'll see what happens.


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## Dibs-h (10 Aug 2010)

mark270981":hwx2x2rg said:


> thats the whole point, the plane hasn't taken off, its very in conact with rolling ground.
> 
> If the plane was weightless then i would totally agree it would take off, but it isn't regardless of how free the wheel bearings are.



The crux of it (for me anyway) is that such a wheel on a moving treadmill - the "leg" doesn't experience much if any in the way of horizontal force. The faster the treadmill - the faster the wheel spins. When you apply thrust to the plane - it will move forward.

I think if you held a small wheel on a treadmill (or a belt sander ) you'd experince for yourself that the horizontal component of force is virtually negligible. It doesn't increase with belt speed.

This very "conundrum" has resulted in some right argi-bargies on internet forums, so I'll bow out & I think we've done rather well. :lol:

Dibs


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## PeterBassett (10 Aug 2010)

mark270981":2d7afkmm said:


> tell ye what.
> 
> tomorrow before i get the workshop i will buy a matchbox car with free wheeling wheels and push it over a beltsander, i am not sure how i will do it in the interests of science ie weight the car down with a brick or something.
> 
> and i'll see what happens.



Please read what I wrote. You are so close to getting it.



Pete":2d7afkmm said:


> Ok, now you agree that the wheels allow the the aircraft to roll freely on the conveyor, to move along it, just turn the engine on and accellerate to take off airspeed.
> *The conveyor is as long as a normal runway.*


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## Anonymous (10 Aug 2010)

Honestly i really do get what you say, i just don't agree with you, (for now).


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## PeterBassett (10 Aug 2010)

mark270981":1nldhnaf said:


> Honestly i really do get what you say, i just don't agree with you, (for now).



I'll leave it there then.


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## RogerS (10 Aug 2010)

So Mark if you agree that a dangled plane will move forward then the wheels on the trolley are free-wheeling? Yes? 

Same as the wheels under the plane...they free-wheel. They couldn't care less if the treadmill under them is stationary, moving forward or moving backward. The rotation of the wheels is irrelevant. 

If the weight of a plane was an issue then as soon as the plane touched down, the wheels would lock solid according to your line of reasoning. Clouds of blue smoke! But that doesn't happen. There's a quick puff of smoke as the wheels are not rotating when the plane touches the ground. Then they start rolling and then the plane comes to a stop.

If weight was an issue then your car wouldn't free-wheel down a hill.


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## bugbear (10 Aug 2010)

PeterBassett":1oyq0ixh said:


> RogerS":1oyq0ixh said:
> 
> 
> > I like that one, Richard.
> ...



I really couldn't believe you when you wrote this. I apologise for doubting you!

Wow.

BugBear


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## PeterBassett (10 Aug 2010)

I've seen it before and I couldn't stop it! We truly are destined to repeat history. 


A lot of it was my fault for not taking no for an answer...  

Sorry everyone.


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## RogerS (10 Aug 2010)

Don't apologise...this is fun!


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## Benchwayze (10 Aug 2010)

Pete is right. 

If you run fast enough, you can run up a downward moving escalator. (And vice versa) 

So it boils down to how much power you can apply I guess. 

Right? 


If a lift is falling down a shaft out of control, could you save yourself by jumping up wards, just before it hits the buffers? 

Serious answers please. and there's one answer I am looking for! 


Hat-Coat-Door! :lol: :lol:


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## PeterBassett (10 Aug 2010)

Theoretically yes, practically no.

You would have to cancel out the downward velocity completely with the force of your jump.

Dependant on how long the lift had been falling, that could be a very high speed.

If you could generate enought force to jump with that velocity at just the right time you could survive.

However if you had muscles that powerfull (you dont) you would just break your legs with the force of the jump.

Superman could do it.


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## SBJ (10 Aug 2010)

I thinnk mythbusters did the lift thing too. If i remember correctly, the advice if you find yourself in that situation, was to lay on a fattie.


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## PeterBassett (10 Aug 2010)

SBJ":byasf2t5 said:


> if you find yourself in that situation, was to lay on a fattie.



As with so many other situations...

I'll get me coat.


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## big soft moose (10 Aug 2010)

Benchwayze":3p2y6zmf said:


> If a lift is falling down a shaft out of control, could you save yourself by jumping up wards, just before it hits the buffers?
> 
> Serious answers please. and there's one answer I am looking for!
> 
> ...



no , because you and everything else inside the lift are travelling at the same velocity - therefore all you will achieve by being inthe air in the lift when it hits the ground is to hit the floor a couple of seconds later at the same velocity.

lying on padding or a fat gett might help a bit but realistically you are pineappled whatever you do.


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## RogerS (10 Aug 2010)

big soft moose":14bcnrmn said:


> Benchwayze":14bcnrmn said:
> 
> 
> > If a lift is falling down a shaft out of control, could you save yourself by jumping up wards, just before it hits the buffers?
> ...



You haven't seen the size of my sister!


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## Karl (10 Aug 2010)

big soft moose":1ijlu6qk said:


> lying on padding or a fat gett might help a bit but realistically you are pineappled whatever you do.



Not so - did you not hear of the skydiver who had a first timer strapped to his back? Main parachute failed, as did the emergency. He deliberately rolled in the air so that he hit the ground first. He died, first timer survived.

Cheers

Karl


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## Benchwayze (10 Aug 2010)

Well yes... 
But the answer I was looking for was:






































When you jump upwards, make sure you are on the stairs.


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## Tom K (11 Aug 2010)

So what is the final conclusion on the plane thing? 
As I see it a Harrier jet requires a couple of ground crew to give the wheels a spin to make it move forwards after vertical take off is that right?


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## RogerS (11 Aug 2010)

Tom K":3ejv3cdp said:


> So what is the final conclusion on the plane thing?
> As I see it a Harrier jet requires a couple of ground crew to give the wheels a spin to make it move forwards after vertical take off is that right?



It takes off. And the ground crew on the Harrier get incinerated.


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## Anonymous (11 Aug 2010)

considering the fact that we have roughly 7 billion people on this earth and comes with that all the machinery houses etc etc.

Is the earth heavier, lighter or the same weight than what it was 200 years ago?


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## big soft moose (11 Aug 2010)

mark270981":lx72ytkj said:


> considering the fact that we have roughly 7 billion people on this earth and comes with that all the machinery houses etc etc.
> 
> Is the earth heavier, lighter or the same weight than what it was 200 years ago?



the planet its self is probably slightly lighter because although the population has increased the food to make the peoples bodies came from the earth (principally), as did the materials to build houses machinery etc.

however in doing all this we have also mined and burned a lot of mineral resources some of which have thus realsed to the atmosphere as gas.

However the weight of the earth in toto , including the atmospheric ball will be mostly unchanged


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## Racers (11 Aug 2010)

Hi,

Well I suppose all the matter that has been burnt and turned into heat and radiated out in to space makes the earth lighter.

Pete


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## bugbear (11 Aug 2010)

Benchwayze":1xofy4jz said:


> Pete is right.
> 
> If you run fast enough, you can run up a downward moving escalator. (And vice versa)
> 
> ...



Not in the case of the plane, no. 

The conveyor CANNOT apply a force to the aircraft - it just spins the wheels.

Meanwhile the aircraft "grips" (so to speak) the air (using propellor or jet) and pulls itself along, and takes off.

The normal case of a treadmill (where you propel yourself by working against the treadmill surface) creates such a strong image in your mind that it's hard to shake it.

BugBear


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## bugbear (11 Aug 2010)

mark270981":yunae9br said:


> considering the fact that we have roughly 7 billion people on this earth and comes with that all the machinery houses etc etc.
> 
> Is the earth heavier, lighter or the same weight than what it was 200 years ago?



Heavier, from accumulated metorites, I think.

The only thing leaving the earth would be some atmosphere. Radiation of EM particles ain't gonna be a factor against those.

Man's activities just shuffle the arrangement of atoms that are already here.

BugBear


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## bugbear (11 Aug 2010)

Racers":182ajz7d said:


> Hi,
> 
> Well I suppose all the matter that has been burnt and turned into heat and radiated out in to space makes the earth lighter.
> 
> Pete



Thankfully very little matter has been turned into heat. That requires a process that is ... quite unusual on the Earth.

I think this thread has thrown some of the confident scientific assertions and opinions in the global warming discussions into quite an interesting light.

BugBear


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## PeterBassett (11 Aug 2010)

Yes, the earth as a total system is heavier and getting heavier. But not by much as a percentage of the total. 

We lose light elements from the atmosphere (helium mostly, hydrogen is too reactive to escape) and radioactive elements lose mass when they decay. We have put a few thousand tons of stuff into space over 50 years. 

All of that is dwarfed by the incoming mass from micro meteorites.

Also space aliens. :shock:


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## RogerS (11 Aug 2010)

Racers":3piuuqno said:


> Hi,
> 
> Well I suppose all the matter that has been burnt and turned into heat and radiated out in to space makes the earth lighter.
> 
> Pete



The earth is considerably lighter as a result of losing all the hot air that this forum generates :lol:


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## bugbear (11 Aug 2010)

RogerS":1uqh4ip2 said:


> Racers":1uqh4ip2 said:
> 
> 
> > Hi,
> ...



No - that's continuously recycled. Round and round and round and round. Very eco-friendly.

BugBear


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## studders (11 Aug 2010)

We have a Big Toe...

Why do we have a Thumb and not a 'Big Finger' ?


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## Tom K (11 Aug 2010)

Coz a thumb and a finger are very different things.


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## Noel (11 Aug 2010)

I'm surprised the old chestnut of the bullet being fired on the plane (not on a treadmill) hasn't come up yet.


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## PeterBassett (11 Aug 2010)

What part of a moving train is always going backwards?


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## RogerS (11 Aug 2010)

PeterBassett":2tfda0xz said:


> What part of a moving train is always going backwards?



The back of the wheels just after they leave the track?


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## big soft moose (11 Aug 2010)

If a tree falls in a forest with no one there to hear it , does it make a sound ? :lol:


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## Anonymous (11 Aug 2010)

i reckon the answer is the

same minus all the satellites and anything else we have thrown into space.

right i haven't done my experiment yet, however i am to change my stance and say the plane takes off.

NOW NOEL cooey over here can we delete this thread like we spoke about whilst i eat my humble pie.


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## Noel (11 Aug 2010)

mark270981":1urplzyg said:


> i reckon the answer is the
> 
> same minus all the satellites and anything else we have thrown into space.
> 
> ...



Sure, no problem............


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## Tom K (12 Aug 2010)

Noel":2qix0xzr said:


> mark270981":2qix0xzr said:
> 
> 
> > i reckon the answer is the
> ...



Surely quoting the humble pie comment is like rubbing his nose in it?


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## Anonymous (12 Aug 2010)

Tom K":8t52ihh1 said:


> Noel":8t52ihh1 said:
> 
> 
> > mark270981":8t52ihh1 said:
> ...



Not really that bothered Tom its just a forum, sometimes its just easier to admit you're wrong, its good banter at best - watch

THE PLANE TAKES OFF

THE PLANE TAKES OFF

THE PLANE TAKES OFF

THE PLANE TAKES OFF

THE PLANE TAKES OFF

THE PLANE TAKES OFF


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## PeterBassett (12 Aug 2010)

RogerS":2n0jvayc said:


> PeterBassett":2n0jvayc said:
> 
> 
> > What part of a moving train is always going backwards?
> ...



If I read you right, then no.


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## RogerS (12 Aug 2010)

Am I warm?


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## Noel (12 Aug 2010)

I'd rather not find out...........


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## Noel (12 Aug 2010)

Tom K":w105ewet said:


> Noel":w105ewet said:
> 
> 
> > mark270981":w105ewet said:
> ...



Hope so!


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## bugbear (12 Aug 2010)

PeterBassett":12afzh8g said:


> What part of a moving train is always going backwards?



There is no single part of a train which is continuously doing that.

However, whatever piece of the flange is at the bottom of the wheel is momentarily going backwards.

BugBear


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## RogerS (12 Aug 2010)

bugbear":1s1ojw78 said:


> PeterBassett":1s1ojw78 said:
> 
> 
> > What part of a moving train is always going backwards?
> ...



That's what I said..only you put it more eloquently! and he said No.


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## PeterBassett (12 Aug 2010)

"The back of the wheels just after they leave the track?"

I didn't read that as flange, and the flang is moving backwars at any point lower than the level of the track, not just when it leaves the track.

BugBear and RogerS are correct.


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## Jake (12 Aug 2010)

mark270981":1cegl1jl said:


> right i haven't done my experiment yet, however i am to change my stance and say the plane takes off.
> 
> NOW NOEL cooey over here can we delete this thread like we spoke about whilst i eat my humble pie.



No, and I'm not letting you delete this either.


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## big soft moose (12 Aug 2010)

PeterBassett":1l46yopg said:


> RogerS":1l46yopg said:
> 
> 
> > PeterBassett":1l46yopg said:
> ...



the locomotive at the rear ? - modern trains usually have a loco on the back end facing backwards to facilitate a quick turn round


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