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

[Tom K]

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:

 

[Jake]

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?

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

 

[PeterBassett]

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)

 

[Anonymous]

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.

 

[PeterBassett]

No, there really is an answer.

There really is.

 

[Sportique]

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

 

[Tom K]

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

 

[Benchwayze]

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

 

[Jacob]

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

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:

 

[Jacob]

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.

 

[Tom K]

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

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

 

[Racers]

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

 

[bugbear]

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

 

[Vormulac]

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.

 

[SBJ]

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

http://mythbustersresults.com/episode97

 

[RogerS]

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

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

 

[RogerS]

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.

 

[PeterBassett]

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

 

[bugbear]

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

Or the lorry load of budgies.

Oops. ;-)

BugBear

 

[PeterBassett]

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.