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Coffee House => Coffee House Boards => CH / Science and Technology => Topic started by: a1200 on February 26, 2006, 02:45:21 AM
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Guys, I have a thread on Amigaworld that needs more brains than ever before:
Here (http://amigaworld.net/modules/newbb/viewtopic.php?topic_id=17861&forum=4) is the thread. It asks the question:
"A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?"
Please comment on here and Amigaworld if you have any insight to this. It might be harder than answering "if the universe is expanding, what's beyond the universe?" question! Well maybe not but it is driving me potty. What is the answer?!?!
When you have made a decision, there is a poll here. (http://amigaworld.net/modules/newbb/viewtopic.php?topic_id=17885&forum=4)
Edit: Title had spelling mistake :roll:
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Since the determinig factor for lift-off is the speed of the plane versus that of the surrounding air, the plane cannot take off, as it is standing still with respect to said air. Question solved.
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I'm not welcome at the Local site.
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I think Cymric is right. The plane needs uplift from the surrounding air as the air passes past the wings.
If the air isn't passing the wings (just staying still) there's no updraft.
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The plane can't take off under those circumstances.
Not going to rejoin AW for a poll though.
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Hum,
AOrg users are too smart....
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The problem is worded perfectly adequately, if one assumes a frame of reference of a point which is static at v=0:
The problem states that the runway moves with the same speed as the plane - if the plane is stationary the runway is also stationary.
The problem DOES NOT state that the runway applies an equal opposing force to the plane as that exerted by the plane engines on the surrounding air.
In the absence of a paint program and the use of algebraic symbols I'm not gonna start drawing free body diagrams, scanning them in, and writing the force-balance equations, but the upshot is:
There is an imbalance of forces - the plane accelerates, the plane takes off.
Step-by-step:
1) The plane begins to move - let's assume it moves at v=30m/s
2) The ground moves in an opposite direction relative to the plane at v=-30m/s.
3) The wheels are free to rotate about their bearings.
4) The wheels rotate as if the plane were moving at 60m/s, ie the planes velocity relative to the moving runway.
5) The plane doesn't give a crap how fast either the ground is moving or the wheels are rotating (Neglecting friction at the bearings - which, because they are bearings is negligible) - the only thing which concerns the plane is its own velocity relative to the air around it which (in simplified terms) creates lift.
6) The only thing the motion of the ground will accomplish is making the wheels rotate faster, and the bearings produce twice as much heat as normal due to friction.
7) The plane will take off.
Phewww! 4 years of studying mechanical engineering, and it seems some of it sticks after all.
Any arguement and I get the algebra out......
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Boot_WB wrote:
5) The only thing the motion of the ground will accomplish is making the wheels rotate faster.
6) The plane will take off.
I trust you meant to insert a 'not' in there somewhere ;-).
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@Cymric
No, but I edited my post, as I realised I couldn't count to 7 after writing the first draft :-D
The problem states the ground moves at the same speed as the plane - if the plane is still, the ground is still - if the plane moves a 5m/s the ground moves at -5m/s - the interface between the ground and plane (ie the wheels) measures their RELATIVE velocity, but the absolute velocity of the plane (in this case assumed relative to the surrounding air) remains unchanged by the ground motion.
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Unfortunately for you, your reasoning is correct up until point 7. The plane does not take off, because there is no force imbalance. There is simply no net force in the vertical direction (a.k.a. lift), so the plane does not move upwards. That is the job of wings, alerons, flaps and slats: transform the horizontal motion of air passing over them into a vertical force which balances gravity, or exceeds it slightly (in which case Newton's Second Law stipulates an accelaration in the upward direction), or is slightly smaller (in which case there is accelaration in the downward direction).
That is also why loss of lift results in such catastrophic accidents. The plane still has lots of horizontal velocity relative to the air, but the areas which are responsible for lift don't do their job any more. The plane is then literally a brick due to impact on the ground within (I think) at most two or three minutes.
Later edit: Ack, ick, phooey. I never got the hang of the differences in torques in freely rotating bodies, or those which were powered to do so, when both are presented with a solid surface. Of course I am completely and utterly WRONG: The plane WILL take off. It's the same as when an airplane would be taking off from a completely iced-over runway. The wheels would just slip over the ice (and in fact, nearly stand still) so you'd lack the ability to steer unless you throttle the engines separately. But take off you will. The morale of the story: don't put a soapbox race car with JAT engine on those rolls used to test car performance in the lab, because you'll create one heck of a hole in the wall...
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Cymric wrote:
Since the determinig factor for lift-off is the speed of the plane versus that of the surrounding air, the plane cannot take off, as it is standing still with respect to said air. Question solved.
But unless the conveyor belt can produce an equal but opposite force to the plane ,it won't be standing still with respect to the surrounding air.
The only interface between the belt and the airframe are the wheels, which aren't powered and are free to rotate at any speed they like (well, upto breaking point), so I fail to see how such an opposing force can be exerted.
Anyway, time for BootWB to break out the algebra, and I'll put some popcorn on.
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In which case it would be moving relative to the belt, which seems to negate the original problem.
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the answer av web.net (http://www.avweb.com/news/columns/191034-1.html)
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Planes are not propelled through their wheels, so the speed of the conveyor belt is irrelevant. The plane will take off.
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Hum,
If we assume that there is air and that the aircraft uses jets (or the propellers do not push air past the wings) the airspeed relative to the aircraft is zero.
The plane cannot takeoff.... (the answer in av web.net is wrong!)
You canna change the laws of physics.
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People are getting confused, but an airplane is not a car - the thrust doesn't come from the wheels. Took me less than a second to decide the correct answer is yes, the jet takes off.
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Lol.
Took me less than a second to decide the correct answer is No.
It’s simple physics.
There is no forward motion. (It does not matter if it comes form wheels, jets, magnets or rubber bands etc)
No forward motion = no air flow = no lift...
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blobrana wrote:
Lol.
Took me less than a second to decide the correct answer is No.
It’s simple physics.
There is no forward motion. (It does not matter if it comes form wheels, jets, magnets or rubber bands etc)
No forward motion = no air flow = no lift...
Wow. You flunked physics, obviously. A plane's motion is due to thrust from the engine/propellor against the frame, not from the motion of the wheels.
The jet engine applies force to the plane, which starts to move. The conveyor belt starts to move the opposite direction at the same speed, but all it's doing it TURNING THE WHEELS! The wheels have NO EFFECT on the speed of the plane (assuming the brake isn't set). The jet engine keeps thrusting the plane forward faster while the conveyor belt keeps turning the wheels faster WITH NO EFFECT ON THE FORWARD MOTION OF THE PLANE! The plane reaches take off speed and takes off.
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Hum,
whoops...
thinking about it further...the plane does move forward...
Assuming that the wheels have no or little friction.
And if it moves forward then there is air flow.
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I think all forums (this one, aw.net & others) are now concluding that the plane WILL TAKE OFF! This has been awesome and I am in agreement now the plane is outta there!
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Hmmmn, I misread the question, I could of sworn it said stationary in there.
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LOL
me too...
My first thought was to add up the two (opposite) vectors to give zero speed.
But the ground vector is a red herring, there is only one vector (the plane)
Doh!
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That's what made me pause a second when I first read it. It's deliberately misleading in an effort to make you think about it in the wrong manner to provoke an incorrect answer. They state the problem in a manner to make people think about a jogger on a treadmill whose net velocity with respect to the ground is 0. That's not what it is, though. This problem would be more like a jogger on a treadmill wearing a jet pack and roller skates. The treadmill would simply turn the wheels of the roller skates while the jet pack blasted the person forward irrespective of the treadmill's speed. :-D :lol:
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Hum,
i hate
(http://static.flickr.com/37/105418464_631420af42_o.gif)
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The plane will take off because it's a harrier jump jet.
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lol
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:lol:
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The plane takes off, and it takes off slightly sooner than it otherwise would have.
The runway will go backwards as fast as the plane moves forwards. The vast length of the moving runway will naturally entrain some air and will drag said air past the plane thus slightly increasing the planes airspeed over what it would have been with a static rnuway.
The runway can only move backwards if the plane is moving forwards.
Imagine the question is asked but the plane does not touch the ground. The plane flies over the runway at 600 km/h. The runway moves backwards at 600 km/h. Does the plane fall out of the sky? Of course not. Wheels down, wheels up, ground speed is irrelevant.
Now, imagine the same question but with a car and ask does the car move? Or just as much fun, does the road move?
Imagine that the car starts to move at 1 km/h. The road starts to move backwards at 1 km/h. Now the car is spinning it's wheels to stay in one place. But now it's velocity is zero. Ergo the runway matches that and the car starts to move. So the road ...
etc
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I believe I can state this better:
its all down to a pressure difference between the air above the wing, and beneath the wing - this is the bernoulli principle: the shape of the wing is such that the topside path is longer than the underside path. the airflow over the top is therefore travelling faster to get past the wing than the air under, so the air pressure is lower, causing lift. in order to best get this, the plane must be moving relative to the body of air. this is also why planes take off into the wind.
therefore, a 'conveyor belt' runway moving in a direction to negate the motion of the aircraft will prevent takeoff - except for this small omission: the wheels dont generate the motion, they just allow free motion: the drive is purely air-based - either a jet or a prop !
Helicopters do exactly the same thing, its just that the 'wings' rotate to produce this pressure difference.
Harriers just do VTO/L by sheer use of thrust. :-)
PS: that was all physics - I did a degree for 3 years, and still like to dabble.
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Now, imagine the same question but with a car and ask does the car move? Or just as much fun, does the road move?
err - how do you think rolling roads work ? those things uised by the automotive industry for measuring such things as power output, and the accuracy of speedometers ?
although I will concede the nitpick that rolling roads are just passive rollers, and not conveyer belts...
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this is for those that still think the plane won't take off
(reposted from a-world)
ok let's rephrase the question in terms of a car.
1 the non-powered front wheels are on the conveyor
2 the powered rear wheels are straddling the conveyer(not on the conveyer)
3 the conveyer is moving the same speed as the car
will the car reach the end of the conveyor or will the conveyor hold it in place? :-)
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The powered rear wheels are not on the conveyer, but the unpowered fronts are. They are unpowered, thus unable to rotate, therefore the conveyer is also stationary.
Because the powered rear wheels are straddling the conveyer (but not in contact with it), then they will either be supported on axle stands and thus the car will remain stationary, or if they are in contact with any kind of surface then friction would propel the car forward (provided that the rear wheels were fed power).
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Agafaster wrote:
Now, imagine the same question but with a car and ask does the car move? Or just as much fun, does the road move?
err - how do you think rolling roads work ? those things uised by the automotive industry for measuring such things as power output, and the accuracy of speedometers ?
If a rolling road is moving fast enough to counter 100% of the cars speed, how fast is the car going?
Take that speed and tell me the opposite of it.
The opposite of that speed is the speed of the conveyer.
Is the speed of the conveyer just calculated sufficient to nullify the wheelspeed of the car?
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So what if the plane was a James Bond style car-plane powered solely through the wheels?
:-D
And how does a helicopter do a loop-the-loop if its sole thrust is downwards and momentum is achieved diagonally?
Maybe it's better to wave your hands about instead of using your boffin equations...
;-)
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@Hyperspeed
If the driven wheels are on the conveyor, James Bond couldn't take off using wheel power alone (see my previous post).
Looping a helicopter depends on lots of things, including the design/stiffness of the rotor blades. Some 'copters can be looped, but others can't and would chop off the tail if it was tried.
Just Google for references on flying loops in helicopters.
Cheers,
JaX
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Here's something that puzzles me:
Is it only aerodynamic objects that heat up on re-entry to the Earth's atmosphere?
Surely a flat surface moving downwards with gravity would be subject to terminal velocity?
Why do Soyuz capsules get hot, surely the speed of air hitting them would slow them down, you don't see parachutists in flames...
... or does it all have to do with the electrically charged ionosphere?
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@Hyperspace
Any object entering the Earth's atmosphere from orbit or from anywhere else in space experiences frictional heating. This depends largely on the approach speed and shape of the object. There are other factors, but these are the main ones. The thin outer atmosphere provides enough friction to evaporate a dust particle travelling at 50 Km/sec, hence we see shooting stars. Objects presenting a large face to the atmosphere will heat up quickly, but they will also be slowed down quickly (hence the shape and composition of re-entry heat sheilds). An aerodynamically efficient shape (like a bullet for example) would not heat so quickly (other factors apart). But it would certainly heat up (like Concorde's nose cone).
If the plunging object has enough substance/mass to not burn up completely, it will eventually be slowed down by the denser lower atmosphere and it might reach terminal velocity before landing on the Earth. Hence we get meteorites.
We don't experience hot parachutes because the velocity is too small to generate enough frictional heating, but it does happen. The surface of the chute will get warmer by maybe a few thousandths of a degree, as do the soles of man/woman's shoes, but not enough to notice. Anyone free-falling from say 250,000 feet will eventually reach terminal velocity but will also feel pretty warm (mainly from the adrenalin I suspect!)
JaX
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Well I was going to mention this...
Not so long ago I went to some news article on the BBC website about a man that wanted to jump out of something like a balloon that was on the edge of space.
I remember it seemed very much like that game MDK...
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I remember seeing a docu about NASA or some other US agency doing just that to test astronaut suits for the space program.
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@odin
A couple of weeks ago, they released an old space suit from the International Space Station to see how long unheated batteries would work in orbit (I'd have thought NASA would know that already...?). The suit was supposed to stay in orbit while transmitting radio signals that anyone with a short wave radio could pick up. They published all the beep sequences so listeners could tell how it was getting along.
Anyway, the batteries packed up almost as soon as the 'SuitSat' was released. So, a bit of a damp squib all round. Anyway, the fully inflated suit is probably still out there, slowly losing altitude through weak atmospheric drag. It too will eventually fall with enough velocity to incinerate.
[EDIT] Here's NASA's take on it. (http://science.nasa.gov/headlines/y2006/26jan_suitsat.htm) Russian idea apparently.
[EDIT] Apparently, the batteries did last an orbit or two. HERE'S (http://wcarc.us/Suitesat/index.htm) someone's recordings! Well, whatever turns you on I suppose.
@Hyperspeed, is this what you're hearing...?
JaX
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by JaXanim:
@Hyperspeed, is this what you're hearing...?
No, I think it must be the spirit world calling me into the light...