Oh my God! That's it! Guys, he has it, that's the answer.Quote:
Originally Posted by Mrthemike
Thanks dude, I can't believe that wasn't covered at all in those 6 pages of amateur physics and nerd hate that you conveniently skipped.
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Oh my God! That's it! Guys, he has it, that's the answer.Quote:
Originally Posted by Mrthemike
Thanks dude, I can't believe that wasn't covered at all in those 6 pages of amateur physics and nerd hate that you conveniently skipped.
Jesus fucking christ.
the treadmill won't even begin to move until the plane moves forwards, it's linked to the plane's forward movement, not the movement of the wheels.
there is no power being applied to the wheels, friction doesn't have anything to do with it, nothing will ever be moving at the speed of light. If the plane isn't moving forwards THROUGH THE AIR nothing will be moving. The entire fucking scenario will be static. we'll hear the fucking crickets.
why the hell would any of you think the wheels would be spinning and the plane wouldn't be moving? what would make the wheels spin? the treadmill? why is the treadmill spinning? because the plane is moving? but if the plane isn't moving, how will the treadmill, that's hooked up to the plane's speed, know that it's time to move -- with enough force through friction to counter the power of those jet engines?
back to the wheelchair/treadmill analogy -- if i'm pulling you forwards off a treadmill, and the treadmill senses your forward speed and counters it with an equal backwards speed, the wheels will turn twice as fast -- but i still won't have any problem pulling you off. IF the treadmill's turning is too much, and i stop moving you forwards, the treadmill will stop turning backwards, and i'll be able to pull you forwards again.
edit: speed of treadmill = speed of jetplane. if speed of treadmill = a billion gajillion million then the speed of the jetplane MUST equal a billion gajillion million. and i think we can all agree that a billion gajillion million is probably fast enough to take off. the wheels will be spinning at two billion gajillion million.
and FINALLY, even if it WAS a car -- forward motion isn't measured by how fast the wheels are spinning, that's just a tool we use to guess. when i'm in a snowbank, with four wheel drive, and all four wheels are spinning at 60 and i'm goin nowhere -- do you think i'm really going 60? (i think this is where most of you are hung up) if this fucking treadmill was hooked up to a car, the car would drive right off the fucking end. wheel speed = forward velocity + opposite velocity of the surface they're on. most of the time the opposite velocity is 0.
Ok, so I was too busy tuning my skis last night to write the proof, but here's the just of it:
Motion of the aircraft as dictated by newton's laws:Quote:
Originally Posted by Newton's laws
F of engine thrust ACTING UPON STATIONARY AIR = 95,000 lbs/engine
Total thrust = 95,000 * 4 = 380,000 lbs
F of rolling friction from the conveyor acting on wheels = u/radius of wheel * m*g = u/2 ft * 910,000 lbs
(assuming 4' diameter wheels, couldn't find actual size in google)
NOTE THAT ROLLING FRICTION IS INDEPENDENT OF WHEEL SPEED
If thrust is greater than rolling friction, the plane will have an unbalanced force (read: plane will be accelerating reltative to air) acting upon it, causing it to move forward down the runway, giving it airflow and lift. It is simple to verify that for the plane to be held still
u/2 * 910,000 lbs = 380,000 lbs
u = .835164 !!!!!
That is 83% of the force applied to a 747 is lost to rolling friction. Imagine being on a bike and only getting 17% of your energy put into forward progress. (A side note, sliding friction of rubber tires is close to .85, the 747 could in theory take off on a regular runway provided it's long enough, with it's wheels completely locked up!) In the real world, rolling friction losses are closer to 0.5%. The plane will accelerate reltative to the stationary air due to an unbalanced force between the engine thrust from the plane acting on the air and the force of friction due to the conveyor moving at any speed.
The conveyor cannot stop the 747. Bow to its might.
http://upload.wikimedia.org/wikipedi...g.taxi.arp.jpg
NOOOOO!!!! Let this thread die
http://forums.flightinfo.com/showthread.php?t=66860
just read this..... it's up to 32 pages.....
fucking engineers
DING DING DING DING DINGQuote:
Originally Posted by shmerham
edit cj001f = pwned
I am impervious to the psyops of the people who can't read :fm:Quote:
Originally Posted by DJSapp
The first pro-takeoff post on that thread is well worded. End of the post quoted below...Quote:
Originally Posted by The General
Quote:
The Plane Will Take Off.
The reason you think it won't, if you are one of those people, is because you are thinking of it like a car. A car with wings WOULD NOT take off in this scenario, because a car's motion comes from traction power from the wheels, and the car must necessarily travel the same distance traveled by the outside of the wheel on the ground. Thus, the car moves at speed 0 relative to someone off the treadmill while the wheels move at speed V (which would be the spedometer reading) and the treadmill moves backwards at speed V.
A plane's power is not generated at the wheels, and thus they can spin freely at 2V, allowing the plane to move forward at speed V (relative to an observer not on the treadmill) while the treadmill moves backwards at V.
The conveyor doesn't matter and there is no spoon. Shit, it doesn't even need to be a 747, a Cessna would probably be able to take off in this scenario.
This is still WRONG. This will only be true if the treadmill is hooked up to the car's speedometer. If the treadmill is hooked up the FORWARD SPEED OF THE CAR then the car will still move, albeit at half the speed registered on the speedometer.Quote:
Originally Posted by dbp
I googled "jet plane treadmill" and found that this thread has been duplicated on:
- about a gazillion physics forums
- the Bad Astronomy forum
- Motonews.com
- Ford XR6 Turbo.com
- PaGaLGuY.com - India's largest MBA forum
- RV.net Open Roads Forum
- and, best of all, the Texas Fishing Forum (where it's hit 38 pages)
The bass fishermen seem to understand physics about as well as everyone else.
I also learned that President Bush has added a treadmill to Air Force One.
Ok, that would be true. Except that because the plane is pushing air to accelerate it is accelerating relative to the air and not to the treadmill. Because of this fact, the treadmill has no impact on it's ability to accelerate the mass of the airplane. Unless the plane is tied down to the ground it cannot apply thrust while on a treadmill and not accelerate. So, if the plane is staked out to the ground, you are correct(and in which case taking off has bigger problems than the treadmill). If it's not tied down, the rest of us are.Quote:
Originally Posted by cj001f
So if a Wasatch Powderturds helicopter is loaded with a pilot, a guide (~180lbs each) and 4 middle aged investment bankers (assume they average 275 lbs each), how many dirty hippies, weighing approximately 120 lbs each, would have to hold on to the to chopper’s skids to prevent it from taking off? Assume the helicopter is powered by a JET engine that produces 1.21 Jigga-Watts of power.
As the problem is written, no takeoff.
Relevent part of the problem:
"This conveyor...tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction)."
Think about getting the plane moving forward in the first place.
Even an infintesimally small amount of forward aircraft motion is 100% canceled by the conveyor's movement in the opposite direction, so the conveyor will speed up to the point where the wheel bearings frictional drag will prevent the plane from moving forward thru the air, therfore the plane will not move a single nanometer forward. So no airspeed, no lift, and no takeoff.
Every argument for YES assumes "speed" is ground-speed as measured by the wheel rotations. Nowhere is this stated in the problem.
In the real world, of course it would take off, but this isn't a real-world problem. It's a "gedankexperiment" (thought experiment).
FUCK THIS THREAD!!!
AAGGGGH.
HOW CAN YOU ALL BE SO STUPID.
The only way the plane can be held stationary by moving a conveyor belt (putting aside the awesome calculation of the small friction created by the friction of the wheel bearings) is by applying the wheel brakes of the plane.
OTOH. IF YOU APPLY THE WHEEL BRAKES ON THE PLANE, YOU DON"T EVEN NEED THE CONVEYOR TO HOLD THE PLANE STATIONARY.
sorry for shouting, but this thread is so stupid, and the question was correctly answered many pages ago.
Pretty much. I guess some people just don't get physics.Quote:
Originally Posted by Core Shot
edg..... Wrong
J>=C because C is only provided by air resistance to the plane's forward motion and the friction inherent in the landingear rolling on the surface. At very low thrusts, the plane might not be able to overcome the friction in the wheels, but as soon as that threshold is past, the plane will accelerate unless it's tied down.
Here's a different angle.
You are on a pair of rollerblades standing on a treadmill.
You hold onto a tow rope and I gun the throttle of my motorcycle which is not on the treadmill (just like the thrust of the airplane engine is independent of the treadmill).
No matter how fast the treadmill spins underneath you, you cannot be held stationary.
I will pull you forward, just as the planes engines will push it forward, independent of the speed of the treadmill.
The treadmill does nothing.
It is a trick question.
The treadmill is irrelevant.
I think somebody needs to actually test this in real life. Who's up for it?Quote:
Originally Posted by Core Shot
More importantly, I just realised that the conveyor doesn't exert a force on the plane (well, negligible), but only spins the wheels.Quote:
Originally Posted by Telenater
edg
Dude, we can fucking read, how can you be so fucking stupid? Every argument for it taking off assumes (in a more physically correct manner) it can move relative to the treadmill (which is true, the friction free wheels don't transmit force to the plane). The only problem is the problem states it can't move relative to the treadmill. The argument is framed improperlyQuote:
Originally Posted by Core Shot
Wow, how did I miss this thread for so long!Quote:
Originally Posted by Cornholio
I'm not going to trudge through the rest of this thread, bu Cornholio is 100% correct. The only reason an airplane moves forward on a runway (therefore producing lift over the wings) is because the thrust of the engines causes the tires to roll against the ground. The equal and opposite force is transferred to the ground. On a perfect (i.e., no loss due to friction) treadmill this force will cause the treadmill to travel with exactly the same speed in the opposite direction and the airplane will not move. No movement means no lift and the airplane will NOT takeoff.
Core,
The treadmill's properties are THE key to the problem.
As the original problem was written, the treadmill would speed up to the point where the (admitedly small in real life) friction of the rolerblade wheels & bearings would counteract the forward pull from the bike, holding the blader (& attached bike) in place. Treadmill speed would go incredibly high, but it is by definition high enough to prevent the rollerblader from achieving ANY forward motion (speed).
Quote:
Originally Posted by Core Shot
Where does it say that the plane cannot move relative to the ground/treadmill? That seems to be the premis of your argument.Quote:
Originally Posted by cj001f
We have some stated premises.Quote:
Originally Posted by The Big One
"The plane moves in one direction"
"conveyor moves in the opposite direction" I think that it's safe to assuem that they're referring to the surface of the conveyor belt moving, not the conveyor it'self (though it doesn't really matter).
"tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction)"
From these we get the following.
Vplane can be nonzero (it states that it's moving and thus not tied to the ground).
Vplane = -Vconveyor
Aplane = -Aconveyor
However, Because the plane is using jet engines, unless it's tied down (which it isn't) it will have a thrust that accelerates the plane forward as long as the force of the thrust more than balances out any drag from the landing gear and air resistance.
We know that the force of friction on a wheel is essentially constant with respect to its speed. We also know that the jet's engines can overcome air resistance under normal circumstances. The only variable that chages since the plane's speed with regards to the air would be the same as under normal thrust is the friction from the wheels which being constant is the same as before regardless of the wheel speed. Therefore (and listen carefully here), the only difference from a normal take off would be that the wheels are spinning twice as fast.
No, no, no, no...... edg got it. This would be true if the plane were accelerating using it's wheels. Then the equal and opposite reaction would apply to the treadmill. In this case, the plane is accelerating with it's jet engines. the equal and opposite reaction is applied to the air mass surrounding the jet. In an environment where the only friction is the rolling friction of the planes's wheels on the conveyor, it the conveyor would not move at all.Quote:
Originally Posted by The AD
Yes, I'm a glutton of punishment.
It is?Quote:
Originally Posted by Telenater
Surely the rolling resistance of the tire and friction of wheel bearing continually increase with speed.
You are missing the point that there is a difference in how speed is measured.Quote:
Originally Posted by TomK
In each hypothetical, there are two different speeds.
If I pull you at 10mph on my bike, the treadmill will turn at 10mph.
You, however, will move 10mph forward (as am I, since I am pulling you with my bike which is not attached to the treadmill). There is not enough wheel bearing friction at 10mph to slow you down at all.
In the plane example, the plane measures airspeed and has air thrust.
The Treadmill measures ground speed, and (other than the scant friction of the bearings) cannot exert force on the plane.
I am really not trying to be obtuse, but where does the original problem state that the plane can't move relative to the treadmill? I think it just says it matches the speed of the plane in the opposite direction. Obviously if the plane can't move forward, there won't be airflow and it can't take off... but 99.9% of the male population knows this. Not a very interesting question.Quote:
Originally Posted by cj001f
edit: oops, I type slow or something.
WRONG.Quote:
Originally Posted by The AD
Airplanes push air. The force from the engines pushes air. There is no force transfered to the ground. This is where the non-flyers are missing the point.
According to your argument, airplanes cannot fly, once they leave the ground, there is no force pushing against the plane.
I was thinking about it a little wrong initially, but I still stick to my original view that the plane will not take off.
Imagine it this way. What if the treadmill starts rolling before the airplane's engines are fired up? In this case the airplane will roll off the back end of the treadmill unless it's engines are fired up to provide a reacting force to the treadmill. So if the engines are fired up and the airplane is then able to maintain its position, or move forward, the treadmill can turn faster to cancel out that force.
You're going to have some very hot wheel bearings, though, because all the planes thrust needs to be dissipated by rolling friction.
That's my position and I'm sticking to it.
at least for now...
The bike does not pull the blader forward at 10MPh, it exerts a force that in the absence of other forces WOULD tow the blader at 10MPh.
But...since the treadmill speed is by definition high enough to use the friction provided by the rollerblade wheels & bearings to prevent any forward motion of the blader getting started in the first place, the motorbike's wheel would spin in place generating a lot of smoke, but there would be no forward motion relative to the surface of the earth. So, the blader stays in place while the treadmill spins underneath.
Think about what would have to happen to get the blader (or plane) moving relative to an outside the sytem observer in the first place. Since the treadmill's speed exactly cancels ALL "plane speed", it must be able to counteract any other forces trying to overcome inertia.
Real world limits and relative power would easily overcome the counteracting forces, but not in this "no limits" thought experiment.
Quote:
Originally Posted by Core Shot
Only true if you neglect friction and nowhere in the problem is that stated as an assumption. As long as the tires are in contact with the ground there will be a frictional force opposing the engine thrust. If the treadmill were free-spinning the friction would be << than the engine thrust and the plane could still take off, but since the treadmill is powered it can theoretically turn the belt fast enough to create a static condition between engine thrust and friction.Quote:
Originally Posted by DJSapp
Where does the question state this?Quote:
Originally Posted by TomK
Close, but not quite. In a non defomational system the force applied by friction is defined as having a maximum of the load times the coefficient of friction. The speed of movement doesn't directly apply. However, other events that accompany the increase in speed can effect it as the objects properties(and therefore coefficient of friction) change. ie as the friction will increase witing a bearing supported axle as the heat of the bearings increases, and as turbulence in the lubricant around the bearings increases, or as bearings rub against one another instead of just the traces. In most situations where you have a wheel attached to bearings, small variations in the weight distribution of the wheel become a problem before friction in the hub does.Quote:
Originally Posted by PNWbrit
frictional losses of the tire on the road due to slippage or deformation are far more difficult to model and I don't have any direct information on it. I made the assumption that the change in friction of the tire on the surface due to speed was similare enough per distance traveled that it would also be close to flat.
Quote:
Originally Posted by The AD
But, the treadmill is limited to exactly the speed of the plane.
While I cannot rule out that wheel friction could become an issue below 2x the take off speed, I suspect that the friction provided even at 2x normal take off speed is still significantly less than that of the jet's thrust. So, losses due to wheel friction might make it take longer to reach takeoff, but it would not prevent it.
AD, see a previous post of mine regarding friction; it's too small to matter here, 747's can take off with the brakes locked (provided the wheels don't explode).Quote:
Originally Posted by The AD
As for the rest of your argument, you are measuring the plane's speed relative to the conveyor. This creates a circular logic loop. By measuring the speed relative to the conveyor, the plane cannot begin moving without violating the statement that the belt speed equals the aircraft speed.
The tires will blow up before friction is an issue. Rubber cannot hold together at those speeds. I mean, gonzo has a router that can do 30,000 rpm fer christs sakeQuote:
Originally Posted by Telenater
Quote:
Originally Posted by DJSapp
Clarification:
Airspeed = 1 to begin moving
conveyor = 1 to match
Wheelspeed = airspeed + conveyorspeed
according to 'stay still' arguments:
wheelspeed = conveyorspeed
Therefore according to this, the conveyor cannot start moving. The logic is flawed.
I drew a diagram representing my logic. Correct? Incorrect?
The setup is correct, the question is where speed is measured from.Quote:
Originally Posted by dbp
I'd say completety incorrect because the motorcycle is pushing against the ground and that's what's causing its forward speed. How about replace the cycle with a really big RC airplane and that's more like it. With this scenario can't you imagine the treadmill spinning fast enough such that both the rollerblader and RC plane are stationary WRT the ground?Quote:
Originally Posted by dbp
Really nice diagram, though, other than that! :)