6

1/29/2008 3:36:40 PM

Quote :

"OH NOES I CAN'T DRIVE HOME THE EARTHS SPINNINGS IN THE WRONG WAYS!!!!""

Lol

1/29/2008 3:36:44 PM

.....

Quote :

"If the surface was frictionless, then the wheels would not roll, they would slide."

the wheel bearing is for all intents and purposes is frictionless in this exercise

[Edited on January 29, 2008 at 3:38 PM. Reason : h]

1/29/2008 3:37:30 PM

Quote :

"If the surface was frictionless, then the wheels would not roll, they would slide."

Come on Bobby.

The bearing inside of the wheel is the "frictionless" connection to the plane.

1/29/2008 3:38:06 PM

The wheel bearings can be frictionless, that changes nothing.

There is still friction between the tire and the surface of the treadmill.

[Edited on January 29, 2008 at 3:38 PM. Reason : asdf]

1/29/2008 3:38:30 PM

SO A PLANE TAKES OFF UP A HILL DURING AN AVALANCE AND INSTEAD OF WHEELS IT HAS SKIS.

(OR SNOWBOARDS BECAUSE ITS A 'COOL' PLANE)

1/29/2008 3:38:53 PM

and how exactly is that wheel causing friction on the plane then? through the frictionless bearing?

Quote :

"The wheel bearings can be frictionless, that changes nothing. There is still friction between the tire and the surface of the treadmill."

yes which only make the wheels spin



[Edited on January 29, 2008 at 3:58 PM. Reason : 35000!]

1/29/2008 3:39:10 PM

And that friction causes the wheel to rotate. It does not however translate into any force parallel to the ground on the plane that could overpower the engines.

Draw the diagram and do the math.

1/29/2008 3:39:24 PM

This is so sad so much time has been donated to this pointless discussion.

(SPOILER ALERT!!!!)













































































































































Its gonna takeoff! HA!

1/29/2008 3:39:37 PM

Quote :

"There is still friction between the tire and the surface of the treadmill."

that just means the tires will rotate as the treadmill tries to pull the plane backwards. it can't due to the inertia of the plane

1/29/2008 3:40:07 PM

Quote :

"This is so sad so much time has been donated to this pointless discussion."

indeed

i'm suffocating in the stupid seeping from this thread

1/29/2008 3:40:21 PM

Quote :

"And that friction causes the wheel to rotate. It does not however translate into any force parallel to the ground on the plane that could overpower the engines."

if the plane's engine is off and you turn the treadmill on, does the plane remain still?

1/29/2008 3:41:20 PM

HAI GUYZ WAT"S GOING ON IN HERE?

1/29/2008 3:41:37 PM

^^ yes (neglecting the miniscule frictional loss in the bearings)

i like to think about it like a dinner table with dishes and stuff w/ a tablecloth. you can yank the tablecloth out and the dishes will remain stationary.

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

1/29/2008 3:41:43 PM

Im actually done here


Its sad that some very smart people cant understand the concepts of this myth.

Both Arab and Joe#s are correct.




[Edited on January 29, 2008 at 3:47 PM. Reason : .]

1/29/2008 3:43:36 PM

i think it's safe to say a actual airliner has enough inertia for this to happen...

the only movement involved is a result of friction through the wheel bearings, in real life it would but not nearly as fast as the treadmill. see: inertial coupling

in a frictionless theoretical exercise joe#'s is spot on

1/29/2008 3:46:17 PM

Quote :

"you can yank the tablecloth out and the dishes will remain stationary."

yes, but what about a prolonged, gentle tug?

1/29/2008 3:48:01 PM

Quote :

"As soon as something pushes the plane hard enough to overcome that initial friction, the plane will be easy to keep in place."

ok, now we're on the same page.

So if the treadmill is moving at 100mph, in order for the plane to remain still, the engines have to push the plane with the same force that would push it 100mph on the treadmill if it was not moving.

Relative to the earth, the plane is not moving. Assuming no wind, relative to the earth, the air is not moving either.

no lift.

1/29/2008 3:48:12 PM

^^ But that is not the point. The plane WILL move backwards on the treadmill/conveyor belt until the pilot powers the engines. At that point, the planes motion, relative to the air around it, will move forward. The conveyor belt has no relation to the air around the plane either.

The wheels only hold the plane up, they do not propel it in any way shape or form. Propulsion comes from the engine through the propeller.

[Edited on January 29, 2008 at 3:52 PM. Reason : you guys are arguing over forces that amount to mice nuts in a room full of cats]

1/29/2008 3:50:41 PM

Quote :

"yes, but what about a prolonged, gentle tug?"

the friction between the dishes and the table cloth is too high, and they would fall off. Like WM said before he edited his post, if you pull the conveyor belt ever so slow the plane will be pulled backwards due to the static friction in the bearings.

1/29/2008 3:50:43 PM

Quote :

"So if the treadmill is moving at 100mph, in order for the plane to remain still, the engines have to push the plane with the same force that would push it 100mph on the treadmill if it was not moving."

That is not what i said at all, because what you are saying is completely untrue.

The treadmill is not pushing on the plane with enough force to move it backwards at 100mph. That is the part that you aren't understanding.

The treadmill is turning the wheels, which, if they were solidly fixed to the plane, would cause torque. Since they are fixed to the plane via bearings, it translates almost ZERO of the treadmills energy to the plane.

1/29/2008 3:51:42 PM

this thread honestly has rekindled my fear of flying

because no matter how you look at it, half of you are fucking stupid* - and that means that half the engineers out there are stupid, and fuck it, i'm not even driving home. i'm walking. or taking a treadmill. or some shit.

*i only say stupid because of how sure everyone is about their opposing answers. i have no idea, i would think it would lift off but am totally unqualified to make the call so im not gonna argue it

1/29/2008 3:51:51 PM

And if the table cloth were a conveyor belt that started randomly spinning super fast then everything on top of it would fly off again.

Its a retarded analogy/example made by someone thats not understanding the basic premise (as seems to be the case with a lot of people here).

1/29/2008 3:51:55 PM

what about a space shuttle on a skateboard?

1/29/2008 3:55:03 PM

^ not in a million years unless the wheels were made with peppermints

1/29/2008 3:55:35 PM

Quote :

"Since they are fixed to the plane via bearings, it translates almost ZERO of the treadmills energy to the plane."

The force applied by the treadmill onto the plane is between the tire and the surface of the treadmill, not at the bearings. The surface of the treadmill is in contact with the tire, not the bearings. The ADDITIONAL friction possibly introduced by the bearings, is absolutely inconsequential for the purposes of this exercise.

1/29/2008 3:58:42 PM

so how is that tire translating any force to the plane?

1/29/2008 4:00:42 PM

I dont know you in person, but I honest to god thought you were smarter than this.

Draw your force diagram and show the force being imparted on the airplane to make it move. Its a simple request that will undoubtedly prove your side of the argument. (If you could actually draw it).


[Edited on January 29, 2008 at 4:02 PM. Reason : .]

1/29/2008 4:01:12 PM

WHERE'S THE CARL FACE WHEN YOU NEED IT?

1/29/2008 4:01:31 PM

DID ANY OF YOU WHO DON'T THINK THE PLANE WILL TAKE OFF EVEN WATCH THE VIDS THAT I POSTED??

lets simplify the question. lets say that a plane in LANDING onto a treadmill that is traveling at the same speed but the opposite direction of the plane. when the plane lands will it magically stop instantaneously??

1/29/2008 4:03:30 PM

i know right....

frankly this isn't the first time i've experienced a engineer completely missing the barn.... the other one makes less $ than I do...

but for bobby not to get this... wow. just wow. they need to teach this in physics 205 is it?

1/29/2008 4:04:49 PM

well, say a plane landed on a boat going the same? direction at 100 mph

[Edited on January 29, 2008 at 4:05 PM. Reason : ?]

1/29/2008 4:05:12 PM

this is also why aircraft carriers recover aircraft by turning into the wind, reduces the planes ground speed (relative to the carrier deck) without stalling the aircraft.

1/29/2008 4:07:24 PM

The vids don't show anything substantial.

All he does is speed the belt up, then apply throttle.

From the reference point of the plane, which is stationary and has a speed of 0, added thrust is going to accelerate it. Its not hard physics or impossible to visualize.

However

If he applied thrust and accelerated the belt in such a way that the force applied forward equaled the force applied by the belt moving in the opposite direction, then the plane would remain stationary.

and wouldn't need a tether

At this point, it won't take off, even though its at full throttle.

1/29/2008 4:08:21 PM

Quote :

"And that friction causes the wheel to rotate. It does not however translate into any force parallel to the ground on the plane that could overpower the engines. Draw the diagram and do the math."

There is a non-zero horizontal force, even with frictionless bearings. Something has to keep the wheels from rolling away. However, that force is only related to the rotational acceleration of the wheels. Once the zero-friction-bearing wheels reach the speed of the treadmill, no force is applied to the plane.

1/29/2008 4:11:47 PM

Quote :

"If he applied thrust and accelerated the belt in such a way that the force applied forward equaled the force applied by the belt moving in the opposite direction, then the plane would remain stationary."

WRONG!! And he demonstrated this by accelerating the tread mill with out increasing thrust...and guess what...the plane didn't go backwards...the wheels just spun faster.

1/29/2008 4:12:38 PM

you know, this whole thing is very similar to my level of experience in my field

i should consider myself an engineer, because i can hang with any csc grad. but thats so much the point. the point is that when i got an opportunity to work in a parallel field (gps/bluetooth/frequency transmission/blahblah) i didnt go in there like OH I KNOW WHATS UP. i sat back and had to relearn the shit. just because i took one class on one thing one time in the course of learning something in the same realm doesnt mean that i just caue i can write programs that i knew shit about triangulation

anyways.

bye, im taking the treadmill home

1/29/2008 4:13:51 PM

Umm, what?

1/29/2008 4:15:07 PM

Sparky you're an idiot.

Just read this webpage and if you don't get it there then piss off.

http://www.straightdope.com/columns/060303.html

BR#2

Quote :

" The conveyor doesn't exert much backward force on the plane, but it does exert some. Everyone intuitively understands this. To return to the analogy in my original column, if you're standing on a treadmill wearing rollerblades while holding a rope attached to the wall in front of you, and the treadmill is switched on, your feet will initially be tugged backwards. Partly this is due to friction in the rollerblade wheel bearings, but partly--this is key--it's because the treadmill is accelerating the rollerblade wheels and in the process imparting some angular (rotary) but some linear (backward) momentum to them. You experience the latter as backward force. Eventually the treadmill reaches a constant speed and the rollerblade wheels cease to accelerate. At this point you can easily haul in the rope and pull yourself forward. But what if the treadmill continues to accelerate? Different story. In principle it's possible to accelerate the treadmill at a rate that will exactly counteract any forward force you care to apply. (This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force.) The only mathematics needed to demonstrate this is the well-known physics axiom F = ma--that is, force equals mass times acceleration. Given that the conveyor exerts some backward force F on the plane, we simply crank up the acceleration as much as necessary to equal any forward force F generated by its engines. Result: The plane stands still and doesn't take off. Welcome to BR #2. "

1/29/2008 4:15:50 PM

get your ass on some rollerblades, pull a rope tied to some shit in front of you, lean really far back and see if your rollerblades don't accelerate like a motherfucker and put you on your ass.

1/29/2008 4:18:02 PM

one more thing i cant help but notice (and point out) before i go. damnit

looking at the time stamps, you guys are barely taking the time to read the previous post, let alone ATTEMPT to comprehend it before you come back and call someone else an idiot.

why dont you guys just make a thread called DUMBASS and call each other dumbass over and over and over and save the fucking trouble. to think, so many grown men here, with kids and shit.

HAA

1/29/2008 4:18:35 PM

no its not about force applied to the plane through the tiny bit of friction

the question involves the raw speed of the belt = speed of the plane

the forces are seperate and UNEQUAL

the jet engines force (for instance) is several thousand lbs

to get that kind of force applied through torque through the nearly frictionless wheels the treadmill will have to move several hundred if not thousand times faster than the planes speed relative to the ground.

this is not hard to get. all this means is that the force coming FROM THE PLANE is IN NO SHAPE OR FORM transmitted TO THE TREADMILL and vice versa

in reality a very VERY TINY amount would get transferred but it wouldnt be significant


sandsanta - i guess you didn't catch this "(This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force.)" part. they say do to the forces present in reality but they don't say just how many times the SPEED of the belt needs to be above what SPEED you're going at.

put a can on its side next to a box on a conveyor belt at the store next time, tell me which reaches the end of the belt first

[Edited on January 29, 2008 at 4:23 PM. Reason : s]

1/29/2008 4:18:45 PM

There's nothing to read here dude

I know I'm right by virtue of not uncontrollably drooling on myself.



[Edited on January 29, 2008 at 4:20 PM. Reason : spellstrike]

1/29/2008 4:19:35 PM

Quote :

"I know I'm right by virtue of not uncontrollably drool on myself."

inpsiring confidence with that one.

1/29/2008 4:20:15 PM

Quote :

"the question involves the raw speed of the belt = speed of the plane"

If the raw speed of the belt = speed of the plane, then relative to the earth, it is not moving.

where does the lift come from?

Until the plane is moving sufficiently fast enough relative to the earth/air, it cannot take off.

[Edited on January 29, 2008 at 4:23 PM. Reason : unless it's a harrier ]

1/29/2008 4:22:26 PM

Quote :

"But what if the treadmill continues to accelerate? Different story. In principle it's possible to accelerate the treadmill at a rate that will exactly counteract any forward force you care to apply. (This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force.) The only mathematics needed to demonstrate this is the well-known physics axiom F = ma--that is, force equals mass times acceleration. Given that the conveyor exerts some backward force F on the plane, we simply crank up the acceleration as much as necessary to equal any forward force F generated by its engines. Result: The plane stands still and doesn't take off. Welcome to BR #2."

that guy is an idiot and hardly reputable. as arab said, the original question said nothing to the affect of the treadmill exerting a counter FORCE. The guy even admitted to this...

Quote :

"This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force."

1/29/2008 4:24:25 PM

Quote :

"So if the treadmill is moving at 100mph, in order for the plane to remain still, the engines have to push the plane with the same force that would push it 100mph on the treadmill if it was not moving. Relative to the earth, the plane is not moving. Assuming no wind, relative to the earth, the air is not moving either."

take 3 scenarios:
1) Assumptions: The surface of the treadmill and the wheels are entirely frictionless.
Therefore, if a plane was sitting on top of the treadmill and the belt started moving, the plane would not move at all. The wheels wouldn't even move. Truly frictionless would mean the belt would just slide right underneath the plane. If the plane was turned off, and the frictionless belt started moving underneath it, nothing would happen. it would just sit there.

As soon as the plane turned on its engines though, it would be propelled forward. As we said, there is no friction between the wheels and the belt, so the engines would produce thrust and would push the plane forward, regardless of how fast the frictionless belt is moving. The thrust would push the plane forward, accellerating until take-off speed.

2) Assumptions: There is surface friction between the wheels and the belt, meaning whenever the belt moves, the wheels will move with it, never losing contact. However, there is zero friction between the wheel bearings and the axles.
Therefore, if a plane was sitting still on the belt, and the belt started moving, the wheels would start to spin. But since there is no friction between the wheels and the axles, the wheels would literally be spinning, but the plane would not move at all. The belt could accelerate as much as it wants, and the wheels would simply spin to keep up, but the plane wouldn't move (if there was no friction between the wheels and the axles)

Once again, as soon as the engines are turned on, they would provide thrust and the plane would move forward, eventually taking off just as in scenario 1. During this acceleration of the plane, the belt could go faster or slower or whatever, and it wouldn't matter. All it would do would spin the wheels faster or slower, but since there is no friction between the wheels and the axles (i.e. between the wheels and the rest of the plane) this wouldn't effect the overall acceleration or take-off of the plane.

3) Now in the "real world scenario". There is surface friction between the wheels and the belt, and there is some friction between the wheels and the axles. This would play out almost exactly as in scenario #2, however a little bit of the thrust of the engines would be required to equalize the friction caused by the spinning wheels. In this real world case, if the plane was sitting there, and the belt started to move slowly backwards, it would stand to reason that the plane would start to move backwards with it.
However, if the engines were turned on a little bit, they would overcome the friction between the wheels and the axles. As you learned in Statics and Dynamics, i'm sure, there is a difference between Static Friction and Rolling Friction, so once the engines provide enough trust to overcome the Static Friction between the wheels and the axles, the wheels will start to spin. Then the thrust can be decreased, and to keep the plane in the same place the thrust only has to match the drag created by the Rolling Friction. If the belt starts to spin faster, then maybe the engines have to compensate a bit, but just enough to match the Rolling Friction so the plane stays in one place. Once these forces are equalized, as soon as the engines are throttled up a little more, then the plane will start to move forward, and after that it will play out just like in scenario 1 and 2.


Now, wheels are obviously built to create as little friction, static and rolling, as possible. Compared to the amount of force necessary to speed a plane up to 100mph, this friction is essentially zero. This is why scenario #2 can be used for most basic calculations of how this myth would play out. However, in real life, something between scenario 2 and 3 would occur. Either way, whenever the plane engines provide enough thrust to over come the relatively tiny amount of friction in the wheels, then the plane will start to move forward relative to the ground, and more importantly relative to the air. It will then continue to accelerate until it reaches take-off speed, regardless of what is happening to the wheels and belt underneath it.

1/29/2008 4:25:01 PM

This whole thread is a departure from the original (vague) question

1/29/2008 4:25:47 PM

the plane does not need a certain ground speed to take off it needs a certain airspeed.

treadmill = ground speed

aircraft thrust = airspeed

otherwise do you really think the plane magically goes a lot faster on one leg of the trans-atlantic flight than the other? yet the plane still flies, yet the ground speed is different! OMG

thank you agentlion

[Edited on January 29, 2008 at 4:28 PM. Reason : -no]

1/29/2008 4:25:56 PM

Quote :

"no, the plane does not need a certain ground speed to take off it needs a certain airspeed."

Unless there is wind, speed relative to the air or ground is the same.

1/29/2008 4:27:41 PM