A jet plane on a large treadmill

[P_McPoser]

I GUARANTEE that bitch is far from frictionless!!!

You're right, your mom DID give me friction dick.

[MeatPuppet]

predicting at least 20 pages before this dies

Go reread my post on page 15.

[MeatPuppet]

friction dick.

Is that an engineering term?

[rideit]

Sounds like a good name for a lava based soap.

[focus]

Wait, so this forward velocity of the plane, it's increasing right? And then the conveyer matches that right? And the wheels do not speed up how now?

Your brain is starting to get scrambled as well.

The wheels have nothing to do with it. The type of plane has nothing to do with it. It doesn't matter if the plane is on skis, or pontoons, or just riding along on its belly. It doesn't matter if the plane is powered by jet engines, by a v8, or by a rubber band. It doesn't matter if it transmits that power through turbines, through a prop, or through its wheels. It doesn't matter if it's a plane, or a car, or a train, or your old aunt Sally.

The conveyer can go faster and faster only when the plane goes faster and faster. The conveyer belt, the plane, and takeoff are not at odds with each other in any way, shape, or form.

The happy little plane says "I'm going to go forwards at 5mph (or 100mph, or 1,000,000mph) now."

The happy little conveyer belt says "hey, you're going 5mph (or X mph). I must go the same speed in the opposite direction."

The happy little wheels (or skis, or pontoons, or whatever) say "Whoa, the plane's going one way @ 5mph, and the conveyer's going the other way at 5mph. I guess I'm going to have to spin (or slide, or drag) at 10mph."

All the happy little speedometers, IAS measurerguys, and pitot tubes are measuring what they measure. What they measure may or may not reflect what's actually going on.

All the disgruntled little enginerds say "I have formulas!"

[Jer]

It's the jet engines that are irrelevant - that's the key! Verry subtle!

As soon as the slightest breeze -- or even a trivial tremor in the earth -- budges the jet infinitesimally forwards or backwards, the conveyor belt will detect that slight rotation of the wheels - and instantly match it.

But by matching that, the conveyor belt will have doubled the rotational speed of the wheels!

So then the belt will double its speed to match the wheels' new speed. And so the wheels' speed will double ...

So the conveyor belt will accelerate at an exponential rate towards the speed of light - but the wheels are moving just a tad faster, so they'll get there first.

... the plane? Oh, it ceases to exist pretty quick, along with the Earth, as both the wheels and the conveyor belt approach infinite mass.

[Rideski]

I'm with the group that says the jet engines do not power the wheels.


Also, no air flow under the wings due to no motion; means no lift, no fly. Unless of course you figure that the jet engines will blast the thing right off of the treadmill, which of course would be the reason the riddle is nonsense.

[P_McPoser]

What is sad is that you're entirely too fucking retarded to realize NO ONE IS ARGUING WITH YOU. WE ALL AGREE. YOU LACK READING COMPREHENSION.

IT'S EASY, DUMBASS, I SAID "PLANE SPEEDS UP, CONVEYER SPEEDS UP, WHEELS SPEED UP" you said "PLANE SPEEDS UP, CONVEYER SPEEDS UP, WHEELS SPEED UP."

The happy little plane says "I'm going to go forwards at 5mph (or 100mph, or 1,000,000mph) now."

The happy little conveyer belt says "hey, you're going 5mph (or X mph). I must go the same speed in the opposite direction."

The happy little wheels (or skis, or pontoons, or whatever) say "Whoa, the plane's going one way @ 5mph, and the conveyer's going the other way at 5mph. I guess I'm going to have to spin (or slide, or drag) at 10mph.

All the disgruntled little enginerds say "I have formulas!"

[focus]

IT'S EASY, DUMBASS, I SAID "PLANE SPEEDS UP, CONVEYER SPEEDS UP, WHEELS SPEED UP" you said "PLANE SPEEDS UP, CONVEYER SPEEDS UP, WHEELS SPEED UP."

Where we differed, dipshit, is I said car speeds up, conveyer speeds up, wheels speed up -- car drives off the end of the conveyer. I understood you to disagree with this.

If you don't...well, then....sorry about that.

(that and we still have people talking about nothing going anywhere and people keep bringing up wheel friction not being able to overcome jet engines....and it all just drives me batshit....)

edit: and we have dumbshits like bertier below me who still don't get it and think Dec 2005 was two years ago.

[Bertier]

edit- i just realized this is a 2 year old 17 page thread... just ignore me.

The jet will continue to accelerate gaining airspeed... The jets push air not ground so who fucking cares if it is on a treadmill.

Gaining airspeed = wrong

The belt negates any forward progress, therefore airspeed/groundspeed/any speed relative to the earth = ZERO.

Jets push air = right

Yeah, jets push air, from the front of engine to the back, but no air is passing over the wings. The wing is the important part for takeoff. I dont give a shit how the plane is propelled, if there is no wind going over the wings, no lift is produced. Jets produce lift in conjunction with wings by pushing the plane, but the belt is countering any forward progress so flying = no.

[Squatch]

I think this guy said it way better than I could

[Shen]

who puts a jet on a treadmill anyways?

I have to fully agree with this.

[Tippster]

It's a Plane. Not a Car. The speed of the ground is irrelevant, since the engines push against the air. An airplane flying west often reaches an airspeed of 500mph, yet due to headwinds the ground speed (airplane relative to ground - not a true velocity vector) is only ~300mph. The plane does not stall, the engines don't get torqued off the wings, there is no connection between the airplane and the ground.

The wheels are the key - they represent a (nearly) zero friction base for the plane to "stand" on - they are not the means for the plane's locomotion (like a car's engine turning the wheels.) The only way they would impede the plane's forward progress is through friction, and that would mean the conveyor belt would have to move MUCH faster than the plane, which is NOT posited in the original question.

The fact that this thread will likely double in length 2 years after inception means that there are a whole slew of new overthinkers on this site.

[focus]

It's a Plane. Not a Car.

I've gotta step away from this...but...dooood. You're still arriving at a (mostly) correct answer in the wrong way.

It doesn't matter if it's a plane or a car. And as I said earlier, the wheel's really don't have shit to do w/ it. The very second anybody brings up frictionless pivots, and pressures, and windspeeds, and wings creating lift and moving through the air independently of the ground -- they've overanalyzed the fucking question and indicate that they don't understand the premise.

[dbp]

I can't remember if this has been posted before, but for those of you who trust Cecil...

The Straight Dope

[SkiED]

I dont get how you guys are so stupid when it comes to this question. A plane works by air flowing over the wings correct? And if that plane is sitting on a treadmill explain to me how there is air passing over the wings providing lift?????

it doesnt matter how much air the engines push, t hat is just to provide forward momentum and thus air over the wings.....but if the plane is essentially sitting still with its engines on full power and its wheels spinning on the tarmac where is the air over the wings coming from???

Some of you people are just so fucking dense its unreal, a plane gets lift from air passing over the wings, how the fuck does that happen if its essentially stationary on the runway???????????????????????? Please someone answer that.

[focus]

Some of you people are just so fucking dense its unreal, a plane gets lift from air passing over the wings, how the fuck does that happen if its essentially stationary on the runway???????????????????????? Please someone answer that.

You're so dense it's unreal.

What's this "essentially stationary" nonsense? Why do you think the plane is stationary? That isn't what's happening here.

[SkiED]

I can't remember if this has been posted before, but for those of you who trust Cecil...

The Straight Dope

Yeah so that answer makes sense, but I took it as the plane is essientlly sitting still on the runway, no matter how fast the plane tries to move forward the conveyor belt keeps it in the same spot. The question is worded poorly. But essentially a plane needs air over its wings to take off, and if the conveyer belt is negating any forward movement(like the question pretty much states) then fuck no its not goin gto take off and you must be a retard to think so.

[SkiED]

I get the wheels dont propel it the engines do, which is unaffected by the treadmill where a car would be affected. but the question pretty much is formed to make you think that if you were looking from ground level where you could see the treadmill it would just look like the plane is sitting still with engines on full power and the wheels spinning really fast.

[focus]

Yeah so that answer makes sense, but I took it as the plane is essientlly sitting still on the runway, no matter how fast the plane tries to move forward the conveyor belt keeps it in the same spot. The question is worded poorly. But essentially a plane needs air over its wings to take off, and if the conveyer belt is negating any forward movement(like the question pretty much states) then fuck no its not goin gto take off and you must be a retard to think so.

But, you fucking IDIOT, the question doesn't state anything about negating the plane's forward speed. You must be a retard to think so.



A car may be affected more by this treadmill than the plane, simply because the plane's propulsion is more efficient in this scenario (relative to other scenarios) -- but a car will still not have any problem moving forwards.

[dbp]

but the question pretty much is formed to make you think that if you were looking from ground level where you could see the treadmill it would just look like the plane is sitting still with engines on full power and the wheels spinning really fast.

I think that is just an assumption you make based on your normal experience with treadmills. That is why many peoples' initial response is the "obvious" and incorrect answer that the plane does not take off. Anybody interested in the question is going to know what makes an airplane fly. As I said last year and in the other thread, the crux is whether the plane moves forward to create the airflow. The question isn't testing your knowledge of the Bernoulli Effect. Jebus.

[DJSapp]

Christ, a new batch of unbelievers. Here comes the big hammer.

For all the believers that the treadmill will accelerate to stop the plane here's the numbers:

Net Force= Net m*a

The 747

Force of friction = coefficient of friction * mass of object * gravity / radius of wheel = -[0.005 (coefficient of rolling friction from wheels) *910,000 lbs (mass of 747) * 32 ft/s^2 (accel. due to gravity) / 2 ft (radius of aircraft wheel) = 72,800 lbs force rolling resistance

Force of Engines pushing against air = mass of object (air) * acceleration = 95,000 lbs force (from Boeing's Website) * 4 engines = 380,000 lbs thrust

GUESS WHAT RETARDS, THE SPEED AT WHICH THE WHEELS ROTATE DOES NOT significantly* INCREASE THE FORCE PUSHING AGAINST THE AIRCRAFT!!!!! THIS IS HIGH SCHOOL PHYSICS!!! *one could make an argument for the thermal expansion of the bearings, heat losses, etc. however, everyone intellegent to understand these concepts understands how minimal they are in regard to the other forces at work here

So according to you guys, the treadmill instantly accelerates to the speed of light, imparting a frictional force of 72,800 lbs. The engines push with 380,000 lbs. There is a net force on the plane. The airframe moves forward with respect to the air (thus creating airspeed) and takes off.

I could continue to pick apart your fucked up faulty logic, but it's like fishing with dynamite in the grocery store. However, there is hope, Fox is starting up a new game show where adults try to answer fifth grade homework questions for big prizes (seriously). Go sign up. http://www.fox.com/areyousmarter/

[Roo]

All the people who think that the plane will not take off must also think that if you were on said treadmill with rollerblades on that there is no way you'd be able to pull yourself forwards along the treadmill by using the handrails.

Think about it.

[philippeR]

OMG, this thread is fuckin brilliant, FKNA !

[focus]

Christ, a new batch of unbelievers. Here comes the big hammer.

You still don't get it.

So according to you guys, the treadmill instantly accelerates to the speed of light, imparting a frictional force of 72,800 lbs.

This is, actually, according to you as well. What you don't seem to get is that the treadmill can only accelerate to the speed of light if the airplane accelerates to the speed of light in the opposite direction. I'm pretty sure that takeoff speed < the speed of light. It doesn't matter that the maximum frictional force < thrust. That is an entirely different discussion.

I could continue to pick apart your fucked up faulty logic, but it's like fishing with dynamite in the grocery store.

You didn't pick apart shit, man. Not to fight w/ you or anything. With that post, you're the last one who should talk about faulty logic.

Imagine holding a dowel rod in between your palms. Applying your interpretation of the question at hand to the dowel rod situation, you are required to keep that dowel rod stationary. It can only rotate in place. If you move your right hand in one direction, your left hand must necessarilly move in the opposite direction at exactly the same speed. According to you, the plane is the dowel rod. Your left hand is the treadmill belt and your right hand is the engines/thrust, and your right hand is a lot stronger/faster than your left hand. This is where you go wrong. Any discussion of forces involved, friction, and thrust is entirely beside the point.

What is actually happening, though, is that you are required to move both hands at the same speed in opposite directions. The dowel rod is the treadmill. If one hand (the plane) isn't moving forwards, the other hand (the treadmill belt), cannot be moving backwards. What happens when any force acts on your right hand to move it forwards?