Physics Question

[giantpinkbunnyhead]

Originally posted by: ta8689
Because if you are a pilot and have ever piloted an aircraft, then you should know that it will take off

Not necessarily... before I had thought about it, I first argued that the plane wouldn't take off. Sometimes the obvious escapes us.

<-----commercial pilot, air cargo, B-727

 

[speg]

Someone call the Mythbusters.

 

[SophalotJack]

Anyone who has thought this through nows that the question is not specific enough to conclude on an answer.

Main thing the question leaves out is if the airplane is moving relative to it's ssurrounds and if the friction, caused by the the rotating wheels, is constantly increased by the conveyer belt to counter any forward progress of the airplane relative to it's surroundings.

 

[Kelemvor]

In most physics questions you assume that there is no friction. This would mean between the wheel's bearing and the wheel and the plane

If this is the case, then imaging the plane not turning on its engines. The conveyor belt could be spinning at 1000 MPH and the wheels would be spinning at 1000MPH but theplane wouldn't care and it would not move. It would not affect the plane at all because the wheels simply spin freely. So to this if you then turn on the jets, the plane will go. The ground speed moving has no effect on the plane if there is no friction.

 

[letdown427]

It oly really seems to be those argueing that the plane won't take off that insist the question is worded wrongly or badly.

Sorry for bumping this back up again. Oh and rayden, that's another good analogy proving it can take off! congratulations! man there's been so many analogies, it's awesomely impressive.

 

[SophalotJack]

Originally posted by: letdown427
It oly really seems to be those argueing that the plane won't take off that insist the question is worded wrongly or badly.

Sorry for bumping this back up again. Oh and rayden, that's another good analogy proving it can take off! congratulations! man there's been so many analogies, it's awesomely impressive.

Yes, and resorting to the midset that the people you don't agree with are simply not understanding the question is hypocritical. And doesn't help you prove your point at all.

WTF kind of world has zero friction? If we had that kind of technology, we could go to the moon and back faster than a road trip to grandma's.


Stop with the left-side vs. right-side crap that all debates eventually fall into... the question leaves open to many possible answers... so nobody AND everybody is right.

 

[ta8689]

Originally posted by: speg
Someone call the Mythbusters.

I put the idea up on their website!

 

[Evadman]

To something that was said earlier, you don't need a wing for an object to fly. With enough thrust you can fly a brick.

Also, back to my point about 400 posts ago. The conveyor is on the gound. The conveyor only effects the wheels, not the air movement.

If the air were moving at the same rate the plane was (such as in a wind tunnel trying to fly with the current instead of against it) then the plane could not take off. But the only thing moving is the conveyor, AKA the ground.

 

[mugs]

Originally posted by: SophalotJack

Originally posted by: letdown427
It oly really seems to be those argueing that the plane won't take off that insist the question is worded wrongly or badly.

Sorry for bumping this back up again. Oh and rayden, that's another good analogy proving it can take off! congratulations! man there's been so many analogies, it's awesomely impressive.

Yes, and resorting to the midset that the people you don't agree with are simply not understanding the question is hypocritical. And doesn't help you prove your point at all.

WTF kind of world has zero friction? If we had that kind of technology, we could go to the moon and back faster than a road trip to grandma's.


Stop with the left-side vs. right-side crap that all debates eventually fall into... the question leaves open to many possible answers... so nobody AND everybody is right.

He's right though, the question gives all of the necessary information.

You raised two questions.

1. Is the plane's speed relative to its surroundings? Doesn't matter. The friction in the wheels doesn't depend on how fast they're rotating.

2. "if the friction, caused by the the rotating wheels, is constantly increased by the conveyer belt to counter any forward progress of the airplane relative to it's surroundings." - The question explicitly states that the speed of the conveyor belt matches the speed of the plane. It does not say that the conveyor belt's speed will infinitely increase in an attempt to slow the plane down, if it even COULD slow the plane down, which it really can't.

WTF kind of world has zero friction? The world in which theoretical physics problems are solved. But even in reality, friction won't keep the plane from moving.

Think about this - in order for the plane to be completely motionless, the conveyor belt must also be motionless. In order for the conveyor belt to even start moving, the plane must be able to move. The conveyor belt cannot hold the plane motionless while it itself is moving. The question does not allow it.

 

[elkinm]

I thought of new analogy. Remember my non motorized treadmill. I try to move forward but the tread moves back and I go nowhere, just like this would work with a car on the conveyor.
Now imagine a huge guy runs in to me from behind. I don't care how fast the conveyor is going, I will end up flying forward. The huge guy is like the planes engines which is like a force outside the system acting on the plane. If would not move forward only if my feet were glued to the tread not allowing me to move.

So unless the wheels are glued, the plane will move forward, the conveyor will go backwards the wheels will spin at double the speed of the plane (or the conveyor). But the plane is moving forward relative to the ground and therefore relative to the air so it will be able to take off.

I really like the skateboard idea. The problem is that the fan probably won't propel it too much. Stick it on a treadmill. I think if the fan is enough to move it, it will move regardless of the speed of the treadmill.

Anybody feel like trying this with a REMOTE CONTROLLED PLANE?

 

[Lovepig]

Vertical Take-off


The answer depends upon the type of plane.

Alternatively the whole conveyor could be brought up to lift-off speeds allowing the plane to get airborne...

 

[SophalotJack]

lol, remote control plane and a treadmill!

That would be hilarious. But I would go with the results of the test.

 

[Rayden]

There really is no reason to try it. It can be solved with some simply physics.

 

[mugs]

You'd still need a really long treadmill with a remote controlled plane, because it would need just as much room to take off as it normally would.

 

[CigarSmokedByClinton]

Its obvious to me the plane would take off.

Assume standard wheels with standard properties of inertia and friction. Assume the takeoff velocity for this particular airplane is 100MPH(wrttg)(With Respect to the Ground).

I think I've come up with some equations to prove this.

Terms: Vpg is Velocity of the plane with respect to the ground
Vpc is the Velocity of the plane with respect to the conveyor
Vcp is the Velocity of the conveyor with respect to the plane
F is the coefficient of friction of the wheel bearing(lets assume 10%, though that's pretty high I'm sure)

Eq 1:Now from the original question, Vcp = -Vpc
Eq 2:We can deduce that Vpg = Vpc(1-F)
Eq 3: Substitute Eq 1 into Eq 2, you get Vpg=Vcp(1-F)

If we need Vpg to be 100MPH for flight, we can solve for Vcp, and consequently we'll get Vpc.

100MPH = -Vcp x (1-.10)

Vcp = -100MPH/.9
Vcp= -111.11111MPH = -Vpc

Therefore the plane will take off, and the conveyor will be running at -111.11MPH with respect to the plane

Let me try to put it in words:

Assume the plane's engines put out enough thrust to accelerate the plane to a velocity of 1 ft/second(wwttc)(with respect to the conveyor). The conveyor would then instantly react by speeding up to match the plane's velocity in the opposite direction(So -1ft/second wwttp). The conveyor would slow the plane down a bit because of the friction and inertia in the wheel bearings, but it would have a minimal effect(lets assume 10%)on the plane's forward velocity(wwttg). The plane would continue forward at a forward velocity of .9 ft/second(wwttg), BUT IT IS STILL MOVING FORWARD. Now the pilot gooses the engines to speed the plane up to 100MPH(wwttc). The conveyor would accelerate to 100MPH(wwttp) in the opposite direction. Due to the friction and inertia from the wheels, the plane will only be going 90MPH(wwttg), so wouldn't take off yet. The pilot adds some more thrust, and at some point the plane will reach the 100MPH(wrttg) it needs for adequate lift.

It seems people are using the argument that the conveyer will accelerate to whatever speed required to maintain a zero velocity for the plane wwttg, but this is not supported in the original question.

 

[CigarSmokedByClinton]

Originally posted by: mugs
You'd still need a really long treadmill with a remote controlled plane, because it would need just as much room to take off as it normally would.

It'd actually need a bit longer of a runway than normal, but you're right

 

[littleprince]

For those that think it won't take off the key here is the treadmil moves in relation to the speed of the aircraft.
It does not move in a speed relative to make forward motion of the aircraft = 0. There is a huge difference. The first is assuming the force of the treadmill is applied only as a drag force.
The 2nd implies that the speed is much greater than what the speed of the aircraft would've been and the force is still applied as a drag but it is so great it is = to the - of the plane speed.

 

[Lovepig]

How about this?

1. Get the plan going 100+ mph in REVERSE
2. Conveyor matches so net speed is zero
3. Slam on the brakes
4. Conveyor also "brakes" to zero
5. Plane is thrust forward at 100+ mph allowing lift



(ok, I know the planes probably dont really go 100+ mph backwards, and they also dont stop instantly, but in my defense, they are also not on big conveyor belts. I'm just going with the highly hypothetical theme here...!)

 

[jagec]

Originally posted by: Chryso
Ok, am I right in thinking that the conveyor and the wheels of the plane will be constantly increasing to an infinite speed as the plane begins to move forward until it is no longer on the conveyor or it leaves the ground?

It...depends. In any realistic setup, the plane will accelerate to takeoff speed. The conveyor will track it, and accelerate backwards at the same rate. The plane's wheels will be spinning at twice "takeoff speed", but the plane will roll down the runway and take off normally. If you were watching through binoculars, you'd notice a VERY slightly longer takeoff roll, but that's it.

If we fit the plane with magical indestructible wheels and bearings, and change the conveyor control software to attempt to keep the plane at a standstill relative to the GROUND (instead of simple proportional control at -1 * plane speed, use PI control or something), we CAN actually keep it from flying. But we're talking about a pretty insane rate of acceleration on the part of the treadmill, and the problem NEVER reaches steady state until 100% of the fuel has been converted into angular momentum in the wheels. That's a lot of energy (actually, the wheels are holding 200% of the energy that was in the fuel, assuming perfectly efficient engines...the conveyor contributes an equal amount of energy.) Mind you, altering the control software is NOT in the parameters of the original question...it's just an intriguing concept. When I saw this problem and finally convinced myself the plane WOULD take off, I thought "is there any way you COULD stop a plane from taking off, just with a conveyor belt?"

Originally posted by: ta8689
omfg are there any other pilots in here? honesly. And yes i have a private pilot's certificate.

I have to point out that being a pilot doesn't really help anyone answer the question, since few pilots have ever had to contend with the scenario being described

 

[gsellis]

Originally posted by: DrPizza
Hmmmm...
B. The idea of a conveyor moving the plane backwards wouldn't work. The plane is providing thrust against the air behind it. i.e. when you turn on the engines, the plane moves relative to the air; not necessarily the ground. If the ground were perfectly frictionless, the tires on the plane would not spin; yet the plane could still accelerate down the runway and take off. Thus, the conveyor moving in the opposite direction relative to the direction the plane is moving would have this effect: The planes wheels would spin faster; the conveyor realistically could *not* keep the plane stationary.

edit: changed the word "land"?? to "take off"

I found this on the AT-Wiki for ownage. Proof on the last page, but this is the first, correct answer. But, of course it is, because it is the Dr.

The net from the wheels and conveyor belt are almost 0 except for friction as they are free wheeling. Thrust moves it forward. This creates airflow across the wings. This generates lift. When lift > weight, flight. And some very warm bearings on the wheel hubs. Consider that a General Electric turbofan on Boeing 757 produces 50.000 lbs of thrust (each) and flows up to about 800mph, even with bad wheels, the bearings and rubber would lose.

And I know what you are thinking, planes weigh more than 100,000lbs. Thrust has to be equal to drag where the forward speed creates enough lift to be greater the weight. Thrust does not need to equal weight unless you want to go ballistic.

Yep, the wheels are just a distraction.

 

[g8wayrebel]

Originally posted by: Chryso

Originally posted by: Armitage
One more time ...

All the naysayers - please tell me what force opposes the thrust of the engines to keep the plane stationary?

Thrust does not equal lift. You need airflow over the wings for lift.
I could put a jet engine on a car but that won't make it fly.

Actually , it will. In the Air Force Times a number of years back tey had an investigation of a plane crash out west. An engineer for the AF had mounted two JATO {Jet Assist Take Off} rockets to an old buick if I recall the make correctly. In any case , he was found well above the ground smashed into the side of a cliff. Aerodynamics allow control of flight, but thrust will lift anything causing it to "fly."
edited for grammar and this:
After a little homework , I apologize to those I slammed
It will in fact take off.

 

[DrPizza]

Originally posted by: g8wayrebel

Originally posted by: Chryso

Originally posted by: Armitage
One more time ...

All the naysayers - please tell me what force opposes the thrust of the engines to keep the plane stationary?

Thrust does not equal lift. You need airflow over the wings for lift.
I could put a jet engine on a car but that won't make it fly.

Actually , it will. In the Air Force Times a number of years back tey had an investigation of a plane crash out west. An engineer for the AF had mounted two JATO {Jet Assist Take Off} rockets to an old buick if I recall the make correctly. In any case , he was found well above the ground smashed into the side of a cliff. Aerodynamics allow control of flight, but thrust will lift anything causing it to "fly."
edited for grammar and this:
After a little homework , I apologize to those I slammed
It will in fact take off.

I thought the jato engines on a vehicle smashed into the side of a cliff were an urban legend... But, at least you know the plane will take off.

 

[g8wayrebel]

I can't say that the Air Force Times didn't misreport it , but I saw the article myself.
It showed the location , gave the man's name , rank and job description , and described the physical attributes of the car , including his fingernails embedded in the steering wheel and the brakes being melted.

 

[blahblah99]

Originally posted by: gsellis

Originally posted by: DrPizza
Hmmmm...
B. The idea of a conveyor moving the plane backwards wouldn't work. The plane is providing thrust against the air behind it. i.e. when you turn on the engines, the plane moves relative to the air; not necessarily the ground. If the ground were perfectly frictionless, the tires on the plane would not spin; yet the plane could still accelerate down the runway and take off. Thus, the conveyor moving in the opposite direction relative to the direction the plane is moving would have this effect: The planes wheels would spin faster; the conveyor realistically could *not* keep the plane stationary.

edit: changed the word "land"?? to "take off"

I found this on the AT-Wiki for ownage. Proof on the last page, but this is the first, correct answer. But, of course it is, because it is the Dr.

The net from the wheels and conveyor belt are almost 0 except for friction as they are free wheeling. Thrust moves it forward. This creates airflow across the wings. This generates lift. When lift > weight, flight. And some very warm bearings on the wheel hubs. Consider that a General Electric turbofan on Boeing 757 produces 50.000 lbs of thrust (each) and flows up to about 800mph, even with bad wheels, the bearings and rubber would lose.

And I know what you are thinking, planes weigh more than 100,000lbs. Thrust has to be equal to drag where the forward speed creates enough lift to be greater the weight. Thrust does not need to equal weight unless you want to go ballistic.

Yep, the wheels are just a distraction.

Yup, it all boils down to F=MA. Without the conveyer belt system, F would equal force of engine - air resistance. With conveyer belt system in place, F is now equal to force of engine - air resistance - friction of wheels & bearings. The wheels can spin 100,000,000 RPMS for all I care... it counts as friction and adds negatively to the sum of the forces involved to accelerate mass....

And if that mass is accelerated over time, its velocity increases enough to allow aerodynamics to lift the plane.

It's 8th grade physics.

 

[gsellis]

Originally posted by: g8wayrebel

Originally posted by: Chryso

Originally posted by: Armitage
One more time ...

All the naysayers - please tell me what force opposes the thrust of the engines to keep the plane stationary?

Thrust does not equal lift. You need airflow over the wings for lift.
I could put a jet engine on a car but that won't make it fly.

Actually , it will. In the Air Force Times a number of years back tey had an investigation of a plane crash out west. An engineer for the AF had mounted two JATO {Jet Assist Take Off} rockets to an old buick if I recall the make correctly. In any case , he was found well above the ground smashed into the side of a cliff. Aerodynamics allow control of flight, but thrust will lift anything causing it to "fly."
edited for grammar and this:
After a little homework , I apologize to those I slammed
It will in fact take off.

Urban Legend. This was the first "Darwin Award" and was UL.