Mythbusters to take on "the plane and the treadmill" conundrum?

[sao123]

The primary reason why the the treadmill could never apply enough force to hold the plane backwards... is the following.

The most force that could be transferred from the belt to holding the plane back, is the frictional force between the free rotating wheel and the axle through the greased bearings. once that force reaches its maximum, the rest is converted into a radial force which accelerated the spin velocity of the wheel. If that maximal frictional force was enough to stop the plane from moving on the backwards treadmill, then it wouldnt take off even on a non-moving runway...

Here's why.
The force of static friction (the force required to make a non-moving object begin to move) is always greater than the force of kinetic friction (the force necessary to keep it moving after it has started)...

The point where the friction force is most, then you have the least resistance from the treadmille (zero)... At some point the treadmille reaches its maximal negative force, and no matter how much faster it goes, it cannot apply any further force to the airplane. (in other words, kinetic friction is not dependent on velocity)


Hopefully this helps you with some further explaination.

 

[exdeath]

Originally posted by: sao123
The primary reason why the the treadmill could never apply enough force to hold the plane backwards... is the following.

The most force that could be transferred from the belt to holding the plane back, is the frictional force between the free rotating wheel and the axle through the greased bearings. once that force reaches its maximum, the rest is converted into a radial force which accelerated the spin velocity of the wheel. If that maximal frictional force was enough to stop the plane from moving on the backwards treadmill, then it wouldnt take off even on a non-moving runway...

Here's why.
The force of static friction (the force required to make a non-moving object begin to move) is always greater than the force of kinetic friction (the force necessary to keep it moving after it has started)...

The point where the friction force is most, then you have the least resistance from the treadmille (zero)... At some point the treadmille reaches its maximal negative force, and no matter how much faster it goes, it cannot apply any further force to the airplane. (in other words, kinetic friction is not dependent on velocity)


Hopefully this helps you with some further explaination.

Rolling friction is static friction, not kinetic friction. Ideally, there is no sliding in the wheel, the bearing rollers, etc. The bearing elements roll instead of slide, due to static friction at the contact points. The applied force then simply works with the static friction to cause a torque on the ball or roller in a bearing and causes it to roll. No need to overcome static friction, and by definition, kinetic friction. Ideally, there is no friction at all of any kind. The whole point of a bearing is to NOT have to overcome the strong static friction AND to not have to deal with sliding kinetic friction. All force is transfered into moving the mass of the object itself, not overcoming friction.

In reality though we know nothing is ever perfect do to the imperfect and uncontrollable nature of surfaces at the atomic level, etc. Where the energy loss comes in is the heating and elastic deformation of the contact surfaces (wheel and ground, bearing and bearing cage/race, etc), undetectable microscopic sliding, the work done moving and heating lubricant, radiation of heat from elastic collisions at the molecular level, etc.

You may get sliding friction when you generate enough heat and cause the parts to expand, in which case we all know that a bearing with any tiny bit of resistance is about 1/1000th as effective. Even so bearing designs take into account thermal expansion at their operating ranges, and often at ranges greater than their application to allow for margin of error and safety. But that is not relavent to this problem as wheel bearings can easily handle the speeds in question, namely twice the stall speed of the plane, with little resistance.

And as stated numerous times, even if the wheels were locked up, provided the plane stayed in balance and didn't nose dive or yaw off the runway, the plane could STILL take off, tires smoking and shreading the whole time. Now this may actually require a lighter load or more engine power, since the combined friction forces of all the locked up tires would present a obvious drag force that cancels out some of the trust force.

For example, the engines put out 1,000,000 N of force, and 20 tires with brakes on cause, lets throw a number out there, 100,000 N combined, that leaves only 900,000 N to accelerate the plane. If the plane requires 950,000 N of force to accelerate to it's wing stall speed in a certain distance, and we can't get 1,050,000 N out of the engines, the only way to take off is to increase the runway distance or lighten the plane so the acceleration comes out to be the same.

 

[James3shin]

The "Chuck Norris-ism" that someone posted was GOLD. I actually laughed out loud when I read that.

 

[DrPizza]

Originally posted by: smack Down

Originally posted by: DrPizza

Originally posted by: BigDH01
LOL, this thread is hilarious. I've been spreading this question throughout the office and most get it wrong at first thought but quickly change their answer.

I just paraphrased the original question but I think I got the spirit. If a giant treadmill the size of a runway existed, and we placed a plane on that runway, and set the treadmill up so that it sped at the plane's velocity but in the opposite direction, would the plane take-off? Most people still thought the plane would remain stationary. A quick explanation and people realized that the plane would take-off. The most useful analogy that I've seen here is the space shuttle analogy. When I used that most people seemed to get the idea.

The office now firmly believes that the plane will take-off given the question that I asked.

Yeah, that's generally the case. The intuition of the majority of people tells them at first that the plane will not move. After a quick explanation, the vast majority realize that the plane WILL move, and thus will take off. The people who don't realize this are either absolute idiots or trolls. I'm finding it hard to tell the difference in this thread.

The vast majority of people will consider the treadmill as matching the speed of the aircraft relative to treadmill surface in which case the plane can't take off. (ie they think of the case of a car driving on a treadmill but staying put relative to ground). So if the question states the plane can't move then it can't take off. If you change the question to the treadmill matching the ground speed then the plane would take off and a car on the treadmill would also advance.

OMG! That's correct! Except for the part about the car. Oh, and the fact that the treadmill matching the speed of the aircraft relative to the treadmill surface is a paradox for anything other than a speed of 0; in other words, suggesting that as the constraints to the problem makes no sense. Paradoxes of this sort are created when something refers to itself; like: "this sentence is false."

 

[randay]

Originally posted by: DrPizza

Originally posted by: smack Down

Originally posted by: DrPizza

Originally posted by: BigDH01
LOL, this thread is hilarious. I've been spreading this question throughout the office and most get it wrong at first thought but quickly change their answer.

I just paraphrased the original question but I think I got the spirit. If a giant treadmill the size of a runway existed, and we placed a plane on that runway, and set the treadmill up so that it sped at the plane's velocity but in the opposite direction, would the plane take-off? Most people still thought the plane would remain stationary. A quick explanation and people realized that the plane would take-off. The most useful analogy that I've seen here is the space shuttle analogy. When I used that most people seemed to get the idea.

The office now firmly believes that the plane will take-off given the question that I asked.

Yeah, that's generally the case. The intuition of the majority of people tells them at first that the plane will not move. After a quick explanation, the vast majority realize that the plane WILL move, and thus will take off. The people who don't realize this are either absolute idiots or trolls. I'm finding it hard to tell the difference in this thread.

The vast majority of people will consider the treadmill as matching the speed of the aircraft relative to treadmill surface in which case the plane can't take off. (ie they think of the case of a car driving on a treadmill but staying put relative to ground). So if the question states the plane can't move then it can't take off. If you change the question to the treadmill matching the ground speed then the plane would take off and a car on the treadmill would also advance.

OMG! That's correct! Except for the part about the car. Oh, and the fact that the treadmill matching the speed of the aircraft relative to the treadmill surface is a paradox for anything other than a speed of 0; in other words, suggesting that as the constraints to the problem makes no sense. Paradoxes of this sort are created when something refers to itself; like: "this sentence is false."

But that sentence is false, because its not true, therefore, the sentence will never gain lift because it doesn't move! no lift = sentence does not take off.

 

[Squisher]

Like I posted in the other thread.

If a plane was flying at 200 mph and you were to hang under the plane and affix a treadmill to the wheels that turned the wheels backwards at 200 mph. Would the plane fall from the sky?

 

[Midlander]

Can't you guys take it outside?

 

[shortylickens]

Originally posted by: James3shin
The "Chuck Norris-ism" that someone posted was GOLD. I actually laughed out loud when I read that.

Yur welcome.
Also, tanks to Merc for the addition.

 

[BillyBatson]

iiiiiiiii have never heard of this myth

 

[flot]

Surely someone out there owns the following things:

- Treadmill
- Rollerblades
- gas powered Leaf Blower
- Camcorder

I leave it as an exercise to the reader to figure out what to do with them.

Bonus points for:
- Cardboard wings and/or superhero costume
- Yelling "no snakes on this plane, MFer" as you fly off the (front of the) treadmill

PS: make sure the rollerblade wheels are well lubricated

 

[feralkid]

Originally posted by: flot
Surely someone out there owns the following things:

- Treadmill
- Rollerblades
- gas powered Leaf Blower
- Camcorder

I leave it as an exercise to the reader to figure out what to do with them.

Bonus points for:
- Cardboard wings and/or superhero costume
- Yelling "no snakes on this plane, MFer" as you fly off the (front of the) treadmill

PS: make sure the rollerblade wheels are well lubricated

Better to use an electric leaf blower and have a second person pull you forward with the cord.

 

[Randum]

you all know a plane stays up due to lift in the wings right? a treadmill makes no sense at all, why would you ever think a treadmill would let a plane fly?

 

[bleuless]

this thread seriously scares me. such a simple "conundrum" yet, causes so much confusion

 

[illusion88]

Arew we still talking about this or is this thread dead?

 

[6000SUX]

Holey moley. I'm shocked this even has to be discussed.

 

[jimbob200521]

Originally posted by: 6000SUX
Holey moley. I'm shocked this even has to be discussed.

QFT

The plane would take off because wheels do not propel or move an airplane, the jets or engines do.

 

[6000SUX]

Originally posted by: jimbob200521

Originally posted by: 6000SUX
Holey moley. I'm shocked this even has to be discussed.

QFT

The plane would take off because wheels do not propel or move an airplane, the jets or engines do.

Yep. For someone to think anything differently they have to commit multiple simultaneous errors of logic (or just fail to think it over). The thing that's distressing is that this is monkey knowledge-- we all know the basics of how planes fly and treadmills work, and the simple physical principles involved.

 

[Dritnul]

god i hate to enter this thread but...
from my understanding of physics the engine pushes backwards on the air which pushes forward on the plane in return however only gravity pushes the plane down and the only upward force is the normal force from the treadmill, which is only enough to stop the plane from sinking into the earth there is no upward force great enough to push the plane into the air

(no airflow-> no lift-> no flight)

thats my take but ill switch sides if some1 could provide proof otherwise

 

[6000SUX]

Originally posted by: Dritnul
god i hate to enter this thread but...
from my understanding of physics the engine pushes backwards on the air which pushes forward on the plane in return however only gravity pushes the plane down and the only upward force is the normal force from the treadmill, which is only enough to stop the plane from sinking into the earth there is no upward force great enough to push the plane into the air

(no airflow-> no lift-> no flight)

thats my take but ill switch sides if some1 could provide proof otherwise

Not quite. The plane thrusts itself forward with its props/engines. The wheels are not involved at all as long as the brakes aren't on (disregarding the slight amount of friction they introduce, which as we all know is not enough to keep the plane from taking off).

Taking off from a treadmill is just the same as taking off from a regular runway-- the treadmill just makes the wheels turn faster as the plane pushes itself forward with its own thrust. When it achieves enough velocity, it takes off due to the lift from the Bernoulli effect, again nothing the treadmill can interfere with.

It would take a very long treadmill for the show to test this, not that it needs a test. The treadmill would have to be as long as the minimum required length for a takeoff runway for the plane, plus a little extra length to account for the slight bit of friction introduced by the treadmill itself.

Your big mistake may become apparent when you think about what happens in the beginning. The plane thrusts and begins to move. The treadmill spins-- but the usual phrasing of the problem says that it spins only enough to counteract the motion of the plane (in other words, at a speed equal to the plane's forward motion). The plane keeps moving forward due to its thrust, and the treadmill keeps increasing speed, doubling the rotational speed of the wheels but doing absolutely nothing else.

Another mistake you made is in supposing that there is no airflow. You are presupposing the thing you "prove"-- that the plane does not move. In fact it does, and there is airflow. It moves because of its own thrust.

 

[Dritnul]

well i guess that makes sense but damn it now im back on the fence again

 

[randay]

i just set up a treadmill and a model airplane and tried this and the airplane didnt move at all! it just sat there motionless while the treadmill held it back! pretty cool.

 

[shortylickens]

Originally posted by: randay
i just set up a treadmill and a model airplane and tried this and the airplane didnt move at all! it just sat there motionless while the treadmill held it back! pretty cool.

We arent going to call shens just yet, but (as was already discussed) we need video and pics and stuff.
This has been gone over ad-nauseum with debate and rhetoric. Now is the time for evidence.

 

[BigDH01]

Originally posted by: darwin333But would you accelerate away from the spot you just took off from? Flying into a 30mph wind with a ground speed of 30mph puts 60mph of wind over the wings which is above the stall speed (with flaps). So in theory you would match the acceleration of the carrier and therefor not seem to move forward or reverse of the spot you took off from, right?

Of course a sudden decrease in wind speed would definitely ruin your day, but I am just curious if my theory is correct.

This is one of those "in theory it works but in real life it doesn't situations." On takeoff, once the plane's wheels leave the carrier you'd start to decelerate if you had the engines off. So you'd have to throttle for 30 knots on the carrier so that when you left the ground you would maintain the 60 knot flow over the wings. This would be nearly impossible to do and you'd be more likely to be giving the plane too much power as opposed to too little and accelerate.

This would be an easier situation on landing as you can actually throttle down and slowly approach the carrier to it's easier to get an idea of how much throttle is actually needed. However, this would never be done because if you are slowly approaching the carrier and low speed and altitude and the wind suddenly changed directions or died down, you'd stall and end up in the drink. The only time I ever had a negative ground speed was when I was a couple of thousand feet up. If I were to stall, big deal. Nose down, throttle up, and away I go.

 

[archiloco]

Originally posted by: shortylickens

Originally posted by: randay
i just set up a treadmill and a model airplane and tried this and the airplane didnt move at all! it just sat there motionless while the treadmill held it back! pretty cool.

We arent going to call shens just yet, but (as was already discussed) we need video and pics and stuff.
This has been gone over ad-nauseum with debate and rhetoric. Now is the time for evidence.

yeah pics/video plz. i am sure you are correct, but need PROOF

 

[LeiZaK]

The plane will not take off, but will crash due to its landing gear being ripped off by the treadmill.