Mythbusters punk'd whole internet

[smack Down]

Originally posted by: jagec

Originally posted by: smack Down

Originally posted by: jagec

Originally posted by: smack Down

Originally posted by: jagec

Originally posted by: smack Down
Don't bother your trying to explain things involving momentum to fools who think it matters where a force is applied to a solid body. You might as wheel teach a dog long division you might have better luck. Just call them idiots and be done.

It DOES matter where the force is applied, since the body is not solid but is composed of a solid portion and an attached, but free-wheeling portion.

You've got the wrong idea about why it works, and the constraints under which it works. I seek to rectify that.

Doesn't matter at the the force must be applies to the frame. F=ma learn it. If your car accelerates the the force must have been applies to it. It doesn't matter where.

That's where you're wrong. The engines apply force at the frame, but the treadmill applies force at the wheels, and can only apply angular force. If the original problem specified massless wheels and no friction, there is NO way the treadmill could keep the plane stationary.

As for the "speed of the wheels" bit, let me specify a simpler to understand control system.

Put a position sensor next to the runway. Hook up your control system to this sensor. Attempt to keep the plane in one place by applying control inputs to the treadmill. It's possible if the treadmill is sufficiently powerful, and if we have wheels of finite (nonzero) mass and/or friction at the hub. If we throw out these constraints, it's impossible. If the plane is kept in one place, it will NOT take off.

If the control system is matching the ground speed of the plane (relative to stationary earth), the plane WILL take off, no ifs, ands, buts, or fancy assumptions necessary.

There are two ways to understand the control system. One way allows the plane to take off, the other does not, but requires an immensely overpowered treadmill to work at all. Otherwise it's like trying to stop a boat from launching by squirting a Super Soaker at the bow.

I didn't no wheels with mass was a "fancy assumptions" and here I've been driving my car around all this time with out massless wheels. Do you know where I can get some of your magical tires so that my plane will get better gas millage.

Read my post, I said that massless wheels was a fancy assumption.:roll:

You still fail to understand the problem. Go do an order-of-magnitude calculation on the thrust provided by 4 standard-size jet engines, and the acceleration necessary to impart an equal force on standard-weight, standard-size aircraft wheels via the angular momentum and friction terms. You'll find it's shockingly high. Clearly the original creator of this problem intended the first definition of the control system, but worded it poorly. Attempting to stop the plane using the second (somewhat bizarre) definition is purely a thought experiment, since it would be impossible to perform in real life.

Of course it is a thought experiment and that is why myth buster will screw it up. I disagree with you that it is the bizarre reading of the question. It is the interpolation must people are to use when they first read the question. Can a treadmill keep an object stationary by matching it speed. The fact that it is a plane is 100 irrelevant.

 

[jagec]

Originally posted by: Skeeedunt
I think this finally makes sense. If the wheels are frictionless, the plane takes of. Otherwise, the conveyor can move the wheels so fast that the downward force of the wheels' rotational inertia is so great that it counteracts the thrust of the engine against the air. Am I right am I right?? Do I get a cookie??

Change "downward" to "backwards", friction actually isn't necessary (but it certainly lowers the treadmill requirements), but besides that :thumbsup:. Note that this is only for the second definition of the control system...in the first definition the plane will take off every time.

 

[smack Down]

Originally posted by: Skeeedunt
I think this finally makes sense. If the wheels are frictionless, the plane takes of. Otherwise, the conveyor can move the wheels so fast that the downward force of the wheels' rotational inertia is so great that it counteracts the thrust of the engine against the air. Am I right am I right?? Do I get a cookie??

Almost but friction is the force that matters as long as the wheels have mass the plane would be unable to take off. If the wheels do not have mass then that would mean it violates the conditions of the problem.

 

[mobobuff]

This episode is actually looking pretty lame...

A simulator? C'mon...

 

[OulOat]

Originally posted by: videogames101

Originally posted by: OulOat

Originally posted by: videogames101
THEY ARE? holy fuck, no better way to say it. THIS IS SPARTA!!!!!!!!!!

EDIT: and for all those noobs out there, it wil not take off. For planes to have a different speed then the wheels there must be MOVING AIR beneath the plane's wings, hence, it's won't take off.


1000 to 1 odds to any takers,,,,

I'll take it. 1000 to 1 odds, everyone here is a witness.

One little flaw in your analysis. A plane moves because the engine(s) push air out of the exhaust, which results in moving air beneath the wings.

are you implyign air is forced under the wings by motors? lol, air is forced under my the movement of the plane, and the plane starts with no movement, while moving on WHEELS. so, as the wheels spin, the treadmill goes opposite, no movement, no air, no lift.

Plane is moved by the engines moving air. As the treadmill spins, the wheels gain speed at twice the speed of the plane (treadmill speed + plane speed). Plane engines push the air, car engines push the ground. Treadmill can only move the ground, not the air.

 

[randay]

Originally posted by: jagec

Originally posted by: randay

Originally posted by: jagec
At that point you CAN actually match the so-called "wheel speed" of the plane with the treadmill.

You can never match treadmill speed with wheel speed. The wheel will always be moving faster then the treadmill. x=y=x+y

Disclaimer: Stopping an airplane from taking off by using a treadmill involves some rather unrealistic assumptions about the power of the treadmill and the robustness of the wheels.

BUT assuming a treadmill of arbitrarily high power, and indestructible wheels of nonzero mass (I'll throw frictionless bearings in there to keep it simple), the treadmill can slow down the plane by speeding up the wheels.

So,
we put a "speed sensor" with a wireless transmitter into the hub of one wheel. The treadmill is designed to match this measured speed in reverse.
Now the plane pulses its engines for a brief second, which causes it to begin moving forward with a small, constant speed. The treadmill accelerates to try and match this speed. The extra force is exerted on the wheels, which have a certain angular momentum. As long as the treadmill continues to accelerate and provide that force, it couples with an equal force at the hub to cause a torque and induce angular acceleration in the wheels. Well, that force at the hub is borrowed from the plane's momentum, since the engines are off. Eventually all of the momentum term is eaten up, the plane comes to a "stop", and the treadmill is now matching the wheel speed. Now that acceleration has stopped, the force on the wheels (and therefore angular acceleration) is back to nil, so the plane will sit in place for forever, barring frictional losses or further engine or treadmill inputs.

Another example: Get a Matchbox car, and one of those flywheel toy cars. Tie a string to the front of the Matchbox car, and measure the force required to keep it stationary on a treadmill. This will be a very small initial force as the wheels accelerate to the treadmill speed, and then an even smaller constant force to compensate for frictional losses. Repeat the experiment with the flywheel car. There will be a fairly substantial initial force as both the wheels and the flywheel accelerate to the treadmill speed, and then a much smaller constant force to compensate for frictional losses (slightly greater than the matchbox car, but a similar order of magnitude). Now accelerate the treadmill on both cars. The matchbox car's force will increase slightly, due to a tiny amount of angular momentum and slowly increasing frictional losses. The flywheel car's force will increase much more, and will remain at a constant high point (slowly increasing due to friction) as the treadmill maintains acceleration.

The way that the treadmill can overcome the force of the engines is through constant acceleration. I believe that once the plane runs out of fuel, the amount of energy tied up in angular momentum in its wheels will be roughly double the amount of energy that was extracted from the fuel (the treadmill contributes an equal amount of force in the opposite direction).

You have not addressed the issue that the treadmill can NEVER match the speed of the wheels relative to itself. Which makes the whole infinite-super-treadmill theory bunk.

Also the problem with what you believe is that you assume that the treadmill will stop once net motion is zero, thats not true since the wheels are still spinning and therefore the treadmill is still accelerating and angular momentum is still building. The treadmill will overcome the thrust of the engines(through angular momentum achieved by infinite acceleration and velocity), and then the airplane will accelerate backwards until it takes off anyway. granted, it will not take off very long since there will be no way to control it backwards, however it will gain an infinite amount of speed and eventually and messily, catch air.

The bottom line is that the treadmill speed = wheel speed interpretation is wrong, and the only correct one is treadmill speed = wind speed. or if you still want to entertain the crazy infinite theory, the only way its true is when wheel speed is zero.

 

[mugs]

Originally posted by: fuzzybabybunny
Looks like they're NOT doing the plane + conveyor belt thing.

It says they are on their website, so if they don't I'm burning their building down. Honestly, how long can this flight simulator crap take?

 

[mordantmonkey]

it depends on what you consider to have "magical" properties in the problem
magic frictionless wheels + real world conveyor belt = plane takes off
magic wheels + magic conveyor = plane takes off
real world wheels + real world conveyor = plane takes off
real world wheels + magic infinite velocity, speed matching conveyor belt = plane doesn't take off* **
*of course wheels will probably have mechanical failure
**also you probably don't need infinite conveyor speed, but rather ridiculously high velocity

 

[smack Down]

Originally posted by: randay

Originally posted by: jagec

Originally posted by: randay

Originally posted by: jagec
At that point you CAN actually match the so-called "wheel speed" of the plane with the treadmill.

You can never match treadmill speed with wheel speed. The wheel will always be moving faster then the treadmill. x=y=x+y

Disclaimer: Stopping an airplane from taking off by using a treadmill involves some rather unrealistic assumptions about the power of the treadmill and the robustness of the wheels.

BUT assuming a treadmill of arbitrarily high power, and indestructible wheels of nonzero mass (I'll throw frictionless bearings in there to keep it simple), the treadmill can slow down the plane by speeding up the wheels.

So,
we put a "speed sensor" with a wireless transmitter into the hub of one wheel. The treadmill is designed to match this measured speed in reverse.
Now the plane pulses its engines for a brief second, which causes it to begin moving forward with a small, constant speed. The treadmill accelerates to try and match this speed. The extra force is exerted on the wheels, which have a certain angular momentum. As long as the treadmill continues to accelerate and provide that force, it couples with an equal force at the hub to cause a torque and induce angular acceleration in the wheels. Well, that force at the hub is borrowed from the plane's momentum, since the engines are off. Eventually all of the momentum term is eaten up, the plane comes to a "stop", and the treadmill is now matching the wheel speed. Now that acceleration has stopped, the force on the wheels (and therefore angular acceleration) is back to nil, so the plane will sit in place for forever, barring frictional losses or further engine or treadmill inputs.

Another example: Get a Matchbox car, and one of those flywheel toy cars. Tie a string to the front of the Matchbox car, and measure the force required to keep it stationary on a treadmill. This will be a very small initial force as the wheels accelerate to the treadmill speed, and then an even smaller constant force to compensate for frictional losses. Repeat the experiment with the flywheel car. There will be a fairly substantial initial force as both the wheels and the flywheel accelerate to the treadmill speed, and then a much smaller constant force to compensate for frictional losses (slightly greater than the matchbox car, but a similar order of magnitude). Now accelerate the treadmill on both cars. The matchbox car's force will increase slightly, due to a tiny amount of angular momentum and slowly increasing frictional losses. The flywheel car's force will increase much more, and will remain at a constant high point (slowly increasing due to friction) as the treadmill maintains acceleration.

The way that the treadmill can overcome the force of the engines is through constant acceleration. I believe that once the plane runs out of fuel, the amount of energy tied up in angular momentum in its wheels will be roughly double the amount of energy that was extracted from the fuel (the treadmill contributes an equal amount of force in the opposite direction).

You have not addressed the issue that the treadmill can NEVER match the speed of the wheels relative to itself. Which makes the whole infinite-super-treadmill theory bunk.

Also the problem with what you believe is that you assume that the treadmill will stop once net motion is zero, thats not true since the wheels are still spinning and therefore the treadmill is still accelerating and angular momentum is still building. The treadmill will overcome the thrust of the engines(through angular momentum achieved by infinite acceleration and velocity), and then the airplane will accelerate backwards until it takes off anyway. granted, it will not take off very long since there will be no way to control it backwards, however it will gain an infinite amount of speed and eventually and messily, catch air.

The bottom line is that the treadmill speed = wheel speed interpretation is wrong, and the only correct one is treadmill speed = wind speed. or if you still want to entertain the crazy infinite theory, the only way its true is when wheel speed is zero.

Are you special or something. Of course the treadmill stops accelerating when it matches the ground speed of the plane. The speed of the treadmill will always be equal to the speed of the plane on the treadmill because that is how the problem define the control system.

Bottom line you failed basic logic.

 

[mobobuff]

Originally posted by: randay

The bottom line is that the treadmill speed = wheel speed interpretation is wrong, and the only correct one is treadmill speed = wind speed. or if you still want to entertain the crazy infinite theory, the only way its true is when wheel speed is zero.

Exactly. The original question was very blunt and didn't specify, but it makes much more sense to assume the treadmill was meant to match ground speed.

This question wasn't thought up by a physicist.

 

[smack Down]

Originally posted by: mordantmonkey
it depends on what you consider to have "magical" properties in the problem
magic frictionless wheels + real world conveyor belt = plane takes off
magic wheels + magic conveyor = plane takes off
real world wheels + real world conveyor = plane takes off
real world wheels + magic infinite velocity, speed matching conveyor belt = plane doesn't take off* **
*of course wheels will probably have mechanical failure
**also you probably don't need infinite conveyor speed, but rather ridiculously high velocity

Right just a small nit pick but it sn't the velocity that matters. Instead it is the acceleration that matters.

 

[ultimatebob]

At least they're proving that Point Break is BS, anyway

 

[jagec]

Originally posted by: smack Down
Of course it is a thought experiment and that is why myth buster will screw it up. I disagree with you that it is the bizarre reading of the question. It is the interpolation must people are to use when they first read the question. Can a treadmill keep an object stationary by matching it speed. The fact that it is a plane is 100 irrelevant.

Considering that lots of the ATOTers use the first interpretation (treadmill matches plane's speed with respect to stationary object), I don't think "most" people are using the second interpretation.

As for the bolded portion, the answer is "it depends". If the wheels are light enough, friction is low enough, and the treadmill does not possess INFINITE power, the answer is "no, it cannot keep an object stationary". Realistic wheels, friction, and treadmill power levels are actually surprisingly close to this approximation. With very heavy wheels, a very weak plane, and/or a disproportionately powerful treadmill, the answer is "yes".

Originally posted by: randay
You have not addressed the issue that the treadmill can NEVER match the speed of the wheels relative to itself. Which makes the whole infinite-super-treadmill theory bunk.

Also the problem with what you believe is that you assume that the treadmill will stop once net motion is zero, thats not true since the wheels are still spinning and therefore the treadmill is still accelerating and angular momentum is still building. The treadmill will overcome the thrust of the engines(through angular momentum achieved by infinite acceleration and velocity), and then the airplane will accelerate backwards until it takes off anyway. granted, it will not take off very long since there will be no way to control it backwards, however it will gain an infinite amount of speed and eventually and messily, catch air.

Yes I have. Take your initial setup: Plane speed with respect to stationary object = P, treadmill speed with respect to stationary object = T, wheel speed = (P + T). If we try to match the treadmill speed to the wheel speed (T = (P + T)), this looks like a paradox until we realize that as long as the treadmill is accelerating, it can interact with the plane and reduce its speed. So the equation becomes (T = (P-AX) + T), where AX is the acceleration of the treadmill (A) times a term (X) which encapsulates the dimensions of the wheels and converts AX into a velocity. The treadmill has to accelerate at a rate where AX = P. If it accelerates faster, the plane moves backwards on the runway. If it accelerates more slowly, the plane moves forwards on the runway. Once the plane runs out of fuel, the treadmill must stop accelerating and continue at a constant speed.

The bolded portion is untrue. I said that the treadmill will stop ACCELERATING when the plane returns to the control position (its initial position on the runway). It will continue moving at a constant speed, with the wheels turning at a constant speed and angular momentum, with no acceleration and no buildup of further forces.

Originally posted by: mordantmonkey
it depends on what you consider to have "magical" properties in the problem
magic frictionless wheels + real world conveyor belt = plane takes off
magic wheels + magic conveyor = plane takes off
real world wheels + real world conveyor = plane takes off
real world wheels + magic infinite velocity, speed matching conveyor belt = plane doesn't take off* **
*of course wheels will probably have mechanical failure
**also you probably don't need infinite conveyor speed, but rather ridiculously high velocity

:thumbsup:

Where "magic wheels", I assume, are defined as frictionless and massless.

 

[Skeeedunt]

Originally posted by: jagec
Change "downward" to "backwards", friction actually isn't necessary (but it certainly lowers the treadmill requirements), but besides that :thumbsup:. Note that this is only for the second definition of the control system...in the first definition the plane will take off every time.

Ahhhh gotcha.

Originally posted by: smack Down
Almost but friction is the force that matters as long as the wheels have mass the plane would be unable to take off. If the wheels do not have mass then that would mean it violates the conditions of the problem.

Yeah I guess the force being applied doesn't really require friction at all since the objects are in some sense full attached/enclosed.

 

[smack Down]

Originally posted by: mobobuff

Originally posted by: randay

The bottom line is that the treadmill speed = wheel speed interpretation is wrong, and the only correct one is treadmill speed = wind speed. or if you still want to entertain the crazy infinite theory, the only way its true is when wheel speed is zero.

Exactly. The original question was very blunt and didn't specify, but it makes much more sense to assume the treadmill was meant to match ground speed.

This question wasn't thought up by a physicist.

No it doesn't the whole idea behind a treadmill is that it matches the speed of the object on it so that the object on it stays in one position.

 

[skulkingghost]

I still see no mention on the conveyor belt :-(

I do believe the whole conveyor belt thing is nonsense, the plane is moved by its engines, as long as they are producing trust against the air, the conveyor belt is of no concern.

 

[Jeff7]

Originally posted by: smack Down
How many times do I need to repeat there is no difference between a car and a plane. The only time it would matter is if we were talking about placing them on rollers.

Lets make this simple.

Do you agree that a car on a stop treadmill will accelerate?
Do you agree that in that case the a force is applied to the body of the car?
Then why do you thing that once the treadmill is started a force is no longer applied to the body of the car?
If you don't think that what makes the plane any different then the car?

Force trace:

Car
Tires push on the ground.
Tires push on the wheels.
Wheels push on the axles.
Axles push on the frame.

If the ground is moving, this will cancel out the forward force created by the tires onto the car's frame.

Airplane
Engines push on the air.
Engines push on the frame.

What the wheels are doing doesn't matter. I don't care if they're being spun forward or backward, at 1rpm, or 100,000 rpm. In the ideal example (ignoring bearing friction), they are exerting no force on the plane. The ground moves beneath them, imparting a force which is converted into rotational momentum, not linear momentum. Since it is rotational only, it does not serve to push the plane forward, nor backward, thus it does not counteract the forward force of the jet engines, which derive their impulse from the air flowing through them. That part is completely independent of the ground's motion.
AND, since the wheels' only momentum is rotational, not linear, they generate no force to oppose that created by the engines.
Result: Net force in the forward direction. a = F/m. Force is positive, acceleration occurs. Plane takes off.

If you still don't get it, I don't know what more can be said.
Go take physics and dynamics courses.

 

[mobobuff]

Originally posted by: smack Down

Originally posted by: mobobuff

Originally posted by: randay

The bottom line is that the treadmill speed = wheel speed interpretation is wrong, and the only correct one is treadmill speed = wind speed. or if you still want to entertain the crazy infinite theory, the only way its true is when wheel speed is zero.

Exactly. The original question was very blunt and didn't specify, but it makes much more sense to assume the treadmill was meant to match ground speed.

This question wasn't thought up by a physicist.

No it doesn't the whole idea behind a treadmill is that it matches the speed of the object on it so that the object on it stays in one position.

I'm just going to assume you're trolling now.

 

[ultimatebob]

Come to think of it, Mythbusters should save plane on a treadmill for the series finale. It could cause so much controversy that they would have to bring back the show for another season!

 

[Flammable]

the simulator is a model of an airbus!

 

[FoBoT]

gack! i just got home!
i don't see anything about the plane/conveyor myth!!!! :Q

 

[Jeff7]

Originally posted by: ultimatebob
Come to think of it, Mythbusters should save plane on a treadmill for the series finale. It could cause so much controversy that they would have to bring back the show for another season!

They could probably turn it into an entire series.


And SageTV was so kind as to crash as soon as it started recording. Wound up missing the first 10 minutes. I do love super-high-quality software.

 

[mobobuff]

Originally posted by: FoBoT
gack! i just got home!
i don't see anything about the plane/conveyor myth!!!! :Q

Doesn't look like they're doing it.

 

[Born2bwire]

Originally posted by: mobobuff

Originally posted by: smack Down

Originally posted by: mobobuff

Originally posted by: randay

The bottom line is that the treadmill speed = wheel speed interpretation is wrong, and the only correct one is treadmill speed = wind speed. or if you still want to entertain the crazy infinite theory, the only way its true is when wheel speed is zero.

Exactly. The original question was very blunt and didn't specify, but it makes much more sense to assume the treadmill was meant to match ground speed.

This question wasn't thought up by a physicist.

No it doesn't the whole idea behind a treadmill is that it matches the speed of the object on it so that the object on it stays in one position.

I'm just going to assume you're trolling now.

First thing I thought of when I saw this thread is that smack down's gonna be in here spouting this crap again.

 

[smack Down]

Originally posted by: Jeff7

Originally posted by: smack Down
How many times do I need to repeat there is no difference between a car and a plane. The only time it would matter is if we were talking about placing them on rollers.

Lets make this simple.

Do you agree that a car on a stop treadmill will accelerate?
Do you agree that in that case the a force is applied to the body of the car?
Then why do you thing that once the treadmill is started a force is no longer applied to the body of the car?
If you don't think that what makes the plane any different then the car?

Force trace:

Car
Tires push on the ground.
Tires push on the wheels.
Wheels push on the axles.
Axles push on the frame.

If the ground is moving, this will cancel out the forward force created by the tires onto the car's frame.

Airplane
Engines push on the air.
Engines push on the frame.

What the wheels are doing doesn't matter. I don't care if they're being spun forward or backward, at 1rpm, or 100,000 rpm. In the ideal example (ignoring bearing friction), they are exerting no force on the plane. The ground moves beneath them, imparting a force which is converted into rotational momentum, not linear momentum. Since it is rotational only, it does not serve to push the plane forward, nor backward, thus it does not counteract the forward force of the jet engines, which derive their impulse from the air flowing through them. That part is completely independent of the ground's motion.
AND, since the wheels' only momentum is rotational, not linear, they generate no force to oppose that created by the engines.
Result: Net force in the forward direction. a = F/m. Force is positive, acceleration occurs. Plane takes off.

If you still don't get it, I don't know what more can be said.
Go take physics and dynamics courses.

What did you smoke to come to that conclusion? Did you mean to say that if the speed of the ground matches the speed of the car with respect to ground but in the opisite direction their is zero movement? If so I agree now again explain how the plane is special.