# Phunny Physics

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#### GregGleason

##### Well-Known Member
I have been entertained a great deal by the answers on the BMABA (Balsa Model Aircraft Builders Association) Facebook page.

This question looks like that it is their version of the "what does CATO mean" on rocket forums.

Greg

just means that the wheels will be going twice as fast at take-off, I believe that we went through all this before....
Rex

Here's some funny physics for you. Did you know that motion is impossible? I can prove it.

In order to move from point A to point B, you must first pass through the midpoint between A and B. So the midpoint becomes the new B, and thus you must also pass thru the midpoint between A and the new B. Since there are an infinite number of midpoints, it is impossible to ever get from one point to another, and thus motion is impossible.

747-200?

747-400?

747-400F?

747-8i?

747-8F?

Here's some funny physics for you. Did you know that motion is impossible? I can prove it.

In order to move from point A to point B, you must first pass through the midpoint between A and B. So the midpoint becomes the new B, and thus you must also pass thru the midpoint between A and the new B. Since there are an infinite number of midpoints, it is impossible to ever get from one point to another, and thus motion is impossible.

Bat-mite whut? The universe DOES revolve around me?

But I think I disagree.......no fly!

The air movement across the wing creates the lift that makes the plane fly. If the plane sits still, no air movement across the wings no lift, no fly.

I have seen an F100 take off directly out of a hanger, but this is more like a rocket launch.

I've also seen planes take off from an aircraft carrier with a very very short runway but again air movement, lift, fly.

High acceleration from restraining the plane and then releasing it can also shorten the required forward movement needed for sustained flight. But again, no movement no flight.

Bernoulli says NO...........

Please teach me if I am wrong.........

But I think I disagree.......no fly!

The air movement across the wing creates the lift that makes the plane fly. If the plane sits still, no air movement across the wings no lift, no fly.

I have seen an F100 take off directly out of a hanger, but this is more like a rocket launch.

I've also seen planes take off from an aircraft carrier with a very very short runway but again air movement, lift, fly.

High acceleration from restraining the plane and then releasing it can also shorten the required forward movement needed for sustained flight. But again, no movement no flight.

Bernoulli says NO...........

Please teach me if I am wrong.........

However the wheels don't provide the motive force on a aircraft. If the wheels are turning, movement is occuring and there is airflow, unless of course the conveyor is turning the wheels.

Same way a seaplane works. Wheels are insignificant and do nothing to assist the movement of the plane.

Anyway, as I said, they proved it on Mythbusters with a real plane. Go find the episode.

If the speeds are matched, the plane doesn't move, or am I not reading it right? For the plane to be moving forward the speeds would have to be unequal.

Jeez....

Wheels are not locked. moving conveyor belt does not "drag" the airplane.

The wheels are (for the sake of argument) friction-less - they spin freely.

If you pull a tablecloth from a heavy object on wheels, the tablecloth will pull out and the object will stay on the table.

If the object was an airplane and it had engines and the engines generated THRUST, you would have to deal with Newtons Laws and the airplane would move forward. The airplane would not care about the speed of the ground under the nearly friction-less wheels.

If the speeds are matched, the plane doesn't move, or am I not reading it right? For the plane to be moving forward the speeds would have to be unequal.

Only if the wheels were what is moving the plane. With a car, it doesn't move. With a plane, it moves and takes off. Watch Mythbusters.

if you were to turn a go-kart into a glider by attaching wings and accelerate up to flight speed using just the wheels, then no you would not move. however if you were at the top of a long hill, then gravity would propel you downhill and up to flight speed. airplanes don't generally have driven wheels, it is like asking which way the smoke blows from an electric locomotive.
Rex

if you were to turn a go-kart into a glider by attaching wings and accelerate up to flight speed using just the wheels, then no you would not move. however if you were at the top of a long hill, then gravity would propel you downhill and up to flight speed. airplanes don't generally have driven wheels, it is like asking which way the smoke blows from an electric locomotive.
Rex

That's funny!

Just to save the google for some people...

The plane's prop/jet engine propel the plane, the wheels just reduce friction with the ground. Thus the plane is able to accelerate, air movement over the wings creates lift, off you go. The ground moving (belt) is a red herring.

or in keeping with the airplane theme, an airliner traveling from Canada to the US is filled with 100 Canadians and 100 Americans, it crashes on the US-Canadian border...where do you bury the survivors?
Rex

or in keeping with the airplane theme, an airliner traveling from Canada to the US is filled with 100 Canadians and 100 Americans, it crashes on the US-Canadian border...where do you bury the survivors?
Rex

With the current people in charge, you sure it wasn't shot down due to the 100 Canadians?

So this is the point were engineers step in and call BS on the physicists and their magic treadmills (seriously, what kind of drivers does that thing have???)

Reading the question statement, the wheels have Forward motion. Forward motion means lift. The plane's engines would have to work a little harder because of some greater rolling resistance though.

Draw a free-body diagram. Taking pure rolling motion (no-slip condition), a point on the wheel in contact with the ground is "not moving" which means the opposite point (ie, the top of the wheel) is moving twice as fast. Its basically an instantaneous speed-lever. For a surface moving backwards "at the same speed as the wheel" (magic treadmills.....), the speed of the ground (-v) is negative that of the hub (v) (the part moving forward). The absolute speed difference is 2*v, and that difference comes out the topside of the wheel giving that point a velocity of 3*v.

The plane will take off, but it won't like you, and your bearings are probably shot.

if you were to turn a go-kart into a glider by attaching wings and accelerate up to flight speed using just the wheels, then no you would not move. however if you were at the top of a long hill, then gravity would propel you downhill and up to flight speed. airplanes don't generally have driven wheels, it is like asking which way the smoke blows from an electric locomotive.
Rex

Our first order of business is clearly to build a go-kart the size of a 747!

Here's some funny physics for you. Did you know that motion is impossible? I can prove it.

In order to move from point A to point B, you must first pass through the midpoint between A and B. So the midpoint becomes the new B, and thus you must also pass thru the midpoint between A and the new B. Since there are an infinite number of midpoints, it is impossible to ever get from one point to another, and thus motion is impossible.

Reminds me of a story about an engineer and a mathematician.

Both were invited to take part in an exercise.
They were taken to one end of a 100 yd hallway and a pizza was placed at the other. They were then told that every 5 minutes, they could advance 50% of the distance between them and the pizza.
The mathematician scoffed, said "This is pointless...We'll never actually get to the pizza!", and left.
The proctor asked the engineer "Do you still wish to participate?"
The engineer replied "Absolutely. Before long I'll be within a tight enough tolerance of the solution for any practical purpose!"

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or in keeping with the airplane theme, an airliner traveling from Canada to the US is filled with 100 Canadians and 100 Americans, it crashes on the US-Canadian border...where do you bury the survivors?
Rex

In the ground.

This is typically asked two ways, 1 where the conveyer is moving at the takeoff speed of the aircraft, the plane has to supply some small power to avoid going backwards but then once the friction is overcome to keep it in position on the moving conveyer, the plane will now move forward with any increase in throttle and take off once proper air speed is attained. The second which is what was posed here is the conveyer can magically increase with any increase in rotational speed of the wheels in which it will not move relative to the ground.

The myth busters tested the simpler problem of just running the conveyer at takeoff speed which means that the wheels just spin twice as fast as normal but the aircraft still accellerates down the conveyer and takes off.

Frank

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why are you burying the survivors....that's sick...you bury the dead people....

In the ground.

or in keeping with the airplane theme, an airliner traveling from Canada to the US is filled with 100 Canadians and 100 Americans, it crashes on the US-Canadian border...where do you bury the survivors?
Rex

it would be impolite to bury the survivors. :wink:

Greg

Actually, you get a bunch of kids to bury them.. in sand.. on the beach they were destined for.. but they must have one hand free for their drinks, as well as their heads to enjoy said drink & take in the views.. and obviously buried not too deep..

(I seriously can't believe this came up.. )

it would be impolite to bury the survivors. :wink:

Greg

Maybe I'm a little twisted that way.:wink:

There are many scenarios. A few are listed:

A) If the plane is moving in relation to the ground, the conveyer belt will accelerate to infinity (S1 (conveyer belt speed) = S1+S2(speed of the plane)). (The wheels speed up as the conveyer belt does) Ignoring the fact that this is impossible, (light limit), the conveyer belt breaks down, catches fire, as do the wheels and bearings. There is now a lot more friction in the system, so the plane probably cannot take off, and one of the following situations will probably happen.

1) The conveyer belt breaks, and with the momentum it has, peices of the belt fling at the plane and destroy it. The plane and fuel burn in the fire. No takeoff.

2) The conveyer belt doesn't break somehow, and the pilot opens up the landing raft/slides. People escape for a while, but the raft melts/burns, as does the airplane itself. The plane doesn't take off, but less caualties.

3) The pilot is pre programmed to take off at all costs. They don't succeed. No takeoff.

4) The conveyer belt flings the plane into the air. Successful takeoff...?

B) No pilot was crazy enough to get into that plane because it was on the conveyer belt of death. No takeoff.

C) The wheels get reinstalled in the opposite direction. Plane successfully takes off.

or in keeping with the airplane theme, an airliner traveling from Canada to the US is filled with 100 Canadians and 100 Americans, it crashes on the US-Canadian border...where do you bury the survivors?
Rex

You bury them in media attention.

Then again, there's that Russian plane that can fly backwards that N. Korea is using to train its paratroopers. The An-2

https://www.bbc.com/future/story/20150415-the-plane-that-can-fly-backwards

I've flown backwards! In a Cessna 152, verified by GPS. It was windy. And many years ago.

Last week I had a ~150kt headwind at altitude, and while we were still moving very fast through the air, the constantly increasing time to the destination made it feel like we were flying backwards.

Both situations illustrate the difference between airspeed and ground speed.

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I still say it can't move if the speed of the belt "exactly" matches the speed of the wheels.

For the plane to move relative to the ground the wheels would have to move at a higher speed than the (seemingly magical) belt. If the speed of the belt is infinite, no amount of thrust will overcome the friction of the wheels turning.

I still say it can't move if the speed of the belt "exactly" matches the speed of the wheels.

For the plane to move relative to the ground the wheels would have to move at a higher speed than the (seemingly magical) belt. If the speed of the belt is infinite, no amount of thrust will overcome the friction of the wheels turning.

Refer to free-body diagrams above.

And I say even then, the takeoff speed of a jet is finite. Which means the treadmill will only get going that fast, and the wheels should only have to turn at twice the angular velocity of a normal takeoff. The bearing grease will be baked, but it could still be possible.

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