OT: Any mensa members here? ;)

[Mark Bournes]

I think this needs to be sent to "mythbusters."

[Mike Teutsch]

Guys Guys Guys!

Come on! It is not that hard. The plane has wheels to roll freely on. If the belt goes one way the wheels roll twice as fast, if it goes the other the wheels don't roll at all. The plane still moves and takes off.

Now I'm like Richard, I'm done posting! :) :) :) And, don't say thank God! :)

Mike

[John Miller]

Originally Posted by Richard Alvarez

The wheels counteract the reverse motion of the conveyor belt by rotating at twice the speed.

The airplane will fly.

Exactly! Had to think this through for a few minutes.

The wheels are, in effect, bearings that decouple the airframe from the conveyor belt.

Now, if the 747 had skids instead of wheels, that would be different....

[Garrison Hayes]

Originally Posted by Mark Bournes

I think this needs to be sent to "mythbusters."

Them guys will actually try that thing out too!

[Joel Chappell]

In the scenario as stated, the conveyor belt reacts to changes in speed of the aircraft, not the speed of the wheels turning.

The folks that have this fictitious airplane flying, (in this fictitous situation), create forward motion by adding outside influences.

If the conveyor belt adjusts it's reverse speed based on the speed of the wheels, or ground speed, thrust would absolutely overtake the actions of the conveyor, forward motion of the aircraft would occur, lift would be created, and the aircraft would fly.

If the conveyor belt adjusts it's reverse speed based on the speed of the aircraft, or airspeed, thrust would not overtake the reverse action of the conveyor, the plane would remain stationary, no lift would be created, and the plane would not fly.

So, depending on how you want this conveyor belt to measure speed determines whether or not it will fly.

The data provided in the scenario is incomplete. It is a hilarious read though, and a great way to kill some time. Although... I've got 2 one hour videos to produce, and a 2 minute commercial to finish before Friday...

Maybe I am not smart enough to understand the complexities of this very simple scenario as I am not a MENSA member, but I did ride the short bus to school on occasion.

[Chris Barcellos]

Joel:

That has been my argument from day one on this thing.... but then I am not mensa either.... but I did stay at a Holiday Inn Express last night.....

[Joel Chappell]

Originally Posted by Chris Barcellos

Joel:

That has been my argument from day one on this thing.... but then I am not mensa either.... but I did stay at a Holiday Inn Express last night.....

I just think the discussion is fascinating. I love hearing different arguments and theories for or against the plane being able to fly.

It is amazing to me which question, riddle or whatever will become a widespread discussion on a forum or other internet sites.

It is apparent to me however, that this forum is attended by some extremely bright and intelligent people. This is an off topic discussion, the really amazing ones are the discussions about true applications in video and it's associated technologies that are accomplished on this forum.

The results in that arena are demonstrated hourly by the members of this forum.

I procured, tested, setup and began a video ministry at my church totally from the information gleaned from the amazing folks here at DVi. From cameras, to tripods to spider braces, steadicams, NLE's plug-ins or whatever, the best in the industry are represented right here. The invaluable contributions by you folks in other threads here saved me countless hours and lot's of dollars. (you can see the results after only 3 weeks at kbbc.org)

So all that don't see eye to eye on planes and conveyors, thanks for your other contributions on this forum.

[Tim Le]

Originally Posted by Joel Chappell

Maybe I am not smart enough to understand the complexities of this very simple scenario as I am not a MENSA member, but I did ride the short bus to school on occasion.

Joel, it's not really about being super smart but just being familiar with physics. And physics is tricky because sometimes your intuition does not always match the physics. For example, if you asked someone what would fall faster in a vacuum, a hammer or a feather, most people would say the hammer because their intuition says it's heavier. But the physics say they will fall at the same rate and this was even proven by one of the Apollo astronauts on the moon.

In this case, it doesn't matter whether the conveyor is measuring the speed of the wheel or the plane. If you're interested, here's a scientific explanation:

To scientifically prove this, a physicist or an engineer would draw a "free body diagram". This is a diagram showing all the forces acting on the plane and in which direction. In this case, the forces in the horizontal direction are what we are interested in and there are only three forces acting in this direction: friction between the conveyor and the wheels, drag from the air and thrust from the jet engines. Friction and drag forces "point" backwards and thrust "points" forward.

So for the plane to remain stationary, the sum of the friction and drag forces would have to equal the thrust force. We already know thrust can overcome drag--a plane does this all the time otherwise it can't move forward. So the question is, can thrust overcome friction? Yes, it can. The plane already does this every time it moves on the ground. The friction is no more than if there was no conveyor there at all or if the conveyor was moving like crazy in the opposite direction. The wheels are just spinning faster. In fact, the friction gets less as the wheels start rolling (just like how it's easier to push a car after you get it going). The equation for friction is f = µN , where f is the friction force, µ is the coefficient of static or kinetic (rolling) friction and N is the normal force to the direction of the friction force (in this case basically the weight of the plane).

[David MD Smith]

Hi All,

this thread is a good way to waste some time. Here's my two cent's anyhow.


In short, it depends.


If there really is no rolling resistance with these wheels then the conveyor has no effect as many have stated. The plane flies (assuming it could fly in the first place).

If there is rolling resistance and this is constant with the wheel speed, again the conveyor has no effect and the plane flies.

If for what ever reason the rolling resistance resistance is not independent of wheel speed (i.e. the wheels have tyres or any such real-world features) then it depends again.

Scenario 1), the increase in rolling resistance due to the wheels rotating at double speed is sufficient that the maximum engine thrust balances the overall resistance (rolling + air) before the plane can reach a ground speed (relative to still ground NOT conveyor and assuming no wind) prior to take off. Plane does not fly but carries on travelling a constant speed.

Scenario 2), increase in rolling resistance is such that the plane has sufficient thrust to reach lift off speed. Plane flies.

Both off these scenarios are depicted on the attached link.
http://www.physics.ox.ac.uk/users/smithdmd/planes.bmp


regards

David Smith

[Chris Barcellos]

Not sure if I understood what you are saying (you must be MENSA), but I have contended that the Gravity force down, creates an a drag on wheels that has to be overcome. I think those that are saying the plane will fly are ignoring the tremendous force of 200 tons of weight on tires and bearings, etc., and the what is need to overcome that. When a plane is sitting on a tarmac, it takes a tremendous thrust just to get the plane rolling. It certainly doesn't. With the plane on a "moving" tarmac, you still have to overcome the same forces. The scenario states that when plane speeds up, conveyor goes faster to counter act. So the way I saw it, at rest, all force on plane is straight down, by virtue of gravitational pull. Then, as engine starts, the forces exerted change the general vector of forces on the plane forward, but then the belt speeds up to counter act the forward thrust, and the forward vector thrusts are counter acted, due to the reverse vector force applied by the sped up conveyor belt... According to the scenario then, at the point the engines reach maximum thrust, the conveyor belt will be counteracting that forward thrust using the downward force of gravity, and the reverse thrust of the conveyor. Airs speed required for take off should never be reached.....

[Pete Bauer]

Originally Posted by Chris Barcellos

When a plane is sitting on a tarmac, it takes a tremendous thrust just to get the plane rolling.

Actually, even at idle thrust, you're well advised to keep your feet on the brakes at the Hold Short line in most jets I've flown. Even if you consider the dynamic drag of the rolling tires, the horizontal force vector is at least an order of magnitude less than the thrust of the engines, probably a couple orders of magnitude at full thrust. Think of the "land speed record" type of jet cars...300+knots ON THE GROUND...ground speed as high as in our experiment except that there's also 300+knots worth of aero drag as well, instead of just 150 knots! If the jet car grew wings, it'd fly.

In any case, as I mentioned several pages ago, this is a thought experiment and the convention is that you don't get to add your own set of variables to thought experiments. Calculating the rolling resistance just wasn't set in the conditions of the riddle.

[Adam Bray]

Easy. It's basic math.

Throw ten rocks on the ground. Then pick up the ten rocks. How many rocks are left on the ground?

Answer: 0

The airplane example is just to get you to think about jet engines, rolling resistance and lift on the wings and blah...blah..blah.. to confuse you. It does not matter if the object going down the belt at 10 mph is a jet with large tires, a car, a skateboard with small tires, a beachball, explosive diarrhea, space shuttle, or Carl Lewis sprinting down it.

10 mph is 10 mph. I could put a "Slip & Slide" on a belt. If I slide down it at 10 mph, and someone turns it on to 10 mph, I'm going to become stationary.

This like the 10 pounds of feathers vs. 10 pounds of steel question.

Am I a member of MENSA now??

[J. Stephen McDonald]

Originally Posted by Adam Bray

It does not matter if the object going down the belt at 10 mph is a jet with large tires, a car, a skateboard with small tires, a beachball, explosive diarrhea, space shuttle, or Carl Lewis sprinting down it.

Actually, it matters a great deal, as the dynamics of these examples are not all the same. This has been amply explained by some of the previous posters, who must be groaning in frustration when they see how their efforts have failed to penetrate the reasoning barrier. What role in evolution do you suppose this barrier has played for Humans? Perhaps it provides a necessary brake on runaway technological development, without which, we would have exterminated ourselves long ago.

I can imagine a comparable dilemna 50,000 years ago, when the inventor of the spear-launcher tried unsuccessfully to convince the other members of the hunting party why it was better to have a doubled throwing range.

[Adam Bray]

This dog won't fly.

[Jim Michael]

You can leave the wheels out of the discussion. Assume the plane has frictionless wheels with no mass. The engines provide thrust as Richard has correctly described. The wings require air flowing over them to provide lift. The engines do not provide this airflow, it is provided by the movement of the aircraft relative to the air. If the aircraft has a rotation speed of 100 knots and there is a tailwind of 10 knots the airplane will rotate at a speed relative to the ground of 110 knots and the pilot will see 100 knots on his airspeed indicator (ASI). If there is a headwind of 10 knots the aircraft will rotate at 90 knots ground speed and the pilot will see 100 knots on his ASI. If there is no wind and a seaplane is on a river flowing at 10 knots and the pilot takes off in the direction opposite the flow of the river he will depart the Earth when his speed relative to the ground is 100 knots and his speed relative to the river is 110 knots. His ASI will register 100 knots.

If the airplane's forward movement is offset by movement in the opposite direction as in the conveyer belt thought experiment then there is no airflow over the wings and therefore no lift. The ASI will register 0 knots and the airplane will not fly.