Physics question/riddle. This is a good one, read it.

[bad_religion_au]

woops i accidently voted no.. i meant yes... stupid blurry eyes

[OZCAL]

How about this:

A Vertical Takeoff or Landing (VTOL) aircraft like a Harrier jump-jet elevates itself vertically into the air. A few feet off the ground, the aircraft finds stasis; apparently, the aircraft is "floating" there in a single position.

Next, the aircraft fires its "forward engines"* and the aircraft begins to move forward (and why wouldn't it?).

Simultaneously, the aircraft lowers its landing gear and the tyres touch Arya's conveyor belt. How will that contact stop the forward progression of the aircraft? It won't (assume a smooth conveyor belt, inflated tyres and grease in the hubs!). The wheels will merely spin faster or slower depending on speed relative to the conveyor belt.

Guys, this isn't Zeno's paradoxes. You are just looking at a red herring (wheel rpm).

I mean, a bicycle card in the spokes sounds like a motorcycle, but when the card falls out, your legs still work, don't they?

*It doesn't really happen just like this but this suffices for illustration.

[Hey Charger]

Definatley flies , and would need the same length of runway if not more , and it wont matter what way the belt is spinning.
Can a hover craft or swamp boat go upstream ???
ofcourse it can !!! ,
same thing as a plane they are all not driven of the ground or through the water but the air. Too easy , give us something harder !!!

[rusmannx]

if the prop/turbine is spinning fast enough to make the plane lift from the ground on a normal runway (assuming 200knots) it doesn't matter what the hell the tires are doing. if the plane was hovering over the ground, (like it was something from Back to the Future) and you fired up the engines, it would start moving forward. now put wheels under it.... no difference.
the plane will begin to move forward once the prop/turbine is spinning fast enough to generate some thrust. the tires/conveyer-belt could be rolling backwards for all we care.

[OZCAL]

Originally Posted by rusty_tlc

By this logic we wouldn't need runways because planes would lift straight off the ground.

In strong winds, they do! Picture a plane in a wind tunnel. It would "fly" in place if the wind in the tunnel was as fast as the speed the plane needed to travel to fly on a calm day.

[OZCAL]

Originally Posted by airon23

so if this conveyor thing works why don't they use it like on aircraft carriers?

...

[Mr.Bryan]

sheesh, i can't beleive there is 5 pages of this.

ok here's how it works cause I'm smarter then everyone.

Plane moves slightly forward when powering up. Belt senses it, and moves backward. Plane keeps moving faster, belt keeps moving faster. Belt eventually gets to like 5000 mph because it's trying to hold back the plane. by this time the plane is almost at the end of the belt becaue the friction of the wheel bearings isnt enough to hold back the plane because the plane generates force that is completely independent of the wheels. Ok, so now we're doing 5000 mph on this belt thing, then the tires burst because the speed it too great, the plane spins, a non moving part touches the belt, and the plane is catipulted backwards and explodes in a huge fireball. So no the plane wouldn't fly.

[OZCAL]

Originally Posted by Mr.Bryan

sheesh, i can't beleive there is 5 pages of this.

ok here's how it works cause I'm smarter then everyone.

Plane moves slightly forward when powering up. Belt senses it, and moves backward. Plane keeps moving faster, belt keeps moving faster. Belt eventually gets to like 5000 mph because it's trying to hold back the plane. by this time the plane is almost at the end of the belt becaue the friction of the wheel bearings isnt enough to hold back the plane because the plane generates force that is completely independent of the wheels. Ok, so now we're doing 5000 mph on this belt thing, then the tires burst because the speed it too great, the plane spins, a non moving part touches the belt, and the plane is catipulted backwards and explodes in a huge fireball. So no the plane wouldn't fly.

And no frequent flyer points.

[bad_religion_au]

Originally Posted by Mr.Bryan

the plane is catipulted

last i checked catipulted was temporary flight... so it still mother fawking fly's

i like my winching off the dyno example

but i'll pay the hovercraft upstream

[Mr.Bryan]

Originally Posted by bad_religion_au

last i checked catipulted was temporary flight... so it still mother fawking fly's

i like my winching off the dyno example

but i'll pay the hovercraft upstream

ok, you do have a point, but if the plane crashes and is hurtled backwards, is it still a flying plane? or would it be a flying fireball?

[bamachem]

plane will take off.

props or jets will impose thrust on the fuselage and push it forward independent of wheel speed. the wheels will turn no matter what the conveyor tries to compensate for, theoretically, the conveyor will quickly accelerate to it's theorhetical max (and if no theorhetical max, then it would accelerate to infinite velocity) since it's increasing speed would have no effect on the plane's motion.

[Arya Ebrahimi]

I'm glad I created such a great thread. I think 5 pages in less than 12 hours might be some kind of record contendor.

BTW, for all the people who think it won't fly, YOU'RE WRONG.

You can stop argueing now.

[LongIsland60]

Wow,

I can’t believe this thread got so big in so short a time – at first I thought it was a revived thread. I also can’t believe there is actually an argument about it also.

Plane no take off. Period.

No airflow over the wing == no lift == plane stay on conveyor belt.

The means of thrust is irrelevant. If it had a drive on the wheels it would not matter. The prop and/or turbine engines make no difference – they do not produce airflow over the wings. (negligible in any case)

I am dumbfounded that there is actually a debate on this.

Think about what might happen if it magically was lifted off the ground. Once the wheel separated from the conveyor belt the following is the state of the problem:

- conveyor belt either instantaneously stops moving depending on the magicalness of the mechanism that senses the 'speed' of the wheels/pplane or it slows down eventually like a normal mechanism.

- plane is above the ground with engines at full power, but NO FORWARD movement relative to the air as the belt was negating all movement relative to the real world. The full power thrust is not enough to instantaneously push the plane forward enough to cause airflow. (it is not a rocket - it relies on wind across the wing to remain in the air.)

- wing stalls and drops to the ground.

- does not matter since the plane would never get off this conveyor belt int he first place.

I am not sure why there is a 'theoretical limit to the belt when it is a contrived example - if that is the case why is there no theoretical limit on the wheel bearings? How can you have a magic belt that senses speed and yet can't get around a limit on its speed?

No flight for this plane/belt combination.

(As an aside )
I have flown in a glider that was moving 'backward' relative to the ground. It is pretty cool to watch a glider "motionless" except for a slow descent in place.

the gound surface has absolutely nothing to do with flight. Just think of what the airspeed indicator in the cockpit would read. zero is the answer. (pitot tube measures force of air flowing into the little tube. there is no air flowing into the tube (ignoring the neglibible amount from the prop (if mounted in front) and the small amount produced by friction from the belt.


edit
OK, I re-read more of the thread - the answer is based on interpretation of the question.
Plane flies if the belt just goes int he opposite direction at the same speed.

Plane doesn't fly if the belt does what one normally does - halt relative motion to the outside world.

Question needs to be clearer, but I am dumb for getting involved.

[e rock]

Originally Posted by LongIsland60

I am dumb

Fixed it for ya


Your assignment is to go back and read the whole thread before posting again.

[bad_religion_au]

Originally Posted by LongIsland60

No airflow over the wing == no lift == plane stay on conveyor belt.
.

why is there no airflow?

airflow is created by the plane moving

what is stopping the plane from moving?

the conveyer belt turning free spinning wheels???????????????

please answer this one sensibly

[bamachem]

Originally Posted by e rock

Fixed it for ya


Your assignment is to go back and read the whole thread before posting again.

good fix. it fits. arrogant while being wrong - priceless.

[LongIsland60]

Originally Posted by bamachem

good fix. it fits. arrogant while being wrong - priceless.

What will the airspeed indicator read inside this plane?


edit - I see - the answer depends on if the belt negates the movement relative to the rest of the world. If that is NOT the case then I do not understand why the riddle was posed. All that riddle would mean is that the plane 'pushed' the ground under it twice as fast as it normally would.

second edit

I misunderstood the question (or perhaps it was not clear in the original post) - I read it to mean relative motion (to the ground) was negated, however that is not the case according to Arya. Simple test is what the airspeed indicator reads.

So the wheels are spinning twice as fast, the ground is going by normal speed and the conveyor belt is moving backward relative to the ground at the same velocity the plane moves forward. Next time we will need all of the information in the riddle...

My apologies for only reading the first 3 pages before replying. If anyone wants a flight in a plane and is in the NY area feel free to PM me. I can show you what happens on a normal runway.

[beaufort-fj60]

Originally Posted by Arya Ebrahimi

a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.

the question is

will the plane take off or not?

Well? Will it?

the question answers itself

[Cube Dweller]

subzali, sisku, I'm now pretty sure you're correct. As I lay in bed thinking about this this morning (Ary, you're number one ), my feeble mind put into a framework I can understand.

Since the wheels of the plane are not driven, they basically coast along in either case- on a fixed runway or on a conveyor. So the wheels would turn at a much faster velocity than on a fixed runway, but assuming no real world problem like heat, friction on the wheels, the plane would still move forward, air would move over the wings, lift would occur, etc etc.

The plane would takeoff.

[Cube Dweller]

The conveyor with limitless speed is a red herring(no offense to Red Herring). The hovercraft/harrier/any vertical liftoff plane is irrelevant to this problem. Normal plane, F16, Cessena 150, whatever.

[beaufort-fj60]

Originally Posted by Cube Dweller

subzali, sisku, I'm now pretty sure you're correct. As I lay in bed thinking about this this morning (Ary, you're number one ), my feeble mind put into a framework I can understand.

Since the wheels of the plane are not driven, they basically coast along in either case- on a fixed runway or on a conveyor. So the wheels would turn at a much faster velocity than on a fixed runway, but assuming no real world problem like heat, friction on the wheels, the plane would still move forward, air would move over the wings, lift would occur, etc etc.

The plane would takeoff.

the earth spins at ~1,035.7 mi/hr, would a plane take off with the spin or against the spin? easy answer

[MaddBaggins]

OK, I never went to college and I barely passed geometry here is how I see the problem.

Lift is required for a plane to fly
Lift is generated by the plane moving forward thru the air, air flowing over and under the wings
Thrust is used to push the plane forward thru the air
If the forward movement of the plane thru the air is negated by the backward movement of the conveyor how can you get air moving over the wing
If you can't get air moving over the wing you can't get lift
If you can't get lift you can't fly.

Thats how my uneducated mind see's this question.

[subzali]

Cube Dweller, you're point back on the previous page got me thinking: and I still haven't resolved it. The key to the plane taking off is displacement relative to a stationary reference frame. The conveyor keeps the plane from having any displacement. Displacement ONLY comes from a velocity differential at the contact surface of two objects. If the tangential velocity of the wheels in one direction equals the velocity of the belt in the opposite direction, the wheels aren't moving anywhere. And they can't because the conveyor ALWAYS HAS to match the wheel speed - that's part of the problem. Denis has a good point, the question is phrased VERY poorly, allowing for inconsistencies of interpretation. But assuming that the wheels' tangential velocity is what the conveyor is matching, the plane cannot move anywhere relative to a stationary reference frame. If it can't move anywhere it's not going to lift off the ground. BTW this new angle of the problem (to me anyway ) stumped my Physics roommate...Ary thanks for dumbfounding two Mines students

Anyone?

[bigndn]

OK, given that the wind speed is negligible, the plane will NOT lift off. There is not adequate air flow around the wings to create lift. Basically, the plane is sitting stationary, therefore no lift and no takeoff.

Imagine this..........you are standing beside a runway watching a plane sitting adjacent to you. There is no wind and plane isn't moving, therefore it isn't creating it's own airflow over the wings.

Same plane (not moving), not tied down, experiencing tornado-like sustained winds.......will lift off due to the airflow around the wings, creating lift

Now imagine that same airplane accelerating down the runway, therefore creating it's own airflow over the wings, creating lift.........it will rise into the air.

[Hayes]

OK, OK.

I couldn't bring myself to read EVERY post on this thread, so please forgive me if I'm annoyingly redundant.

I have to comment because I teach high school physics--I'm no physicist, but this is just the thing that we have to concentrate on with the kids.

The real challenge is interpreting the vague phrasing of the problem.

First, velocities of both objects need to be stated RELATIVE to something. The ground? The conveyor? The (assumed) stationary atmosphere?

We're not given this information, so we have to go with a literal translation:

"as the plane moves the conveyor moves but in the opposite direction."

"The plane moves." That answers the question right there. Plane moves forward at 500 knots relative to earth, runway moves backward at 500 knots relative to earth, wheels spin forward at 1000 knots relative to earth. Plane takes off. Nothing complicated.

I think folks are also forgetting how and where the forward force is generated:

A propeller generates force through the atmosphere, a jet generates force directly on the plane. The ground has nothing to do with it (exept for a little bit of friction).
IF the plane generated its forward force through its wheels, you could cancel that force with an opposite force transmitted through the conveyor.

Plane takes off.

Mr. Hayes
1st period physics.