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

[sisukid1975]

Originally Posted by import silvia

No forces were given in the problem. Newton's first law wins.

Newton's third law wins.

The plane moves relative to the air, because it's engines are moving air.

Think of how a plane moves when it's flying, not on the ground.

Why does it move forwad in the air when it's flying? Because it's moving relative to the air. It's power to move comes from displacing air. That's where the third law comes in.

Now, imagine a plane flying... it's just flying along.... Flying though the air at 500 miles per hour, happy as a clam. then, it lowers it's wheels.... what happens? If you factor out drag, nothing: it keeps flying through the air, right?

Then what would happen if the wheels touched a surface while the plane was still flying at 500 mph through the air? They would touch the surface and being spinning, but the plane is still flying through the air, right?

So then what would happen if the surface the wheels were touching moved the opposite direction of the plane? Nothing-- the plane is still flying through the air, right?

That's where the key is, you have to visualize what makes the plane move, and waht it is moving relative to, and where it's power to do so is generated, and it isn't the wheels spinning on the surface.


Fred

[cruiseroutfit]

Think about it this way...

A plan moves one direction at 100mph
A converyor moves in an opposite direction of 100mph
The wheels of the plane move at a speed of 200mph (the 100 of the conveyor, and the 100 of the plane).

THE PLANE FLY'S

I proposed this to a room full of ME students and a Professor (4 degrees from MIT). Unanimously (sp?) the plane flies...

[baxter650]

Originally Posted by NorCalDoug

I'm in the "it's not going to fly" camp. And I'm firmly rooted here. As already mentioned, flight is a result of thrust and lift -- lift is generated by air speeding over and under the wing.

If the forward thrust is negated by the backward movement of the conveyor, there is no net thrust -- therefore no air movement under the wings.

Think about running on a treadmill. When you're running on a treadmill...do you feel the wind in your face? Nope. No wind...no lift...no fly.

Fine now strap on a jet pack and lite that sucker off, do you fell the wind now!

[rusty_tlc]

Originally Posted by subzali

sisukid was on the right track. Draw a Free-body diagram and examine the forces. Plane's engines create forward thrust (force). Now assuming the wheels are frictionless on their bearings (which they are pretty close to being), then the ONLY forces on the wheels are the weight of the plane pointing down, the normal forces from the ground, pointing exactly upwards to counteract the weight of the plane, and a frictional force on the wheel, acting at the surface of the wheel and creating a moment. The frictional forces from the conveyor on the wheels ONLY causes them to rotate about their (close to frictionless) axle. Since this axle is pert near frictionless, there is NO latent force from the conveyor to counteract the thrust from the engines. Voila there is a net force in the x direction (if that's the axis you choose, relative to a stationary object, like the earth), and according to F=ma the plane accelerates regardless of the conveyor. Once its speed relative to the stationary ground is high enough, enough air will be passing over the wings to create a pressure differential (lift), and the plane will take off, its tires on fire because they're spinning so fast.

Never argue with a School of Mines student

When you start selling these frictionless axles and tires with no rolling resistance I want some. Maybe you could draw a vector diagram or freebody diagram for us. I really want to understand how a plane could become airborn if there is no forward motion.

[Arya Ebrahimi]

rusty, if there is no forward motion why is the runway moving?

[OZCAL]

The plane flies. Actually, the wing flies; the fuselage and the wheels are along for the ride, and the wing doesn't care how fast the wheels are spinning. The conveyer belt could be going the same direction as the plane and keeping the wheels static for all the wings care.

[rockclmbr]

I think we should ask the Myth Busters. The best way is to actually prove it by doing it. Now, who has a plane we can borrow

[baxter650]

Originally Posted by Cube Dweller

I agree with LoveTractor and Whiteshark. Ary wrote "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."

Since a PLANE requires the relative velocity of AIR to be greater on the top of the wing than on the bottom to generate lift, since the plane is stationary, there is no velocity differential and thus no pressure differential to lift the plane.

Rockets are different, they fly based on thrust generated by the gases created during combustion of propellant.

Draw a freebody diagram

Maybe you guys can get a group rate on a make up phisics class.

[wesintl]

I created a little freebody diagram here and my math as I went along on the white board.

[e9999]

Originally Posted by Cube Dweller

Just like a prof e9999. We're talking about reality here, not some master's level fluids course. Even if we're using an F16 as an example, there's no way turbulence created by the moving treadmill will cause enough wind velocity to create enough lift.

mmm.... turbulence has nothing to do with that .... ya got an F!

[Arleaux]

If what you are proposing is true, then why don't we design a 60' aircraft carrier w/ a conveyor belt on it?

[T Y L E R]

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?

First off if the moveable runway is perfect then the plane will not 'move' as you suggest. At least not with any horizontal vector. Secondly, the plane will take off .. like d'uh .. unless you keep it on that dumbass stallway for it's entire life.





TY


PS: How did I miss this the first time around? Must have been a stealth aircraft.

[sisukid1975]

Originally Posted by Cube Dweller

I agree with LoveTractor and Whiteshark. Ary wrote "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."

Since a PLANE requires the relative velocity of AIR to be greater on the top of the wing than on the bottom to generate lift, since the plane is stationary, there is no velocity differential and thus no pressure differential to lift the plane.

Rockets are different, they fly based on thrust generated by the gases created during combustion of propellant.

Draw a freebody diagram

No, a rocket is not different. It works on the same basic law of physics that a jet engine or propeller uses, the only thing different is the mechanics by which it generates that thrust.

Look at it this way. It's called an "aircraft." It moves though the air. It's not called a "runwaycraft" for a reason! It moves even when it's off the runway. How can that be? Because it does not rely on the runway for motion.

If you take your cruiser off a sweet jump, it loses velocity when it's wheels are not in contact with the ground. Because it 's forward motion comes from the wheels rotation aginst that ground. But what happens when a plane becomes airborn? It doesn't lose speed, it usually gains it. Why? Why does a plane not need the runway in order to maintain it's speed?

Fred

[GetsUThere]

I'm going to vote for no. A plane has to have airspeed to get lift, which I don't think it's going to have. It'll probably have just about as much airspeed as a car on a dyno.

Plus if that really worked, aircraft carriers would be a lot smaller... they'd just put a conveyer there instead of a flight deck and let the planes float off

[sisukid1975]

Originally Posted by GetsUThere

I'm going to vote for no. A plane has to have airspeed to get lift, which I don't think it's going to have. It'll probably have just about as much airspeed as a car on a dyno.

OK. So what would happen if you put a plane on the same dyno you use for your car?

You put the wheels of the plane on the little rollers, then crank up 4 General Electric High-bypass turbofans. Is that plane going to stay there on the rollers of the dyno? Why or why not?

Fred

[GetsUThere]

Yes, it's going to stay there IMO, although the conveyer thing works better... in reality a plane would probably face plant on a vehicle dyno or just break something

[rusty_tlc]

Originally Posted by Arya Ebrahimi

For all intents and purposes they eliminate it since the brakes are not on. If they were not effective at reducing the friction to a value that is negligible, planes would not be able to take off, or the plane's engines would have to generate a force greater than the static friction force of the locked up tires on the runway.

Cube Dweller, I will take this into my physics class tomorrow, although I'm confident the plane will take off.

Wish I weren't going to bed right now.

How much force does the plane exert on the ground?
How is this affected by the engine thrust?

If I had a big enough engine I could make my FJ40 fly, in the real world(TM) planes fly primarly because of lift generate by the wing surface not thrust from the engine. With no air speed a wing has no lift.

[T Y L E R]

I don't care how much thrust an aircraft has, it seems to me that if it can't express that as forward momentuum then it's dead on the ground. There has to be forward movement so that you achieve lift .. air over the wing .. faster over than under bla bla ..


TY

[sisukid1975]

Originally Posted by GetsUThere

Yes, it's going to stay there IMO

No, those engines are putting out 25,000 pounds of thrust each. and they're acting against what? What does a plane engine impart force to?

Fred

[sisukid1975]

Originally Posted by T Y L E R

I don't care how much thrust an aircraft has, it seems to me that if it can't express that as forward momentuum then it's dead on the ground. There has to be forward movement so that you achieve lift .. air over the wing .. faster over than under bla bla ..


TY

But what does the plane move though? When a plane flies, what is it flying through?

[T Y L E R]

Originally Posted by sisukid1975

No, those engines are putting out 25,000 pounds of thrust each. and they're acting against what? What does a plane engine impart force to?

Fred

I think you mean what are those engines pissing away all that energy on .. after all they're being usurped by the conveyor thingy.

[T Y L E R]

Originally Posted by sisukid1975

But what does the plane move though? When a plane flies, what is it flying through?

I think you're confused . Thrust or no thrust , you have NO air over the wing. It's not like you're blowing air from a huge fan over the wings.

[sisukid1975]

Originally Posted by T Y L E R

I think you mean what are those engines pissing away all that energy on .. after all they're being usurped by the conveyor thingy.

No, no..... OK, lets say we can magnetically levitate the plane over the runway no wheels necessary, it just floats a few feet over the runway. Will it take off then? Why or why not?

Fred

[GetsUThere]

Originally Posted by sisukid1975

No, those engines are putting out 25,000 pounds of thrust each. and they're acting against what? What does a plane engine impart force to?

Fred

I'm with Ty on this one. I just don't think the thrust matters. If the plane's not moving relative to the air around it, it's not flying.