Plane on a treadmill

[Dean]

Calgon, take me away!  

[miguel]

i had another thought (yes, i'm obsessing over this).

the treadmill is not producing momentum...the plane is sitting still in relation to the space that surrounds it.  

the "shoving the treadmill runner" theory was mentioned earlier...when a human is on a treadmill and someone shoves him, he will fall not because of his momentum, but rather he'll fall because he's a bi-ped that is on the brink of balance whenever he stands.  he would fall because he was pushed and he looses his balance, not because he's carrying momentum.

have you ever been running on a treadmill when all of the sudden you jump off and put your feet on the sides of the treamill so you're off the belt?  you don't fall, right?  that's because you have no momentum...your body is not in motion so it will not stay in motion.

his feet are in motion, but his body (the greater mass) is at rest.  so in essence, the planes wheels are in motion, but the fuselage and wings are at rest.

[Dean]

A treadmill works for a human because a human has to move forward by exerting thrust on the ground.  If you make the ground move backward at the same speed the human is trying to move forward, the force is effectively cancelled.

A plane does not exert force through it's wheels to move it forward.  Therefore, the treadmill CANNOT cancel out the force.  The engines exert their force against the atmosphere, which is not moving.  Therefore the plane MUST move forward, regardless of whether the treadmill is going the same speed as the plane or 10 times the speed of the plane.

Ok, I'm done.  

[Dean]

Ok, while eating my pizza for lunch I had another idea how I might explain this to you.

You get on the treadmill...with rollerskates on.  You start the treadmill up and set it to 10 MPH.  You hold yourself by the rails so you don't move.

So, the ground (treadmill) is going by at 10 MPH (a good, fast run, right?), but you'd hardly exerting any force at all to keep yourself still.  Now, push yourself forward with your hands.  What?!?  You just moved forward with very little force, yet the ground is going under you at 10 MPH!  How did you do that?!  Because you exerted your force on the rails, which are not moving relative to you.  

It doesn't matter how fast the treadmill is going - your rollerskates are just going to spin faster.  And you'll still be able to push yourself forward no matter the speed.

Same for the airplane.  It's not pushing forward against the ground - it's pushing forward against the air around it.

[miguel]

hey...you said you were done!

go eat more pizza!  

[miguel]

Ok, while eating my pizza for lunch I had another idea how I might explain this to you.

You get on the treadmill...with rollerskates on.  You start the treadmill up and set it to 10 MPH.  You hold yourself by the rails so you don't move.

So, the ground (treadmill) is going by at 10 MPH (a good, fast run, right?), but you'd hardly exerting any force at all to keep yourself still.  Now, push yourself forward with your hands.  What?!?  You just moved forward with very little force, yet the ground is going under you at 10 MPH!  How did you do that?!  Because you exerted your force on the rails, which are not moving relative to you.  

It doesn't matter how fast the treadmill is going - your rollerskates are just going to spin faster.  And you'll still be able to push yourself forward no matter the speed.

Same for the airplane.  It's not pushing forward against the ground - it's pushing forward against the air around it.

i must disagree.  i'd say that if the roller skater jumps off the treadmill, he won't go anywhere.  the spinning mass of the tiny wheels are outweighed by the mass of the human (who is not in motion) and will therefore stop spinning when they hit stable gound.

[darkrose]

I say if a guy with rollerskates is on a treadmill, then he's just plain nuts to begin with!  

Someone mentioned beer.  It's Friday!  To quote the words of the great Homer:

beer!beer!beer!beer!beer!beer!

I digress to my statement about Mythbusters.  Let them try out the theory, then they can blow up the airplane, and it'll be done with!  

[miguel]

i just tried to get my daughter to jump on our treadmill with skates.  my wife said no.  

[Dean]

Mmmm, beer.

[darkrose]

Whoohoo!

[waverider]

First, I just wanted to thank The Ston for posting this original question, as it has now consumed my every waking thought today!  Ya, thanks a lot Ston, thanks a lot!!    

For most of the day I was on the band wagon that the plane would not generate any lift and thus, would not take off.  But the more I read Dean's posts, the more I felt he was right: the plane WILL take off.  I started to think of this in terms of physics and from what little I retained, decided there were three vectors of force acting on the plane: a downward vector related to plane mass and gravity, a forward vector caused from the engine's thrust against air and a reverse vector caused by the very small frictional forces of the free spinning wheels on the ground imparted from the conveyor belt.  I figure the forward vector would be much much greater than either of the other two vectors and thus the plane must lift off regardless of how fast the conveyor belt was spinning.  As the thrust in the engine builds, the downward gravity vector will reduce, which also reduces any force the conveyor belt can transfer to the airplane's free spinning wheels.

Still not completely convinced I 'googled' the question and found the following explanation on the "Advanced Physics Forum":

"There are four forces that govern an airplane at any given time: lift, weight, thrust and drag. Lift and weight oppose each other but can be disregarded in this question because all we are talking about it whether or not the airplane can accelerate in reference to the surrounding air. Once we get acceleration and airflow we can get the lift and weight but we don't need to talk about that now.

The throttle of the airplane is advanced and propeller/jet of the airplane produces thrust. We now have a forward vector, say 500 pounds of force, for a small cessna. In order for the plane to remain stationary an equal and opposite force needs to be introduced. Everybody follow? This opposite force would be drag.

We need to account for 500 pounds of drag. Right now the only drag is the force of friction in the wheel bearing. Has anyone here ever tried to push a cessna? It's not very hard...maybe 50 pounds of force at the most. So now we have 450 pounds of force acting in the forward direction. The conveyor belt itself does not impart any friction or drag to the airplane. It will accelerate, gain airspeed, and take off

Let's think of it a different way. Let's say the plane is landing. It approaches the runway at 100 knots and is 1 foot above the ground. As posed in the original question the conveyor belt is moving in an equal and opposite direction. So it's moving 100 knots backwards. As soon as the plane touches down, what happens? nothing...it continues its rollout as if the runway were stationary. It doesn't just automatically come to a stop. The airspeed indicator of the plane would read 100 but the wheel speed would be 200. (apologies to Youens at flightinfo.com)

People cannot separate a car/bike/walking on a treadmill with an airplane. Same thing would happen if you put the airplane on ice and hit the throttle, it will still accelerate at the exact same rate as on a runway (not a safe operation however). Try that with a car/bike/your foot and you'll get drastically different results."

[lrh]

I would like to know if they are serving dinner on the plane or do you have to buy one of their bag lunches.  It's a long flight to Honolulu you know.

[miguel]

I would like to know if they are serving dinner on the plane or do you have to buy one of their bag lunches.  It's a long flight to Honolulu you know.

LOL!!!

on the "food for sale" flights, we usually buy a sandwich and take it on the plane.  last may, flying home from Kaua'i, a flight attendant saw tomatoes in my sandwich and with a loud voice says, "how did you get tomatoes past agricultural inspection?  that's a big no-no!"  then she called another attendant to come and look at my tomatoes!  :lol:

[darkrose]

then she called another attendant to come and look at my tomatoes!  :lol:  

[mashean]

I say use a harrier and this isn't even an issue!  It'll take off straight up