Imagine a plane is sitting on a treadmill

Did I get the job?

He said crack engineers, not engineers on crack.

who said it slowed the plane down?

No, it stays still relative to the ground, PM me if you want a video, I've got it on my desk here. It will only move if the friction in the wheel bearings is enough to accelerate the car/plane.

If the engines were switched off the plane would accelerate slowly backwards (while the wheels started to spin) due to the friction in the wheel bearings.

It's simple physics and resolving forces ... come on!

You did! Stopped is slowed down, isn't it?

Backwards...due to the small friction in the bearings etc

Edit: Poos, SGB27 beat me to it

If they were never switched on in the first place, it would stay still as the conveyor belt only moves if the plane does.

the conveyor belt moves relative to the plane not the wheels.

the plane will be doing x and the belt -x. the wheels (poor buggers) as they are freewheeling will be doing 2x.

the planes relative position is x + -x

exactly.

Yeah but they ARE switched on

The plane will fly and that's all there is to it - wheels will turn almost twice as fast as on a normal take off, but airspeed (crucially) will remain the same. End of.

And you try telling that to everyone else *sigh*

Not totally convinced by Cecil's answer (though the more I think about it the more it makes sense)

Has anyone actually tried this?

Me like this one. It gets better as you think.

At first I would have bet my left testicle that it would never take off - that the plane would be motionless with spinning wheels and noisy engines.

Then I thought...

After some consideration - I am thinking it is pretty much IDENTICAL to a plane that is already in flight. A plane in flight has no wheels, no conveyor belt, no friction at all underneath but it still flys - and if you put your foot on the gas it goes quicker.

Which tells you there must be a force from the conveyer belt acting on the plane`s gravitational force.

You need to think about force vectors, the plane has two forces acting on it, gravity and its engine thrust. The force vector will be towards the ground unless another force acts against gravity, hence the need for lift from the wings or pointing the engines downward (VTOL).

And what is the plane doing relative to the air? Bearing in mind it's pumping a lot of air out behind it? Hmmm?

Heres something for you "Non Beleivers" to consider... In your interpretation of the question, what frame of reference is the treadmill using to decide what speed to turn?

a. the planes speed relative to a stationary observer
or
b. the planes speed relative to an arbritary point on the surface of the treadmill?

A link to an expert opinion (assuming the self proclaimed "smartest man in the world" is indeed an expert) has been posted but there are no shortage of people, "experts" included, that disagree. So... keep going.

BTW: I've asked that the link and quotes of the link be removed for the sake of discussion. That also means you shouldn't post any new links to the answer (assuming there is one).

So if there is no air in space...what do space rocket engines push against??

No, it's not identical. There IS a difference: there is a difference in velocity between the wings and craft itself. For the craft on the conveyor there is no difference.

stopped realtive to us. err isnt the plane a car until it takes off?

Rockets don't push, the forward motion is a reaction to the force of the gases pumped out behind as in Newton's law 'every action has an equal and opposite re-action'.

nope, its a jet car

No because the wheels aren't driven.

The thrust within the rocket chamber pushes in all directions. Because there is nothing to push against in the nozzle, the net thrust is on the object - hence opposite to the direction to the exhasut.

I hope they aren't in charge of anything important...

Still no-one has explained, in detail, the force that the treadmil exerts on the plane to stop it moving forward with respect to a stationary (to the observer) object.

Once someone can do this convincingly, I will concede that the plane will not take off.

tom, i'd say a.

cant be arsed to type. heres a picture. i even used two colours to show position of belt bit

BTW DW what xmas fun!

Let's examine the question.
"Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction. EDIT: the engines are running at take-off thrust, the brakes are off, etc. Everything is normal save for the fact the plane is on a treadmill.

Can the plane take off?"
If the plane moves down the convayor/runway at 100mph, the convayor goes the opposite direction at 100mph, the wheels are spinning at 200mph but the plane is still moving at 100mph.
What's so hard about that?

Of course it can't take off without any aerodynamic lift.

Notwithstanding, the pilot and co-pilot will be screaming, "AHHHH!!! FUCKIN' 'ELL!!! The runway just started fuckin' rollin' underneath us! HELP!!! HELP!!! MAYDAY!!!"

if your answer is a, then the treadmill would not be turning at all as the planes speed relative to a stationary observer (Answer a) is 0 to begin with.. Where is all that thrust going now? Considering that the brakes are off.

No. Difference between a car and a plane is that the car NEEDS the friction under its wheels to make it operate. If the car is on a conveyor belt - it has lost all its means of forward motion.

Imagine if you will that you are as big as the plane. You are lying on your back on a skateboard, and with your legs compressed against a brick wall - ready to THRUST against the BRICK WALL. The conveyor belt under your skateboard is rolling the opposite way to try and stop you.

You then stretch your legs out - will you move away from the wall? Too damn right you will.

I was also convinced it would not take off until I thought about it this way:
You have a single free-wheeling wheel on an axle. The wheel is on a conveyer belt. Switch on belt, wheel moves backwards.
Then you get two people to hold the axle at each end, either side of the conveyor belt. Switch on belt. Wheel spins, but with minimum force you can keep the wheel in the same position.
If you start to walk forward it should be pretty easy to rol the wheel forward, no matter how fast the belt
is travelling, assuming good bearings with little friction.
Once you overcome the friction the wheel/axle assembly will not move backwards.

Another way of looking at it. The thrust of the jet and the speed of the conveyer belt are two forces that are causing the wheels to spin. They do not cancel each other out as some people seem to think.

Whether it flies or not is irrelevant. The question is how is the lift provided to get the plane off the ground. No air flowing over the wings, no lift

The conveyer belt must be exerting a force on the plane because if it wasn't the plane would accelerate downwards.

There will, I would say, be slightly more power required to reach takeoff or it will take the plane longer to reach rotation speed thanks to the extra, small, amounts of friction in the wheels of the plane. However, the jet engines act on the air, not on the ground, so this will only be, in effect, slightly more than usual, and won't stop the jet getting off the ground.

If the treadmill is turning, always, at twice the speed of the jet, the wheels of the jet will be turning twice as fast as normal, but the airspeed will be the same as normal, thus the plane will take off as normal.

By, errr, moving along relative to the wind, thus getting air speed, thus getting lift

The plane moves forward relative to the air, which is stationary relative to the earth, except there's a giant conveyor going underneath it. So the plane has air and ground speed and takes off normally.

I have a small physics question to ask to determine if my equation is correct, and then I'll be able to answer the original question in full:

Are there any terrorists on this flight?

Obviously not, terrorists always counteract fake-treadmill-forces. Duh!

The problem here is that there are two reference frames: one on the conveyor belt and one NOT on the conveyor belt. To a person standing on the conveyor belt, the plane moves past you at take-off speed. But to a person NOT on the conveyor belt, this person appears to be moving BACKWARD from the aircraft.

I am assuming that the air speed is 0 w.r.t. the person NOT on the conveyor belt.

So, as I said before: there is nothing to stop the plane from moving forward, w.r.t the conveyor belt. The speed of the one cancels the speed of the other with respect to the person not on the conveyor belt. Since the airspeed is 0 relative to the person not on the conveyor belt, the aircraft is not moving w.r.t. the air. No air flow over the wings, no thrust.

Hey Gideon. What happens if we rotate your diagram and make the "plane" take off vertically? (we will call this invention a WOCKET for now).

Will the treadmill stop the Wocket?

so does a plane! a plane will not role forward on concreate if it has no wheels. the car analogy is rubbish as the ground in that analogy has no momentum. the belt moves with the speed of the plane the wheels are irrelevant. imagine there were no wheels. wheels have bugger all to do with lift. imagine it was a body of moving water and the plane was on a flaot?

@ JerseyBoy

No lift you mean. There's loads of thrust thrusting the plane forward relative to the air and ground, which does generate lift, so you're still wrong.

That would happen if you throttled the engines down so that they only provided enough thrust to counteract the forces of friction in the wheel bearings & tyres (and if the conveyor was just tootling along at that speed regardless of the planes speed).

But as the question is posed, the plane has full take-off thrust. The plane is in fact moving forward relative to the stationary observer as there is no force stopping it from doing this?

What force is the treadmill exerting on the plane to stop it moving forward wrt a stationary observer? WHAT FORCE???

Nice JE.

Sin,

Engineers go completely mad and create a massive conveyor belt to be used instead of a tried and tested runway. Then, there are pilots and other aircrew, plus the aircraft's owners who are equally mad enough to put a plane on this conveyor belt/runway thing and try to take off.

And you are worried about terrorists?

Loads of thrust - we all accept that, where is the lift coming from. Ground speed a grizillion miles an hour, airr speed zero. What gets the front wheel off the belt?

Dude - I think you are losing the plot now.

First you say the plane will not role (sic) forward on concrete with no wheels - and then you say what if it was on a flaot (sic)?

You say the wheels have bugger all to do with lift, but they also have bugger all to do with the ability of an aeroplane to move forwards. A plane gets its forward motion from pushing against the air.

It won't? How about planes that take off on skis, or floats?

Edit: well that doesn't make sense does it