A harmless little riddle

Hiya wrote:

Just as when you run on a treadmill, your legs are cycling, but your body does not actually have any horizontal displacement about the sagittal plane.

they are not the same. on a running treadmill, propulsion is produced by pushing off of the treadmill. an airplane pushes off of the wall.

to make an accurate comparison, start running on a treadmill with a big CO2 cartidge tied to your waist pointing backwards. light it off, and it'll push your waist forward relative to the treadmill.

tank wrote:

2 the question was about airplanes not jets.

whether it's a prop or a jet doesn't matter. thrust is produced by exerting force on the air, not the ground. I know of no plane that powers it's wheels during takeoff.

You all need to realize that when a plane takes off, the runway is only there to hold it up, not to push it forward.

It amazes me how many times people can chime in with the correct analysis, and some of you just don't get it. It's really funny that those with minds that work like bricks are calling those who have figured it out 'retards' and 'morons.'

That's why I'll chime in only once. It seems like this will go on forever, until you some of you actually try it.

carryall wrote:

This experiment has actually been done.

Urban myth. I heard that Mikey kid drank coke and pop-rocks and his stomach blew up.

[qutoe] The plane takes off. [/quote]

No it doesnt

I don't really care if anyone believes it or not

Obviously you do since you took the time to post it here.

OK

Here's your real world example.

Raise your hand if you've ever stuck your hand out a car window while driving and enjoyed the feeling of it being lifted up by a power beyond your control. That is how an airplane gets off the ground.

Now, find a treadmill that will go fast enough, and will fit a car (this hypothetical airport runway treadmill would suffice), and drive the car up to a speed of 60 mph, which is more than enough to raise your hand while driving on a highway. Stick your hand out the window. Guess what. No lift.

No lift = no takeoff.

Feel free to put the car in neutral and tie it to a wall with a rope if you like, or on roller skates, or facing east. Will not affect the lift.

If a plane requires a still surface to use its wheels, then how do sea planes work? Or ski planes?

I think people are still not understanding how a plane uses its wheels. I can understand how that is a difficult concept; I didn't know until I was eleven years old that the wheels did not push the plane forward!

Wrong line of thinking, I'm afraid.

Put the car in neutral, and strap a jet engine to it. Fire it up, and the car will start moving forward relative to the ground. Your hand will feel lift force.

You were close with the rope, but to create a force similar to thrust, the rope would have to be pulled and thus would pull the car forward. Merely tying the rope to the front of the treadmill is not creating force.

this is conservation of momentum:

the engines exert a force on the air that rushes through them. the increase in the air's momentum toward the back of the plane is equal and opposite to the plane's change in momentum forward.

because the wheels have no power going to them, whether they are on a treadmill or on asphalt, or whether they even exist is irrelevant. air goes back, plane goes forward, just like the recoil of a gun.

the riddler wrote:

An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?

Give my vote for it'll take off - the question says the plane IS MOVING - so that's a given. So once the plane is moving at 150mph it'll will lift off, the treadmill is going in the opposite direction at 150mph (forcing the engines to work twice as hard as on a runway). The question says the treadmill is going the same speed as the plane is moving not the same speed as the engines would normally produce to go 150 on a stationary runway. Of course I'm assuming "is moving" means is moving forward relative to the ground and air no the wheels are moving at 150mph.

Will a plane parked on a runway take off in a 150mph wind?

the riddler wrote:

An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?

The key phrase is at the same speed the plane is moving. No matter what speed the plane goes, the treadmill matches it, and thus relative to the air around the plane, it is not going anywhere. Relative to the treadmill, yes, but to the air which provides the lift, no. No moving air = no lift.

Sea planes and ski planes take off by reaching a fast enough speed so that the air moving under the wings is moving by fast enough to provide lift.

It doesn't matter where the force moving the plane forward comes from, be it a jet engine or powered wheels or a wizard's spell. If you've ever watched any race car video, you've probably seen one flip up from the front, cause by too much lift underneath the car. The fact of the matter is, the airplane on the treadmill is not moving through any air, no matter how much force it gets from the engine or how fast the treadmill goes, and will thus not get any lift and will not be able to take off.

I think it will fly, but the propellers will have to be going much much faster than for an ordinary plane.

A few points:

Assume it is a prop plane, with a big propeller in front of it. Planes get lift from wind going over the wings, as stated in earlier posts.

Take away the treadmill. If an ordinary plane is in the air, propeller spinning, but with zero forward velocity, it stalls, and falls.

However, I could imagine the propeller spinning SO fast that the wind goes very fast over the wings and generates lift -- the wind would come solely from the propeller. But this would require the propeller to go much faster than a normal airplane engine would be capable of going.

It's funny how many people think it will take off - and how strongly they defend that contention.

There is no way that plane is taking off. So what if the wheels are spinning at 300mph. If the plane is not moving relative to the air then there is no lift.

Seems like the people are making some invalid assumptions about what would actually be occurring.

Those that feel that the plane will not take off assume that because it is on a treadmill that the air surrounding it will be still. But think about it. Would you want to be standing in front of the airplane even if it is on a treadmill?

OK, so the real question is could an airplane create enough "wind" to generate the lift necessary to take off? I would imagine that a normal airplane could not do this. That is, on a treadmill, it would reach some maximum "air" speed that would be insufficient for takeoff.

But then how do they fly normally? Well, because the airplane is also moving relative to the bulk air, thus the force generated by the engines increases the velocity. That is, instead of sucking in "still" air it is sucking in air with a certain relative velocity. On the treadmill, the engines would always be sucking in "still" air (unless you design some sort of wind tunnel, but I have no idea about that).

Another way to think about it is like this. Say on a normal airplane you speed up to 100 mph and then cut off the engines. No despite the slowdown due to drag and friction, there is still close to 100mph air flowing by the wings, at least for a while. Now what happens on the treadmill?

On the treadmill, the airplane is responsible for creating all of the wind necessary for lift since it doesn't get any "for free" from its relative motion. Well, of course it's not really for free. In essence, on the treadmill, the kinetic energy that the airplane would have had if it were in normal forward motion goes into spinning the treadmill.

let me qualify this by dispelling the myths of the arguments pro-take off. I understand that the force for the airplane comes from the engine, and that the wheels on provide a low friction platform that the airplane rests on when on the ground. However, the low friction platform only ceases to be a platform when the plane moves fast enough relative to the air around it that it acquires enough lift to take off from the ground. While the plane may reach speeds far in excess of the necessary speed for takeoff on a normal runway, the problem states that the treadmill will maintain the same speed, and the airplane will not move in relation to the air around it.

The speed of the airplane and the speed of the treadmill have no real effect on the outcome of this problem. it is the speed that the air moves through the lift mechanism created by the wings. The airplane doesn't actually go anywhere, thus there is no air acting on the lift mechanism which would take the weight of the plane off of it's low-friction platform of wheels, and as such the full weight of the plane remains on the wheels and on the treadmill.

Seems like the people are making some invalid assumptions about what would actually be occurring.

Those that feel that the plane will not take off assume that because it is on a treadmill that the air surrounding it will be still. But think about it. Would you want to be standing in front of the airplane even if it is on a treadmill?

OK, so the real question is could an airplane create enough "wind" to generate the lift necessary to take off? I would imagine that a normal airplane could not do this. That is, on a treadmill, it would reach some maximum "air" speed that would be insufficient for takeoff.

But then how do they fly normally? Well, because the airplane is also moving relative to the bulk air, thus the force generated by the engines increases the velocity. That is, instead of sucking in "still" air it is sucking in air with a certain relative velocity. On the treadmill, the engines would always be sucking in "still" air (unless you design some sort of wind tunnel, but I have no idea about that).

Another way to think about it is like this. Say on a normal airplane you speed up to 100 mph and then cut off the engines. No despite the slowdown due to drag and friction, there is still close to 100mph air flowing by the wings, at least for a while. Now what happens on the treadmill?

On the treadmill, the airplane is responsible for creating all of the wind necessary for lift since it doesn't get any "for free" from its relative motion. Well, of course it's not really for free. In essence, on the treadmill, the kinetic energy that the airplane would have had if it were in normal forward motion goes into spinning the treadmill.

ok aerospace guy (yeah right!), if you think you are correct then answer this riddle:

There's a boy. This boy's favorite thing to do is get on his skateboard and have his dad tow him around town by a rope attached to the bumper of his dad's car. The boy and his dad come across this infamous treadmill and get an idea. The boy will put the skateboard on the treadmill, hop on the skateboard, pick up the rope, and have his dad hit the gas. Whatever speed the boy starts to move forward will be matched in the opposite direction by the treadmill.

Now to simplify things lets say the rope is indestructable, and inelastic. Let's pretend also that the skateboard has special bindings making it impossible for the kid to fly off of it, and that his dad's car is similarly indestructable, meaning the bumper will not break off.

So the dad hits the gas....WHAT HAPPENS?????

What the f*** are you talking about?

thank you.

Amazed wrote:

What the f*** are you talking about?

If you think that if a force causing a plane to move forward will be negated by a treadmill moving backwards then you are in quite a dilemma with the question I presented to aerospace guy. How would it be possible for the car to pull the boy forward if the boy is on a treadmill that matches his speed going backward? If the car started to move forward it couldn't because that means the boy would start to move forward and he can't because his speed is negated by the treadmill. But if nothing could move forward in that way then the car's wheels would be screatching on the asphault while the car didn't move at all. But that too wouldn't make sense because if the car isn't moving then the boy isn't moving and the treadmill isn't moving. So where's the force coming from that holds the car in place???? It is an absurdity. An absurdity defended with the same logic has been used by everybody who says it will not fly.

Ok think about the tredmill riddle. You have the skateboad and the car sitting stationary. now while the car is in park turn on the tredmill and the wheels move. The wheels will go as fast as the tredmill and vise versa, so if you want to make it easier to think about it, imagin a skatebaord without wheels on the ground(not a tredmill)...Now what would happen? The same thing as a skateboard with wheels on a tredmill. I'll let somone else say it and get the credit

That example would only work if the car were also on the treadmill. If the car is not on the treadmill, then the boy on the skateboard will be going faster than the treadmill, which is not the situation posed by the riddle.

The treadmill moves at the same speed the airplane is moving. When a treadmill moves at the same speed as a runner, the runner doesn't go anywhere. If the runner goes 10 mph, and the treadmill goes 10 mph, the runner goes nowhere relative to the air around him.