Lets take flying out of the equation. Lets say you have a jet powered car, and the engine makes it go 1 mph. Is your contention that the conveyor belt cannot possibly slow down the jet-powered car at that point?

hd

via friction from the belt.

absolutely. I've even said that the conveyor belt will need to keep accelerating (go back to my example of powering the belt with a mass and pulley system, so that it keeps accelerating)

I address it constantly. Since the wheels are free rolling the only force working against it is the friction from the wheel to the axle. This is a minimal force and easily overcome by the jet engine which is not working against the treadmill but the air around the plane.

ok, but you realize that it is impossible for the belt to "keep accelerating" right? i mean, it can't accelerate forever.

ok, but you realize that it is impossible for the jet engines to keep applying a force, right?

obviously, as I've stated many times, we are in the realm of hypothetical/theoretical here. If the jet and treadmill have the same power (or, ability to apply a force...which means accelerate), the treadmill will keep the jet from taking off.

Like i said, it's a tough bias to deal with, because in the real world, jets have more power than treadmills. but if we let them have the same power, the treadmill will win.

I'm not trying to say that we could ruin all of the worlds airports by installing huge treadmills or anything.

hd

but the friction keeps increasing as the wheels spin. even though the jet doesn't apply power to the wheels, the wheels can be used to slow down the jet.

I stole this from another site and dont claim it as my own.

My father, George Springer, is a Stanford professor and past chair of the Aero/Astro (Aeronautical and Astronautical Engineering) Department, so I thought I'd send the puzzle on to the experts. My father has 50+ years of experience in structures, aeronautics and fluid dynamics, I figured it would be obvious to him. Turns out it is a poorly worded and more subtle problem than it seems at first reading (as many have commented). After a couple of days, and discussion about it with another member of the department who is THE guy - the world's foremost expert - on this kind of thing, the bottom line is that the plane will move, even if the treadmill goes backwards, and will therefore take off. Here is the explanation (with demonstration) that he sent to me:
The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.

So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!

When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.

the engines don't have to keep applying a force! they can simply wait for the treadmill to stop accelerating and then apply force and take off easily. like i said it has nothing to do with the power of the jet or treadmill.

the REAL PROBLEM with these threads is you have people on BOTH SIDES OF THE ISSUE who have no idea what they are really talking about. even the people who are right most of the time have no clue why.

there are MANY problems with this.

1.) Finger on the car, pulling paper out from underneath it: First, this is such a small scale, the normal person won't be able to tell subtle differences in forces.

Imagine doing this with a tarp underneath a 4x4 that is not in gear. Will you still be able to "easily" hold the 4x4 in place with a finger? no. Will you still be able to "easily" pull out the tarp? no.

If you try to pull the paper out faster (with more acceleration, not velocity), do you think it would become harder to hold the car in place? If so, then you see my point. If not, then we agree to disagree.

2.) The matchbox car being pulled by a string. IF you pull it one with with a string and pull the treadmill the other way with any sort of velocity, the wheels will start to slip. Everything I've said has had the assumption that the wheels of the jet won't slip on the treadmill.

3.) he talks about the "main forward thrust". Again, he is falling victim to the bias that the jet is more powerful than everyday treadmills. If they have the same acceleration ability, then the treadmill will win.

THese are three huge problems that negate the adwar's long response that popped into my sleep deprived head immediately.

I really think that the big issue here where we are just two ships passing in the night is the fact that I'm assuming that the treadmill will have as much accelerating power as the jet. If it's a standard jet and a standard 24 hour fitness treadmill, the jet will win. If they have the same power, the treadmill will win.

the treadmills don't have to keep applying a force! they can simply wait for the jet to start accelerating and then apply force and stop the take off easily.

The reason why the treadmill wins when they have the same power ability is because the jet needs to be accelerating more than the treadmill for an extended period of time to achieve takeoff.

I've concluded one thing indisputably....

If you are an insomniac like I am, posting ad nauseum in this thread will help you get tired. I'm going to bed for the first time in a few days now. I'll revisit this when I wake up. Maybe then I'll see where I'm wrong, or a better way to show why I'm right.

But I'm still not totally convinced I'm right (but I'm getting there...)

hd

i just don't know what to say anymore

Once again, this is wrong. When you apply a force to the bottom of the wheel via accelerating the treadmill, most of that force will be turned into rotation yes, but there is still a small linear component acting against the axle due to the rotational inertia of the wheels. The faster you accelerate the treadmill, the more the engines will be counteracted. This is *in addition to* any linear force backwards due to friction of the bearing. The friction will be a constant force, but the other linear component will be proportional to the acceleration of the treadmill.

humdinger, this is wrong. Once the wheel is moving at all, the friction is constant until the mechanisms of the wheel bearing actually break down. The force friction applies doesn't depend on the velocity of the object being acted upon.

Is this actually right? I'm a little confused. The acceleration part of friction is in the normal force, right? Friction force (F) = u (friction coefficient) x N (normal force). The only thing linear acceleration (velocity actually) affects is u. i.e. the faster the wheels spin, the larger u is. Since gravity is always accelerating, and as long as u is large enough (from a super fast treadmill), why couldn't the treadmill counteract the plane? Obviously in a real world, sans infinitely fast tradmills and with a properly maintained plane, u would never be large enough. Sorry, it's been a long ass time since I did anything with mechanical type physics.

humdinger- While it's possible to counteract an infinitely large amount of thrust with a treadmill, that's not the question.

1. To negate that amount of thrust through friction on the bearings will almost certainly cause the landing gear to break.

2. The question says nothing about counteracting. "If a treadmill holds a plane in place is it moving?" is a Zen parable, not a legitimate question.

Velocity doesn't affect the coefficient of friction (other than going from the static coefficient to the kinetic coefficient). μ only depends on the materials and shape of the two surfaces.

Thanks Jdub (Josh) and Humdinger

Unlike you folks, I went to sleep last night....lol

Anyway, I watched the video several times. Great job in such a short time-frame guys!!!! What I did notice is that the pinewood car did NOT stay stationary (but preety darn close). If you view the video it was moving forward slowly as the paper was being pulled. It wasn't much, but it was still moving. Regardless, I think if you had a longer piece of paper, you could hold the car in place if you kept increasing the speed of the paper. Thar is a big "if."

This brings us to the point that Daryn has been making. And that is the role of acceleration. In order to keep the car stationary, the paper (or treadmill) must ALWAYS be accelerating.

anyone who doesnt understand that the plane will fly and has read all of this thread, should ensure they never have kids. The risk to the gene pool is too great.

imo

The only way the conveyor belt would have any effect on either scenario, is if there was a brake on the wheels supplying enough friction to overcome the forces propelling the wagon down or the airplane forward.

Introducing friction in this way changes the experiment.

lol, how did this thread happen again?

Wrong, as I've explained like 5 times now.

No, I'm not wrong. Your posts deal with the rotational inertia of the wheels. Unless the wheels make up a significant percentage of the mass of the plane, the amount of force going in to turning the wheels is insignificant.

I've skimmed this thread.

I want to shoot myself in the face.

I think the problem here is that people think of the wheels as something other than a way for the plane to move down the runway with the least amount of resistance.

If the plane was on skis and the conveyor belt was made of snow you'd get the same result: The plane would take off. The amount of friction under the skis is negligible.