Originally posted by hitech
Eskimo2: Even in your theoretical case, the conveyor and wheel speed do not equal a change in force.
Visualize yourself on a pair of roller skates standing on a moving side walk like in an air port. You are motionless relative to the building holding yourself motionless by holding a rope. On the end of the rope is a scale measuring how hard you are pulling, lets say 10 lb of force.
Speed up the side walk , the scale will read 10 lb's.
Slow down the side walk, the scale will still read 10 lb's.
Take you theoretical wheels and conveyor, scale still reads 10 lb's.
Yea, I got that Hitech; that’s what I’ve been saying all along.
What has been ignored by most of us though is that it takes energy to increase a wheel’s rpm. When an aircraft lands, its wheels go from 0 to 100 mph in a second. Even if the pilot greases the landing with nearly no vertical decent, the touchdown bump the get the wheels up to an outside diameter speed of 100 mph exists, and can be felt. The amount of energy is very small compared to the inertia of the plane and the power produced by the motor, but it does slow the plane down a tad.
One time I dropped a bowling ball out of a car window; we were going 70 mph on a deserted desert highway. The bowling ball’s speed was cut significantly within in a second or two. Why? Because some of the 70 mph of non-rotational inertia had to be redistributed to rotational inertia. In fact, the formula for the rotational inertia of a sphere: 2/5 IR squared comes to mind (back from high school physics; not sure why I remember that one).
Reconsider the hypothetical question where the wheel and treadmill can be accelerated up to infinity and still hold together, note that we are just talking about the wheel, not the plane. In fact, imagine that we are only talking about an aircraft wheel with say a mass of 100 lbs and no aircraft, sitting on a treadmill. Turn the treadmill on at 10 mph suddenly and the wheel will start rolling back. It will drift back until its rpm gets up to 10 mph. If you were holding a rope, tied to a mass-less axle through this wheel, however, you could hold it in place. You would feel a tug at first, then once it was up to 10 mph you would feel almost nothing and could hold it in place with one finger.
Now imagine that at the end of 1 second the treadmill is going 10 mph, at the end of 2 seconds it is up to 20 and so on. In this situation you would feel a constant pull. This force would be insignificant to a big aircraft engine, but to you, you would definitely feel it. Now exaggerate the snot out of this rpm example; if the wheel and treadmill can be accelerated up to infinity very quickly and still hold together (and not burn up) we could be talking about big forces. If this treadmill and wheel accelerated up to 1,000,000 rpm in 1 second, the rope would tear your arm off.
