DCer wrote:
My question is this, and please answer if you know:
The thrust is provided by the engines using air like a rope, we all know this...
But because the wheels ARE needed for the absence - or near absence - of friction, how can the plane continue to move forward if the treadmill matches the wheels' speed?
I mean, due to gravity, the wheels ARE inded necessary for the plane to move forward, right? I guess my question is, if the wheels are spnning at 300mph, and so is the treadmill, how can the plane move forward, assuming the treadmill always matches the speed of the wheels?
thanks
There are two pairs of forces at work in this problem: gravity and lift, and thrust and drag.
Gravity obviously pulls the plane down toward the ground.
Lift is needed to overcome gravity and carry the plane into the air. As many on both sides have correctly stated, lift is generated by the movement of air over the wings (airspeed).
Thrust is provided by the plane engines. That is the engines and the engines alone push the plane foward. This is true for all planes whether they have skis, floats, or wheels. Thrust, when a sufficient amount is applied, moves the plane foward and produces both airspeed and groundspeed (though they may not be the same).
Drag, because the wheels on a plane's landing gear are free spinning and are designed to be as close to frictionless as realistically possible, comes from only one source in this problem, and that is air resistance.
Since the runway (be it treadmill or standard runway) cannot apply a drag force to the plane via. the near-frictionless wheels (unless the brakes are engaged), the runway can not oppose the applied thrust. Since there is no force to oppose the applied thrust the plane will move foward. Once the plane has attain a sufficient airspeed the magnitude of the lift force, produced by the motion of air over the planes wings, will become larger than the magnitude of the gravitational force and the plane will take off.
Think about how a seaplane takes off against a tide or current.