they actually did a pretty good job of wording it in my opinion, as even after 6 pages, we're still reading things into it.
"It is then possible to get the plane to move the opposite direction of the conveyor so that to an observer, the airplane doesn't move." yes, and then, you can increase the thrust from the engines, and get the plane to move a bit more.
the state you referred to is when the constant frictional force provided by the conveyor is equal to the thrust of the engines. you said yourself the fricional force is constant, and that is what provides the --> force, thrust provdiing the <-- force. so pilot can then increase thrust, to saaay, <----- force, and friction is still only ->> force, as it is constant, as you said. plane moves faster. plane continues to move faster, at a rate governed by the resultant force, which will equal force due to thrust minus resistance from the conveyor. this reultant force is of course = to the mass of the plane multiplied by the acceleration, and as this resultant force is now >0 (as thrust is bigger than resistance) the acceleration is equal to this resultant force divided by the planes mass. even if this is very small, it is acceleration. and as such, the plane will eventually reach the speed necessary for it to take off.
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"Bartolo" 0